Localization Of Target Proteins Research Articles

Tumor Necrosis Factor Receptor-associated Factor 6 (TRAF6) Associates with Huntingtin Protein and Promotes Its Atypical Ubiquitination to Enhance Aggregate Formation

Huntington disease (HD) is a neurodegenerative disorder caused by an expansion of polyglutamines in the first exon of huntingtin (HTT), which confers aggregation-promoting properties to amino-terminal fragments of the protein (N-HTT). Mutant N-HTT aggregates are enriched for ubiquitin and contain ubiquitin E3 ligases, thus suggesting a role for ubiquitination in aggregate formation. Here, we report that tumor necrosis factor receptor-associated factor 6 (TRAF6) binds to WT and polyQ-expanded N-HTT in vitro as well as to endogenous full-length proteins in mouse and human brain in vivo. Endogenous TRAF6 is recruited to cellular inclusions formed by mutant N-HTT. Transient overexpression of TRAF6 promotes WT and mutant N-HTT atypical ubiquitination with Lys6, Lys27, and Lys29 linkage formation. Both interaction and ubiquitination seem to be independent from polyQ length. In cultured cells, TRAF6 enhances mutant N-HTT aggregate formation, whereas it has no effect on WT N-HTT protein localization. Mutant N-HTT inclusions are enriched for ubiquitin staining only when TRAF6 and Lys6, Lys27, and Lys29 ubiquitin mutants are expressed. Finally, we show that TRAF6 is up-regulated in post-mortem brains from HD patients where it is found in the insoluble fraction. These results suggest that TRAF6 atypical ubiquitination warrants investigation in HD pathogenesis.

The construction and sub-cellular localization analysis of novel mycobacterial membrane-anchored expression vector

Objective To construct mycobacterial membrane-anchored expression vector and to analyze expression level and sub-cellualr localization of exogenous target protein. Methods Based on the mycobacterial intracellular expression vector pMFA42 which contained a strong promoter of pfurAma mutant, the signal sequence of Mycobacterium tuberculosis(Mtb) 19×103 lipoprotein (19SS) was synthesized and was then cloned into the downstream of pfurAma mutant to generate the mycobacterial membrane-anchored expression vector pMFA42M. The coding gene of enhanced green fluorescent protein(EGFP) was amplified by PCR, and then sub-cloned into these two vectors described above to construct recombinant EGFP fused and membrane-anchored strains, respectively. The coding genes of Mtb immuno-dominant antigens Ag85A and its chimera Ag856A2 were then sub-cloned intothe membrane-anchored construct pMFA42MG to produce recombinant Mtb antigen EGFP fused-expression strains. After that, expression levels and sub-cellualr localization of exogenous target protein were further analyzed by Western blot and flow cytometry sorting(FCS), and the fluorescence intensities of recombinant EGFP- expressed strains were observed in vitro directly and after transfection of murine macrophage cell line RAW264.7. Results The novel mycobacterial membrane-anchored expression vector was constructed successfully by introduction of signal sequence of Mtb 19×103 lipoprotein. Using of EGFP as model antigen, exogenous target protein was demonstrated to be expressed with high level and could be anchored into cell membrane of recombinant mycobaterial strains. Conclusion A novel mycobacterial membrane-anchored expression vector was constructed successfully to research recombinant BCG and functions of mycobacterial membrane proteins, and the constructed EGFP-expressed recombinant strains could also be used to research cytophagy in cell model and mycobacterial colony and translocation in animal immunization as model indicator bacteria. Key words: Mycobacteria; EGFP; Membrane-anchored expression vector; Sub-cellular localization

Production and single-step purification of EGFP and a biotinylated version of the Human Rhinovirus 14 3C protease

The fluorescent reporter enhanced Green Fluorescent Protein (EGFP) has been used for assaying a wide range of biological activities ranging from gene expression, or localization of target proteins through to intermolecular interactions. However, over-production of this protein in Escherichia coli has resulted in the presence of inclusion bodies, which complicates recovery of the protein in significant quantities. In this paper, we describe a single-step method for isolating the protein from a Glutathione-S-Transferase (GST) fusion protein, release of the EGFP protein from the fusion was demonstrated using a biotinylated variant of Human Rhinovirus 14 3C protease that we have also constructed. We also suggest the potential uses of the biotinylated protease for bionanotechnology and synthetic biology.

M153R Mutation in a pH-Sensitive Green Fluorescent Protein Stabilizes Its Fusion Proteins

BackgroundGreen fluorescent protein (GFP) and its fusion proteins have been used extensively to monitor and analyze a wide range of biological processes. However, proteolytic cleavage often removes GFP from its fusion proteins, not only causing a poor signal-to-noise ratio of the fluorescent images but also leading to wrong interpretations.Methodology/Principal FindingsHere, we report that the M153R mutation in a ratiometric pH-sensitive GFP, pHluorin, significantly stabilizes its fusion products while the mutant protein still retaining a marked pH dependence of 410/470 nm excitation ratio of fluorescence intensity. The M153R mutation increases the brightness in vivo but does not affect the 410/470-nm excitation ratios at various pH values.Conclusions/SignificanceSince the pHluorin(M153R) probe can be directly fused to the target proteins, we suggest that it will be a potentially powerful tool for the measurement of local pH in living cells as well as for the analysis of subcellular localization of target proteins.

Metabolic role of osteocalcin in healthy postmenopausal women

The objective of this study was to investigate effects of insulin-like growth factor 1 (IGF1) on proliferation, wound healing and differentiation processes of human periodontal ligament (PDL) cells under inflammatory conditions and whether the protective, anabolic effects of IGF1 can attenuate unfavorable effects of interleukin-1β (IL-1β).Inflammation was mimicked through cell stimulation with IL-1β. PDL cells were characterized in respect to the presence of components of the IGF system and the responsive potential on IL-1β incubation. Gene expression levels were analyzed by quantitative real-time PCR. Cellular localization of target proteins was visualized using fluorescent-based immunohistochemistry. Effects on cell division were investigated by proliferation assays. Wound healing was analyzed using light microscopic techniques. Differentiation was quantified by measuring biomineralization and osteoblast-specific alkaline phosphatase enzyme activity.PDL cell proliferation and wound healing were positively affected by IGF1 and the combination of IGF1 with IL-1β, while only IL-1β showed negative effects. Biomineralization was enhanced by IGF1, IL-1β, and the combination of both stimulants. Osteoblast differentiation was increased by IL-1β and the combination of IL-1β with IGF1, whereas only IGF1 negatively affected ALP activity. Phosphorylation of p38 was regulated by IL-1β and IGF1.The data presented in this work showed a potential of IGF1 to improve wound healing and proliferation processes and to sustain cell differentiation under inflammatory stimuli in PDL cells.

Characterization of SENP7, a SUMO-2/3-specific isopeptidase

The modification of proteins by SUMO (small ubiquitin-related modifier) plays important roles in regulating the activity, stability and cellular localization of target proteins. Similar to ubiquitination, SUMO modification is a dynamic process that can be reversed by SENPs [SUMO-1/sentrin/SMT3 (suppressor of mif two 3 homologue 1)-specific peptidases]. To date, six SENPs have been discovered in humans, although knowledge of their regulation, specificity and biological functions is limited. In the present study, we report that SENP7 has a restricted substrate specificity, being unable to process SUMO precursors and displaying paralogue-specific isopeptidase activity. The C-terminal catalytic domain of SENP7 efficiently depolymerized poly-SUMO-2 chains but had undetectable activity against poly-SUMO-1 chains. SENP7 also displayed isopeptidase activity against di-SUMO-2- and SUMO-2-modified RanGAP1 (Ran GTPase-activating protein 1) but had limited activity against SUMO-1-modified RanGAP1. in vivo, full-length SENP7 was localized to the nucleoplasm and preferentially reduced the accumulation of high-molecular-mass conjugates of SUMO-2 and SUMO-3 compared with SUMO-1. Small interfering RNA-mediated ablation of SENP7 expression led to the accumulation of high-molecular-mass SUMO-2 species and to the accumulation of promyelocytic leukaemia protein in subnuclear bodies. These findings suggest that SENP7 acts as a SUMO-2/3-specific protease that is likely to regulate the metabolism of poly-SUMO-2/3 rather than SUMO-1 conjugation in vivo.

Ubiquitylation in innate and adaptive immunity

Protein ubiquitylation has emerged as a key mechanism that regulates immune responses. Much like phosphorylation, ubiquitylation is a reversible covalent modification that regulates the stability, activity and localization of target proteins. As such, ubiquitylation regulates the development of the immune system and many phases of the immune response, including its initiation, propagation and termination. Recent work has shown that several ubiquitin ligases help to prevent the immune system from attacking self tissues. The dysfunction of several ubiquitin ligases has been linked to autoimmune diseases.

Pax8 protein stability is controlled by sumoylation

The transcription factor Pax8 is involved in the morphogenesis of the thyroid gland and in the maintenance of the differentiated thyroid phenotype. Despite the critical role played by Pax8 during thyroid development and differentiation, very little is known of its post-translational modifications and how these modifications may regulate its activity. We focused our attention on the study of a specific post-translational modification, i.e., sumoylation. Sumoylation is a dynamic and reversible process regulating gene expression by altering transcription factor stability, protein-protein interaction and subcellular localization of target proteins. The analysis of Pax8 protein sequence revealed the presence of one sumoylation consensus motif (psiKxE), strongly conserved among mammals, amphibians, and fish. We demonstrated that Pax8 is sumoylated by the addition of a single small ubiquitin-like modifier (SUMO) molecule on its lysine residue 309 and that Pax8(K309R), a substitution mutant in which the candidate lysine is replaced with an arginine, is no longer modified by SUMO. In addition, we analyzed whether protein inhibitor of activated signal transducers and activators of transcription (PIASy), a member of the PIAS STAT family of proteins, could function as a SUMO ligase and we demonstrated that indeed PIASy is able to increase the fraction of sumoylated Pax8. Interestingly, we show that Pax8 is targeted in the SUMO nuclear bodies, which are structures that regulate the nucleoplasmic concentration of transcription factors by SUMO trapping. Finally, we report here that the steady-state protein level of Pax8 is controlled by sumoylation.

Cerebral ischemia/stroke and small ubiquitin‐like modifier (SUMO) conjugation – a new target for therapeutic intervention?

Transient cerebral ischemia/stroke activates various post-translational protein modifications such as phosphorylation and ubiquitin conjugation that are believed to play a major role in the pathological process triggered by an interruption of blood supply and culminating in cell death. A new system of post-translational protein modification has been identified, termed as small ubiquitin-like modifier (SUMO) conjugation. Like ubiquitin, SUMO is conjugated to the lysine residue of target proteins in a complex process. This review summarizes observations from recent experiments focusing on the effect of cerebral ischemia on SUMO conjugation. Transient global and focal cerebral ischemia both induced a rapid, dramatic and long-lasting rise in levels of SUMO2/3 conjugation. After transient focal cerebral ischemia, SUMO conjugation was particularly prominent in neurons located at the border of the ischemic territory where SUMO-conjugated proteins translocated to the nucleus. Many SUMO conjugation target proteins are transcription factors and sumoylation has been shown to have a major impact on the activity, stability, and cellular localization of target proteins. The rise in levels of SUMO-conjugated proteins is therefore likely to have a major effect on the fate of post-ischemic neurons. The sumoylation process could provide an exciting new target for therapeutic intervention.

An Essential Role for 14-3-3 Proteins in Brassinosteroid Signal Transduction in Arabidopsis

Brassinosteroids (BRs) are essential hormones for plant growth and development. BRs regulate gene expression by inducing dephosphorylation of two key transcription factors, BZR1 and BZR2/BES1, through a signal transduction pathway that involves cell-surface receptors (BRI1 and BAK1) and a GSK3 kinase (BIN2). How BR-regulated phosphorylation controls the activities of BZR1/BZR2 is not fully understood. Here, we show that BIN2-catalyzed phosphorylation of BZR1/BZR2 not only inhibits DNA binding, but also promotes binding to the 14-3-3 proteins. Mutations of a BIN2-phosphorylation site in BZR1 abolish 14-3-3 binding and lead to increased nuclear localization of BZR1 protein and enhanced BR responses in transgenic plants. Further, BR deficiency increases cytoplasmic localization, and BR treatment induces rapid nuclear localization of BZR1/BZR2. Thus, 14-3-3 binding is required for efficient inhibition of phosphorylated BR transcription factors, largely through cytoplasmic retention. This study demonstrates that multiple mechanisms are required for BR regulation of gene expression and plant growth.

Systematic Identification and Analysis of Mammalian Small Ubiquitin-like Modifier Substrates

Small ubiquitin-like modifier (SUMO) regulates diverse cellular processes through its reversible, covalent attachment to target proteins. Many SUMO substrates are involved in transcription and chromatin structure. Sumoylation appears to regulate the functions of target proteins by changing their subcellular localization, increasing their stability, and/or mediating their binding to other proteins. Using an in vitro expression cloning approach, we have identified 40 human SUMO1 substrates. The spectrum of human SUMO1 substrates identified in our screen suggests general roles of sumoylation in transcription, chromosome structure, and RNA processing. We have validated the sumoylation of 24 substrates in living cells. Analysis of this panel of SUMO substrates leads to the following observations. 1) Sumoylation is more efficient in vitro than in living cells. Polysumoylation occurs on several substrates in vitro. 2) SUMO isopeptidases have little substrate specificity. 3) The SUMO ligases, PIAS1 and PIASxbeta, have broader substrate specificities than does PIASy. 4) Although SUMO1 and SUMO2 are equally efficiently conjugated to a given substrate in vitro, SUMO1 conjugation is more efficient in vivo. 5) Most SUMO substrates localize to the nucleus, and sumoylation does not generally affect their subcellular localization. Therefore, sumoylation appears to regulate the functions of its substrates through multiple, context-dependent mechanisms.

Construction of system for localization of target protein in yeast periplasm using invertase

Construction of system for localization of target protein in yeast periplasm using invertase

Construction of system for localization of target protein in yeast periplasm using invertase

We constructed a novel system for periplasmic localization of target proteins, using yeast external invertase (INV) as anchor protein, in which the C- or N-terminal of the target protein was fused to the invertase and the fusion proteins expressed under the control of the constitutive glyceraldehyde-3-phosphate dehydrogenase promoter (GAPDH). Unlike in conventional cell-surface display, the system enables the target fusion protein to localize in yeast periplasm in a free state. As a model, enhanced green fluorescence protein (EGFP) was localized in yeast periplasm using the new system. Yeast-periplasm localization of INV-EGFP and EGFP-INV fusion proteins was confirmed by fluorescence microscopy and immunoblotting: green fluorescence was observed at the cell outline and, in western blot analysis, most fusion proteins were detected in the cell-surface fraction, indicating that the fusion proteins had been transported to the cell-surface layer. In addition, in both C- and N-terminal fusion, invertase showed activity, indicating dimer formation. These results demonstrate that invertase is a useful anchor for localizing target protein in the yeast periplasm.

Cellular retinaldehyde-binding protein interacts with ERM-binding phosphoprotein 50 in retinal pigment epithelium.

To characterize mechanisms of apical localization of visual cycle components in retinal pigment epithelium (RPE) by the identification of cellular retinaldehyde-binding protein (CRALBP) interaction partners. An overlay assay was used to detect interactions of CRALBP with components of RPE microsomes. Interacting proteins were identified with two-dimensional (2D)-PAGE and liquid chromatography tandem mass spectrometry (LC MS/MS). Protein interactions were characterized by affinity chromatography, peptide competition, and expression of protein domains. Protein colocalization in mouse retina was examined using double-label immunocytochemistry and confocal microscopy. CRALBP bound to a 54-kDa protein in RPE microsomes, which was identified as ERM (ezrin, radixin, moesin)-binding phosphoprotein 50 (EBP50), a PDZ domain protein, also known as sodium/hydrogen exchanger regulatory factory type 1 (NHERF-1). EBP50 and ezrin in solubilized microsomes bound to CRALBP-agarose but not to a control agarose column. CRALBP bound to both recombinant PDZ domains of EBP50 but not to the C-terminal ezrin-binding domain. In outer retina, EBP50 and ezrin were localized to RPE and Müller apical processes. CRALBP was distributed throughout both RPE and Müller cells, including their apical processes. RM proteins are multivalent linkers that connect plasma membrane proteins with the cortical actin cytoskeleton. EBP50 interacts with ERM family members through a C-terminal domain and binds targets such as CRALBP through its PDZ domains, thus contributing to an apical localization of target proteins. Our results provide a structural basis for apical localization of a retinoid-processing complex in RPE cells and offer insight into the cell biology of retinoid processing and trafficking in RPE.

Ubiquitin and the synapse.

Post-translational modification by the attachment of ubiquitin seems to have a crucial role in regulating synaptic structure and function. By controlling the stability, activity and localization of target proteins, this versatile regulatory system can shape the pattern, activity and plasticity of synaptic connections. Here, we discuss the myriad ways in which ubiquitin functions to sculpt synapses during development, and to remodel synapses for the acquisition and storage of memory.

Targeting the targets of Type III effector proteins secreted by phytopathogenic bacteria

Targeting the targets of Type III effector proteins secreted by phytopathogenic bacteria

PDZ-mediated interactions retain the epithelial GABA transporter on the basolateral surface of polarized epithelial cells

The PDZ target motifs located in the C-terminal end of many receptors and ion channels mediate protein-protein interactions by binding to specific PDZ-containing proteins. These interactions are involved in the localization of surface proteins on specialized membrane domains of neuronal and epithelial cells. However, the molecular mechanism responsible for this PDZ protein-dependent polarized localization is still unclear. This study first demonstrated that the epithelial gamma-aminobutyric acid (GABA) transporter (BGT-1) contains a PDZ target motif that mediates the interaction with the PDZ protein LIN-7 in Madin-Darby canine kidney (MDCK) cells, and then investigated the role of this interaction in the basolateral localization of the transporter. It was found that although the transporters from which the PDZ target motif was deleted were still targeted to the basolateral surface, they were not retained but internalized in an endosomal recycling compartment. Furthermore, an interfering BGT peptide determined the intracellular relocation of the native transporter. These data indicate that interactions with PDZ proteins determine the polarized surface localization of target proteins by means of retention and not targeting mechanisms. PDZ proteins may, therefore, act as a sort of membrane protein sorting machinery which, by recognizing retention signals (the PDZ target sequences), prevents protein internalization.