Fibronectin Type III Domain Research Articles

Glycoside hydrolase family 5 (GH5) from Paenibacillus curdlanolyticus B-6 (PcGH5) is a modular protein consisting of a GH5 catalytic domain and three non-catalytic domains (NCDs): a family 11 carbohydrate-binding module (CBM11), a fibronectin type 3 (Fn3), and a family 3 carbohydrate-binding module (CBM3). PcGH5 exhibits unique properties compared with other GH5 members; it shows multifunctional endo-cellulase, endo-xylanase, endo-mannanase, and endo-1,3-1,4-β-glucanase activities. To understand the function of each NCD, recombinant full-length PcGH5 and truncated variants were analyzed. Compared with the GH5 catalytic domain alone, stepwise addition of CBM11, Fn3, and CBM3 enhanced substrate binding and improved enzymatic activities towards polysaccharides, as confirmed by Fourier transform infrared spectroscopy (FTIR). Each NCD contributed distinctly: CBM11 bound 1,3-1,4-β-glucan, xylan and mannan with limited branching; Fn3 bound only cellulose; and CBM3 significantly improved binding to 1,3-1,4-β-glucan and highly branched xylan and mannan. There was a higher percentage of surface-exposed aromatic amino acids in CBM11 and CBM3 of PcGH5, important for hydrophobic interactions with sugar rings, compared to other CBM11 and CBM3 members. Unlike other CBMs, sequence alignment and structural modelling revealed that PcGH5 CBM11 and CBM3 have extra and/or more surface-exposed aromatic amino acids, which could interact with various oligosaccharide ligands such as hexose (cellotetraose and mannotetraose) and pentose (xylotetraose) via hydrophobic interactions, affecting enzyme activity. Mutagenesis confirmed Trp17 (CBM11) and Trp51 (CBM3) as key residues for insoluble substrates binding and enzymatic enhancement. Therefore, these aromatic amino acids are key factors improving substrates binding and enzymatic activities of PcGH5 towards different β-1,4 glycosidic polysaccharide substrates.

ABSTRACT The obscurin family, containing the giant protein OBSCN (obscurin, cytoskeletal calmodulin and titin-interacting RhoGEF) and its closely related OBSL1 (obscurin like cytoskeletal adaptor 1) as well as SPEG (striated muscle enriched protein kinase) are a group of intracellular proteins that contain serially linked immunoglobulin (Ig) and fibronectin type III (Fn3) domains, along with signaling modules such as protein kinase domains. Hence, obscurins harbor multi-faceted roles for the architecture and organization of cell- and organelle membrane proteins. Besides mediating cellular signaling and promoting protein homeostasis, obscurin proteins are also proposed to act as versatile cytoskeletal linkers. Due to their close homology, many functions for OBSCN are evolutionary conserved in OBSL1 and SPEG. However, their expression patterns differ widely, with OBSL1 being ubiquitously expressed in all cell types, while OBSCN and SPEG are more restricted to cross-striated muscles. Recent evidence indicates that autophagy-linked peptidases of the ATG4 family interact with the cytoskeletal adapter proteins OBSL1 and OBSCN. Peptidases of the ATG4 family process Atg8-family proteins (e.g. LC3) in their immature state (i.e. as pro-peptides like pro-LC3) or their bioactive lipidated state (i.e. LC3-II) and facilitate their conversion to the delipidated state (i.e. LC3-I). Loss of interaction between ATG4 peptidases and obscurin family proteins affects cellular macroautophagy/autophagy and mitophagy, leading in situations of cellular stress to depletion of ATG4 and accumulation of the lipidated state for Atg8-family proteins (e.g. LC3-II).

LRIT3 is a leucine-rich repeat (LRR) protein that is expressed in the retina, and its absence causes complete congenital stationary night blindness (cCSNB), a genetically diverse disorder characterized by impaired low-light vision, myopia, and nystagmus. LRIT3 is expressed in rod and cone photoreceptors, and it trans-synaptically organizes the assembly of the glutamate signaling complex, the signalplex, on depolarizing bipolar cells (DBCs). LRIT3 is a single-pass membrane protein with extracellular LRR, IG, and FN3 domains. The mechanism by which LRIT3 controls postsynaptic receptor organization remains unknown. We address this by using rAAV to express deletion constructs in LRIT3 knockout retinas and examining LRIT3 trafficking, as well as the structural and functional recovery of the signalplex in DBCs. We show the LRR domain is required for trafficking LRIT3 to the synapse in cones, but not rod photoreceptors, although it is needed for reassembly and function of the rod BC signalplex. Neither the IG nor the FN3 domain is needed for synaptic localization of LRIT3. However, the IG domain is required for the localization of TRPM1 to the signalplex and thus function. The FN3 domain is not necessary for either DBC signalplex assembly or function. Our data demonstrates that the LRR and IG domains of LRIT3 are crucial for TRPM1 localization and retinal function, with the LRR domain playing a key role in the differential function of LRIT3 at rod and cone synapses. Notably, our results show that restoring Nyctalopin localization to the DBC signalplex alone is insufficient to restore TRPM1 expression. Based on our findings, we propose a model in which the LRR domain trans-synaptically binds with Nyctalopin, while the IG domain interacts with TRPM1.

Fibronectin (FN), a primary component of the extracellular matrix (ECM), features multiple structural domains closely linked to various cellular behaviors, including migration, spreading, adhesion, and proliferation. The FN3 domain, which contains the RGD sequence, is critical in tissue repair because it enables interaction with integrin receptors on the cell surface. However, the large molecular weight of wild-type FN presents challenges for its large-scale production through heterologous expression. Therefore, this study focused on cloning the FN3 functional domain of full-length FN for expression and validation. This study selected Bacillus subtilis as the expression host due to its prominent advantages, including efficient protein secretion, absence of endotoxins, and minimal codon bias. The recombinant vector pHT43-FN3 was successfully constructed through homologous recombination technology and transformed into Bacillus subtilis WB800N. The FN3 protein was successfully expressed after induction with IPTG. Following purification of the recombinant FN protein using a His-tag nickel column, SDS-PAGE analysis showed that the molecular weight of FN3 was approximately 27.3 kDa. Western blot analysis confirmed the correct expression of FN3, and the BCA protein assay kit determined a protein yield of 5.4 mg/L. CCK8 testing demonstrated the good biocompatibility of FN3. In vitro cell experiments showed that FN3 significantly promoted cell migration at a 20 μg/mL concentration and enhanced cell adhesion at 10 μg/mL. In summary, this study successfully utilized Bacillus subtilis to express the FN3 functional domain peptide from FN protein and has validated its ability to promote cell migration and adhesion. These findings not only provide a strategy for the expression of FN protein in B. subtilis, but also establish an experimental foundation for the potential application of FN3 protein in tissue repair fields such as cutaneous wound healing and cartilage regeneration.

The Interleukin-12 (IL-12) family ligand subunits (IL-12s) and receptor subunits (IL-12Rs) constitute pivotal regulators of immune homeostasis, with direct implications in autoimmune pathologies and oncogenesis. Through phylogenetic reconstruction, synteny analysis, and sequence alignment across 400+ animal species, we delineated the evolutionary trajectories and functional diversification of these immune mediators. Phylogenetic analysis revealed IL-12Rs originated prior to the mollusk era (514-686.2 million years ago, Mya), while ligand subunits p19/p28 emerged during the mammalian and avian epoch (180-225 Mya). Structural characterization identified three evolutionarily invariant signature motifs within the fibronectin type III (fn3) domain essential for receptor-ligand interface stability. Furthermore, phylogenetically ultra-conserved residue and motif configurations were mapped, revealing candidate therapeutic epitopes. These findings establish an evolutionary framework explaining functional conservation/divergence in IL-12 signaling components. The identified ancient receptor architectures coupled with derived ligand innovations provide a blueprint for cross-species immunotherapy design targeting conserved interaction interfaces. The conserved molecular signatures offer dual utility in developing precision therapies and interspecies disease management strategies, particularly for translational applications across human medicine, agriculture, and aquaculture.

This study aimed to clarify the sequence characteristics and spatiotemporal expression patterns of the insulin-like growth factor 1 receptor (IGF1R) gene in Alopex lagopus (Arctic fox), thereby addressing the existing knowledge gap regarding IGF1R-mediated growth regulation in this species. The findings establish a crucial foundation for subsequent investigations into the correlation between this gene and Arctic fox growth traits. Specific primers were designed based on the cDNA sequence of the canine IGF1R gene (Accession No. XM_545828). The full-length coding sequence (CDS) of the Arctic fox IGF1R gene (1617 bp, encoding 538 amino acids) was successfully cloned via RT-PCR. Phylogenetic analysis using the Unweighted Pair Group Method with Arithmetic Mean (UPGMA) algorithm revealed a 99% sequence homology in the IGF1R gene between the Arctic fox and canine, confirmed their closest evolutionary relationship. Protein characterization showed that the IGF1R protein has a molecular weight of 60.62 kDa (theoretical isoelectric point pI = 5.15), containing one fibronectin type-III domain and one tyrosine kinase domain, classifying it as an acidic hydrophilic transmembrane protein. Phosphorylation site prediction identified 27 phosphorylation sites, with secondary structures dominated by α-helices (26.39%) and random coils (52.79%). The IGF1R gene displayed significant tissue-specific expression variations across 12 examined tissues in Arctic foxes: highest expression levels in testis, minimal expression in stomach, and no detectable expression in duodenum. Spatiotemporal expression analysis revealed that in 2-, 4-, and 6-month-old individuals, hepatic IGF1R exhibited a progressive increase, testicular expression reached peak levels at 6 months, and skeletal muscle demonstrated transient upregulation peaking at 4 months. These spatiotemporal expression patterns suggest that IGF1R may participate in metabolism and organ developmental processes during critical growth stages of Arctic foxes through tissue-specific regulation.

Autoimmune nodopathy (AN), a newly recognized disease entity, is an immune-mediated polyneuropathy involving autoantibodies against cell adhesion molecules located in nodes of Ranvier and paranodal regions, such as neurofascin 186 (NF186) and neurofascin 155 (NF155). The present study aimed to identify the epitopes for autoantibodies against NF155 in a large cohort of Japanese patients with anti-NF155 antibody-positive (anti-NF155+) AN. Human embryonic kidney 293 cells stably expressing NF155, NF186, or the third to fourth fibronectin type III domain region (Fn3-Fn4) of NF155, as well as cells transiently expressing Fn3, Fn4, or the shorter Fn3-Fn4 region of NF155, were developed. Western blotting and flow cytometric cell-based assay (CBA) analyses were performed to determine the expression levels of the proteins and identify their target epitopes in serum samples from 100 IgG4 anti-NF155+ patients, four non-IgG4 anti-NF155+ patients, and eight healthy controls. The expression levels of NF186, NF155, Fn3-Fn4 of NF155, and the other truncation variants of NF155 were confirmed by western blotting and flow cytometric CBA. Flow cytometric CBA analysis showed that the autoantibodies in all 104 anti-NF155+ patients bound to Fn3-Fn4. No autoantibodies reacted with NF186, Fn4, or shorter Fn3-Fn4, although the autoantibodies in one IgG4 anti-NF155+ patient (1.0%) recognized Fn3 in addition to Fn3-Fn4. Western blotting analysis of representative samples generally reproduced the CBA results. The present study involving a large cohort of patients clarified that the primary epitope for anti-NF155 antibodies is located in the Fn3-Fn4 region, but not in the Fn3 or Fn4 domains alone.

Fibronectin type-III domain containing protein-5 (FNDC5), predominantly expressed in skeletal muscles, encodes FNDC5 transmembrane-protein. A segment of this protein is cleaved and secreted into blood as irisin, which promotes browning of white adipose tissue, leading to energy expenditure. It functions synergistically with fibroblast growth factor-21 (FGF21). Irisin is considered as a potential target for treating obesity-related disorders. Likewise, FNDC5 variations can contribute to development of such disorders. This study aimed to identify putative non-synonymous single nucleotide polymorphisms (nsSNPs) of human FNDC5, potentially impacting FNDC5-FGF21 interaction. Sequence and structure based computational tools were used to identify nsSNPs of FNDC5, which revealed eight nsSNPs as being most deleterious (N39K, R78H, R209H, T124I, L150P, L156V, V83M, and T86I). Molecular-docking was performed to analyze the impact of FNDC5 mutations on wild-type and mutant FNDC5-FGF21 complexes, revealing that T124I (rs185141197) and L150P (rs377741902) showed higher buried surface area (BSA) than wild-type. Following this, molecular dynamic (MD) simulation further affirmed the findings and revealed that T124I induced conformational changes in the irisin domain of FNDC5, which may significantly affect its binding with protein FGF21, potentially impairing synergistic effects of FNDC5 and FGF21 on adipocyte browning and increasing risk for developing obesity and related disorders.

Autoinhibition of cMyBP-C by its middle domains.

The discovery that sponges (Porifera) can fully regenerate from aggregates of dissociated cells launched them as one of the earliest experimental models to study the evolution of cell adhesion and allorecognition in animals. This process depends on an extracellular glycoprotein complex called the Aggregation Factor (AF), which is composed of proteins thought to be unique to sponges. We used quantitative proteomics to identify additional AF components and interacting proteins in the classical model, Clathria prolifera, and compared them to proteins involved in cell interactions in Bilateria. Our results confirm MAFp3/p4 proteins as the primary components of the AF but implicate related proteins with calx-beta and wreath domains as additional components. Using AlphaFold, we unveiled close structural similarities of AF components to protein domains in other animals, previously masked by the mutational decay of sequence similarity. The wreath domain, believed to be unique to the AF, was predicted to contain a central beta-sandwich of the same organization as the vWFD domain (also found in extracellular, gel-forming glycoproteins in other animals). Additionally, many copurified proteins share a conserved C-terminus, containing divergent immunoglobulin (Ig) and Fn3 domains predicted to serve as an AF-interaction interface. One of these proteins, MAF-associated protein 1, resembles Ig superfamily cell adhesion molecules and we hypothesize that it may function to link the AF to the surface of cells. Our results highlight the existence of an ancient toolkit of conserved protein domains regulating cell-cell and cell-extracellular matrix protein interactions in all animals, and likely reflect a common origin of cell adhesion and allorecognition.

In this study, we report the molecular and enzymatic characterisation of Spg103, a novel bifunctional β-glucanase from the marine bacterium Streptomyces sp. J103. Recombinant Spg103 (rSpg103) functioned optimally at 60 °C and pH 6. Notably, Spg103 exhibited distinct stability properties, with increased activity in the presence of Na+ and EDTA. Spg103 displays both lichenase and cellobiohydrolase activity. Despite possessing a GH5 cellulase domain, FN3 and CBM3 domains characteristic of cellulases and CBHs, biochemical assays showed that rSpg103 exhibited higher activity towards mixed β-1,3-1,4-glucan such as barley β-glucan and lichenan than towards beta-1,4-linkages. The endolytic activity of the enzyme was confirmed by TLC and UPLC-MS analyses, which identified cellotriose as the main hydrolysis product. In addition, Spg103 exhibited an exo-type activity, selectively releasing cellobiose units from cellooligosaccharides, which is characteristic of cellobiohydrolases. These results demonstrate the potential of Spg103 for a variety of biotechnological applications, particularly those requiring tailor-made enzymatic degradation of mixed-linked β-glucans. This study provides a basis for further structural and functional investigations of the bifunctional enzyme and highlights Spg103 as a promising candidate for industrial applications.

Pathogenic variants in the fibronectin type III domain of leptin receptor: Molecular dynamics simulation and structural analysis

Lignocellulolytic clostridia employ multiple pairs of alternative σ/anti-σ (SigI/RsgI) factors to regulate cellulosomal components for substrate-specific degradation of cellulosic biomass. The current model has proposed that RsgIs use a sensor domain to bind specific extracellular lignocellulosic components and activate cognate SigIs to initiate expression of corresponding cellulosomal enzyme genes, while expression of scaffoldins can be initiated by several different SigIs. Pseudobacteroides cellulosolvens contains the most complex known cellulosome system and the highest number of SigI-RsgI regulons yet discovered. However, the function of many RsgI sensor domains and their relationship with the various enzyme types are not fully understood. Here, we report that RsgI4 from P. cellulosolvens employs a C-terminal module that bears distant similarity to the fibronectin type III (Fn3) domain and serves as the sensor domain. Substrate-binding analysis revealed that the Fn3-like domain of RsgI4 represents a novel carbohydrate-binding module (CBM) that binds to a wide range of polysaccharide types. Structure determination further revealed that the Fn3-like domain belongs to the type B group of CBMs with a predicted concave face for substrate binding. Promoter sequence analysis of cellulosomal genes revealed that SigI4 is responsible for cellulosomal regulation of major scaffoldins rather than enzymes, consistent with the broad substrate specificity of the RsgI4 sensor domain. Notably, scaffoldins are invariably required as cellulosome components regardless of the substrate type. These findings suggest that the intricate cellulosome system of P. cellulosolvens comprises a more elaborate regulation mechanism than other bacteria and thus expands the paradigm of cellulosome regulation.

Obesity is a complex disease that has become increasingly prevalent. While obesity itself is not new, its widespread occurrence is a more recent concern. Stimulating brown adipose tissue (BAT) and promoting the browning of white adipose tissue (bWAT) have shown promise as therapeutic targets to increase energy expenditure and counteract weight gain. This study aimed to investigate two main aspects. First, we examined how obesity affects the expression of the fibronectin type-III domain containing 5 (FNDC5) and uncoupling protein 1 (UCP1) genes in male Wistar rats. Second, we assessed the effects of six weeks of aerobic exercise, exposure to cold water, and the combination of both on the expression of the FNDC5 and UCP1 genes in obese male Wistar rats. In this experiment, 25 male Wistar rats were randomly assigned to five groups (5 rats per group) after inducing obesity. The groups included: A control group (C), an obesity group (O), an obesity group exposed to cold water (OC), an obesity group engaged in aerobic exercise (OE), and an obesity group exposed to both cold water and aerobic exercise (OCE). The aerobic exercise sessions lasted 30 - 60 minutes, with a speed of 15 - 25 meters per minute. The cold water exposure protocol involved shallow water (2 - 4 cm) with a temperature of 14 - 18°C. The OCE group performed both aerobic and cold water exercises in each session. The expression of the FNDC5 gene in the soleus muscle and the FNDC5 and UCP1 genes in subcutaneous fat was evaluated using Real-Time PCR. All statistical analyses were performed using SPSS software version 16, with a significance level set at P ≤ 0.05. Obesity significantly increased the expression of the FNDC5 gene (P = 0.008). After six weeks of aerobic exercise (P = 0.016) or cold water exposure (P = 0.016), there was a significant decrease in FNDC5 gene expression. Surprisingly, the combination of both interventions did not result in a significant effect (P = 0.75). On the other hand, none of the interventions-whether aerobic exercise, cold water exposure, or their combination-had a significant effect on the expression of the UCP1 gene (P > 0.05). The increase in FNDC5 gene expression caused by obesity may serve as a compensatory mechanism to cope with the condition. However, both cold water exposure and aerobic exercise appear to mitigate this increase in FNDC5 gene expression through enhanced thermogenesis.

The glycoside hydrolase family 3 (GH3) β-glucosidases from filamentous fungi are crucial industrial enzymes facilitating the complete degradation of lignocellulose, by converting cello-oligosaccharides and cellobiose into glucose. Understanding the diverse domain organization is essential for elucidating their biological roles and potential biotechnological applications. This research delves into the variability of domain organization within GH3 β-glucosidases. Two distinct configurations were identified in fungal GH3 β-glucosidases, one comprising solely the GH3 catalytic domain, and another incorporating the GH3 domain with a C-terminal fibronectin type III (Fn3) domain. Notably, Streptomyces filamentous bacteria showcased a separate clade of GH3 proteins linking the GH3 domain to a carbohydrate binding module from family 2 (CBM2). As a first step to be able to explore the role of accessory domains in β-glucosidase activity, a screening system utilizing the well-characterised Aspergillus niger β-glucosidase gene (bglA) in bglA deletion mutant host was developed. Based on this screening system, reintroducing the native GH3-Fn3 gene successfully expressed the gene allowing detection of the protein using different enzymatic assays. Further investigation into the role of the accessory domains in GH3 family proteins, including those from Streptomyces, will be required to design improved chimeric β-glucosidases enzymes for industrial application.

Considering the current growing interest in new and improved enzymes for use in a variety of applications, the present study aimed to characterize a novel detergent-stable serine alkaline protease from the extremophilic actinobacterium Microbacterium metallidurans TL13 (MmSP) using a combined in silico and experimental approach. The MmSP showed a close phylogenetic relationship with high molecular weight S8 peptidases of Microbacterium species. Moreover, its physical and chemical parameters computed using Expasy's ProtParam tool revealed that MmSP is hydrophilic, halophilic and thermo-alkali stable. 3D structure modelling and functional prediction of TL13 serine protease resulted in the detection of five characteristic domains: [catalytic subtilase domain, fibronectin (Fn) type-III domain, peptidase inhibitor I9, protease-associated (PA) domain and bacterial Ig-like domain (group 3)], as well as the three amino acid residues [aspartate (D182), histidine (H272) and serine (S604)] in the catalytic subtilase domain. The extremophilic strain TL13 was tested for protease production using agricultural wastes/by-products as carbon substrates. Maximum enzyme activity (390U/gds) was obtained at 8th day fermentation on potato peel medium. Extracellular extract was concentrated and partially purified using ammonium sulfate precipitation methodology (1.58 folds purification fold). The optimal pH, temperature and salinity of MmSP were 9, 60°C and 1M NaCl, respectively. The MmSP protease showed broad pH stability, thermal stability, salt tolerance and detergent compatibility. In order to achieve the maximum stain removal efficacy by the TL 13 serine protease, the operation conditions were optimized using a Box-Behnken Design (BBD) with four variables, namely, time (15-75min), temperature (30-60°C), MmSP enzyme concentration (5-10U/mL) and pH (7-11). The maximum stain removal yield (95 ± 4%) obtained under the optimal enzymatic operation conditions (treatment with 7.5U/mL of MmSP during 30min at 32°C and pH9) was in good agreement with the value predicted by the regression model (98 ± %), which prove the validity of the fitted model. In conclusion, MmSP appears to be a good candidate for industrial applications, particularly in laundry detergent formulations, due to its high hydrophilicity, alkali-halo-stability, detergent compatibility and stain removal efficiency.

Synthetic binding proteins are human-made binding proteins that use non-antibody proteins as the starting scaffold. Molecular display technologies, such as phage display, enable the construction of large combinatorial libraries and their efficient sorting and, thus, are crucial for the development of synthetic binding proteins. Monobodies are the founding system of a set of synthetic binding proteins based on the fibronectin type III (FN3) domain. Since the original report in 1998, the monobody and related FN3-based systems have steadily been refined, and current methods are capable of rapidly generating potent and selective binding molecules to even challenging targets. The FN3 domain is small (∼90 amino acids) and autonomous and is structurally similar to the conventional immunoglobulin (Ig) domain. Unlike the Ig domain, however, the FN3 lacks a disulfide bond but is highly stable. These attributes of FN3 present unique opportunities and challenges in the design of phage and other display systems, combinatorial libraries, and library sorting strategies. This article reviews key technological innovations in the establishment of our monobody development pipeline, with an emphasis on phage display methodology. These give insights into the molecular mechanisms underlying molecular display technologies and protein-protein interactions, which should be broadly applicable to diverse systems intended for generating high-performance binding proteins.

This study delves into the functional and structural implications of non-synonymous single nucleotide polymorphisms (nsSNPs) within the Prolactin Receptor (PRLR) gene. Thirteen deleterious nsSNPs were identified through bioinformatics tools, with SIFT predicting 168 out of 395 nsSNPs as detrimental, exhibiting tolerance index (TI) scores ranging from 0 to 0.05. Polyphen2 assigned likelihood scores >0.99 to all 13 nsSNPs, indicating high probability of harm, while Panther scores classified most nsSNPs as ‘probably damaging’, with specific mutations like W218R scoring 0.74, suggesting a higher impact. Stability analysis using DDG I-Mutant and DDG Mupro consistently predicted decreased stability for all mutations, with CUPSAT indicating mutations like V125G and W218R significantly decreasing stability. Structural analysis through DynaMut predicted destabilization for all mutations except L196I and L292H. MutPred2 highlighted structural alterations for all nsSNPs except L196I, L293V, R315W, and S353N. Domain analysis revealed key mutations within essential functional domains, with five nsSNPs located within Fibronectin type-III domains. Bayesian analysis through ConSurf identified 9 critical residues, with 11 nsSNPs exhibiting notably high conservation. STRING analysis unveiled a complex interaction network, indicating involvement in vital biological processes like lactation. Molecular dynamics (MD) simulations, spanning 100 nanoseconds, elucidated structural dynamics induced by detrimental missense SNPs. Post-translational modification (PTM) analysis identified specific mutations, such as R351, involved in methylation, while S353 was implicated in phosphorylation and glycosylation. These findings offer comprehensive insights into the molecular and phenotypic effects of deleterious nsSNPs in the PRLR gene, crucial for selective breeding. Communicated by Ramaswamy H. Sarma

Over the past three decades a diverse collection of small protein domains have been used as scaffolds to generate general-purpose protein-binding reagents using a variety of protein display and enrichment technologies. To expand the repertoire of scaffolds and protein surfaces that might serve this purpose, we have explored the utility of (i) a pair of anti-parallel alpha-helices in a small highly disulfide-bonded 4-helix bundle, the CC4 domain from Reversion-inducing Cysteine-rich Protein with Kazal Motifs (RECK), and (ii) a concave beta-sheet surface and two adjacent loops in the human FN3 domain, the scaffold for the widely used monobody platform. Using M13 phage display and Next Generation Sequencing (NGS), we observe that, in both systems, libraries of ∼30 million variants contain binding proteins with affinities in the low uM range for baits corresponding to the extracellular domains of multiple mammalian proteins. CC4- and FN3-based binding proteins were fused to the N- and/or C-termini of Fc domains and used for immunostaining of transfected cells. Additionally, FN3-based binding proteins were inserted into VP1 of AAV to direct AAV infection to cells expressing a defined surface receptor. Finally, FN3-based binding proteins were insertion into the Pvc13 tail fiber protein of an extracellular contractile injection system particle to direct protein cargo delivery to cells expressing a defined surface receptor. These experiments support the utility of CC4 helices B and C and of FN3 beta-strands C, D, and F together with adjacent loops CD and FG as surfaces for engineering general-purpose protein-binding reagents.

IntroductionThe lack of regenerative capacity of the central nervous system is one of the major challenges nowadays. The knowledge of guidance cues that trigger differentiation, proliferation, and migration of neural stem and progenitor cells is one key element in regenerative medicine. The extracellular matrix protein tenascin-C (Tnc) is a promising candidate to regulate cell fate due to its expression in the developing central nervous system and in the adult neural stem cell niches. Of special interest are the alternatively spliced fibronectin type III (FnIII) domains of Tnc whose combinatorial diversity could theoretically generate up to 64 isoforms in the mouse. A total of 27 isoforms have already been discovered in the developing brain, among others the domain combinations A1D, CD, and A124BCD.MethodsIn the present study, these domains as well as the combination of the constitutively expressed FnIII domains 7 and 8 (78) were expressed in Chinese hamster ovary cells as pseudo-antibodies fused to the Fc-fragment of a human immunoglobulin G antibody. The fusion proteins were presented to primary mouse neural stem/progenitor cells (NSPCs) grown as neurospheres, either as coated culture substrates or as soluble additives in vitro. The influence of the domains on the differentiation, proliferation and migration of NSPCs was analyzed.ResultsWe observed that the domain combination A124BCD promoted the differentiation of neurons and oligodendrocytes, whereas the domain A1D supported astrocyte differentiation. The constitutively expressed domain 78 had a proliferation and migration stimulating impact. Moreover, most effects were seen only in one of the presentation modes but not in both, suggesting different effects of the Tnc domains in two- and three-dimensional cultures.DiscussionThis knowledge about the different effect of the Tnc domains might be used to create artificial three-dimensional environments for cell transplantation. Hydrogels spiked with Tnc-domains might represent a promising tool in regenerative medicine.