Rab-interacting Lysosomal Protein Research Articles

RILP Induces Cholesterol Accumulation in Lysosomes by Inhibiting Endoplasmic Reticulum-Endolysosome Interactions.

Endoplasmic reticulum (ER)-endolysosome interactions regulate cholesterol exchange between the ER and the endolysosome. ER-endolysosome membrane contact sites mediate the ER-endolysosome interaction. VAP-ORP1L (vesicle-associated membrane protein-associated protein- OSBP-related protein 1L) interaction forms the major contact site between the ER and the lysosome, which is regulated by Rab7. RILP (Rab7-interacting lysosomal protein) is the downstream effector of Rab7, but its role in the organelle interaction between the ER and the lysosome is not clear. In this study, we found RILP interacts with ORP1L to competitively inhibit the formation of the VAP-ORP1L contact site. Immunofluorescence microscopy revealed that RILP induces late endosome/lysosome clustering, which reduces the contact of endolysosomes with the ER, interfering with the ER-endolysosome interaction. Further examination demonstrated that over-expression of RILP results in the accumulation of cholesterol in the clustered endolysosomes, which triggers cellular autophagy depending on RILP. Our results suggest that RILP interferes with the ER-endolysosome interaction to inhibit cholesterol flow from the endolysosome to the ER, which feedbacks to trigger autophagy.

Legionella effectors SidC/SdcA ubiquitinate multiple small GTPases and SNARE proteins to promote phagosomal maturation

Protein ubiquitination is one of the most important posttranslational modifications (PTMs) in eukaryotes and is involved in the regulation of almost all cellular signaling pathways. The intracellular bacterial pathogen Legionella pneumophila translocates at least 26 effectors to hijack host ubiquitination signaling via distinct mechanisms. Among these effectors, SidC/SdcA are novel E3 ubiquitin ligases with the adoption of a Cys-His-Asp catalytic triad. SidC/SdcA are critical for the recruitment of endoplasmic reticulum (ER)-derived vesicles to the Legionella-containing vacuole (LCV). However, the ubiquitination targets of SidC/SdcA are largely unknown, which restricts our understanding of the mechanisms used by these effectors to hijack the vesicle trafficking pathway. Here, we demonstrated that multiple Rab small GTPases and target soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNARE) proteins are bona fide ubiquitination substrates of SidC/SdcA. SidC/SdcA-mediated ubiquitination of syntaxin 3 and syntaxin 4 promotes their unconventional pairing with the vesicle-SNARE protein Sec22b, thereby contributing to the membrane fusion of ER-derived vesicles with the phagosome. In addition, our data reveal that ubiquitination of Rab7 by SidC/SdcA is critical for its association with the LCV membrane. Rab7 ubiquitination could impair its binding with the downstream effector Rab-interacting lysosomal protein (RILP), which partially explains why LCVs avoid fusion with lysosomes despite the acquisition of Rab7. Taken together, our study reveals the biological mechanisms employed by SidC/SdcA to promote the maturation of the LCVs.

Autophagy initiation triggers p150Glued–AP-2β interaction on the lysosomes and facilitates their transport

The endocytic adaptor protein 2 (AP-2) complex binds dynactin as part of its noncanonical function, which is necessary for dynein-driven autophagosome transport along microtubules in neuronal axons. The absence of this AP-2-dependent transport causes neuronal morphology simplification and neurodegeneration. The mechanisms that lead to formation of the AP-2-dynactin complex have not been studied to date. However, the inhibition of mammalian/mechanistic target of rapamycin complex 1 (mTORC1) enhances the transport of newly formed autophagosomes by influencing the biogenesis and protein interactions of Rab-interacting lysosomal protein (RILP), another dynein cargo adaptor. We tested effects of mTORC1 inhibition on interactions between the AP-2 and dynactin complexes, with a focus on their two essential subunits, AP-2β and p150Glued. We found that the mTORC1 inhibitor rapamycin enhanced p150Glued–AP-2β complex formation in both neurons and non-neuronal cells. Additional analysis revealed that the p150Glued–AP-2β interaction was indirect and required integrity of the dynactin complex. In non-neuronal cells rapamycin-driven enhancement of the p150Glued–AP-2β interaction also required the presence of cytoplasmic linker protein 170 (CLIP-170), the activation of autophagy, and an undisturbed endolysosomal system. The rapamycin-dependent p150Glued–AP-2β interaction occurred on lysosomal-associated membrane protein 1 (Lamp-1)-positive organelles but without the need for autolysosome formation. Rapamycin treatment also increased the acidification and number of acidic organelles and increased speed of the long-distance retrograde movement of Lamp-1-positive organelles. Altogether, our results indicate that autophagy regulates the p150Glued–AP-2β interaction, possibly to coordinate sufficient motor-adaptor complex availability for effective lysosome transport.

Disruption of Golgi markers by two RILP-directed shRNAs in neurons: A new role for RILP or a neuron-specific off-target phenotype?

In neurons, degradation of dendritic cargos requires RAB7 and dynein-mediated retrograde transport to somatic lysosomes. To test if the dynein adapter RAB-interacting lysosomal protein (RILP) mediated the recruitment of dynein to late endosomes for retrograde transport in dendrites, we obtained several knockdown reagents previously validated in non-neuronal cells. Striking endosomal phenotypes elicited by one shRILP plasmid were not reproduced by another one. Furthermore, we discovered a profound depletion of Golgi/TGN markers for both shRILP plasmids. This Golgi disruption was only observed in neurons and could not be rescued by re-expression of RILP. This Golgi phenotype was also not found in neurons treated with siRILP or gRILP/Cas9. Lastly, we tested if a different RAB protein that interacts with RILP, namely the Golgi-associated RAB34, might be responsible for the loss of Golgi markers. Expression of a dominant-negative RAB34 did indeed cause changes in Golgi staining in a small subset of neurons but manifested as fragmentation rather than loss of staining. Unlike in non-neuronal cells, interference with RAB34 did not cause dispersal of lysosomes in neurons. Based on multiple lines of experimentation, we conclude that the neuronal Golgi phenotype observed with shRILP is likely off-target in this cell type specifically. Any observed disruptions of endosomal trafficking caused by shRILP in neurons might thus be downstream of Golgi disruption. It would be interesting to identify the actual target for this neuronal Golgi phenotype. Cell type-specific off-target phenotypes therefore likely occur in neurons, necessitating revalidation of reagents that were previously validated in other cell types.

A Novel Role for RILP in Regulating Osteoclastogenesis and Bone Resorption

Increased bone resorption caused by excessive number or activity of osteoclasts is the main cause of osteoporosis. Osteoclasts are multinucleated cells that are formed by the fusion of precursor cells. Although osteoclasts are primarily characterized by bone resorption, our understanding of the mechanisms that regulate the formation and function of osteoclasts is poor. Here we showed that the expression level of Rab interacting lysosomal protein (RILP) was strongly induced by receptor activator of NF-κB ligand in mouse bone marrow macrophages. Inhibition of RILP expression induced a drastic decrease in the number, size, F-actin ring formation of osteoclasts, and the expression level of osteoclast-related genes. Functionally, inhibition of RILP reduced the migration of preosteoclasts through PI3K-Akt signaling and suppressed bone resorption by inhibiting the secretion of lysosome cathepsin K. Treatments with siRNA-RILP attenuated pathologic bone loss in disease models induced by lipopolysaccharide. Thus, this work indicates that RILP plays an important role in the formation and bone resorption function of osteoclasts and may have a therapeutic potential to treat bone diseases caused by excessive or hyperactive osteoclasts.

Prognostic value of RILPL2 and its correlation with tumor immune microenvironment and glycolysis in non-small cell lung cancer

ABSTRACT Rab-interacting lysosomal protein – like 2 (RILPL2) has been reported to be associated with prognosis and tumor biological functions in breast cancer and endometrial carcinoma. However, its expression and functional role in non-small cell lung cancer (NSCLC) remain unclear. The expression and clinical data of lung adenocarcinoma (LUAD) and lung squamous carcinoma (LUSC) were downloaded from the TCGA database. The expression of RILPL2 in NSCLC cell lines was verified by the Western blot. We used online databases and bioinformatics analysis tools to explore its prognostic value, potential biological functions, and correlations with tumor immune microenvironment.The expression of RILPL2 was significantly lower in NSCLC compared with adjacent normal tissues. Low RILPL2 expression was associated with worse overall survival (OS) in NSCLC. The GO analysis showed RILPL2 was comprehensively involved in immune activity. RILPL2 expression was significantly positively correlated with the infiltration levels of B cells, CD8+T cells, CD4+T cells, macrophages, neutrophils, dendritic cells (P < 0.001), and it was also significantly positively correlated with programmed cell death ligand 1 (PD-L1/CD274) (P < 0.001). High RILPL2 expression could predict better immunotherapy response and prognosis in the immunotherapy cohort. The GSEA analysis showed low RILPL2 expression was associated with glycolysis process in LUAD, which was verified in vitro.These results showed RILPL2 expression was correlated with prognosis, tumor microenvironment, and immunotherapy response in NSCLC. Besides, RILPL2 may regulate glycolysis in LUAD.

RILP inhibits proliferation, migration, and invasion of PC3 prostate cancer cells

RILP (Rab-interacting lysosomal protein) is a key regulator of lysosomal transport and a potential tumor suppressor. However, the role of RILP in prostate cancer and the underlying mechanism of RILP in regulating the proliferation, migration, and invasion of prostate cancer cells remain to be studied. In this study, we confirmed RalGDS (Ral guanine nucleotide dissociation stimulator) as the interaction partner of RILP in PC3 prostate cancer cells. Immunofluorescence microscopy showed that RILP recruits RalGDS to the lysosomal compartment. We found that RILP inhibits the activation of RalA and downstream effector RalBP1, and negatively regulates the downstream molecular phosphorylation of Ras. We showed that RILP inhibits the proliferation, migration, and invasion of PC3 prostate cancer cells, which may give rise to novel ideas for cancer treatment.

Amyloid-β oligomers block lysosomal targeting of miRNPs to prevent miRNP recycling and target repression in glial cells.

Upon exposure to amyloid-β oligomers (Aβ1-42), glial cells start expressing proinflammatory cytokines, despite an increase in levels of repressive microRNAs (miRNAs). Exploring the mechanism of this potential immunity of target cytokine mRNAs against repressive miRNAs in amyloid-β-exposed glial cells, we have identified differential compartmentalization of repressive miRNAs in glial cells that explains this aberrant miRNA function. In Aβ1-42-treated cells, whereas target mRNAs were found to be associated with polysomes attached to endoplasmic reticulum (ER), the miRNA ribonucleoprotein complexes (miRNPs) were found to be present predominantly with endosomes that failed to recycle to ER-attached polysomes, preventing repression of mRNA targets. Aβ1-42 oligomers, by masking Rab7a proteins on endosomal surfaces, affected Rab7a interaction with Rab-interacting lysosomal protein (RILP), restricting the lysosomal targeting and recycling of miRNPs. RNA-processing body (P-body) localization of the miRNPs was found to be enhanced in amyloid-β-treated cells as a consequence of enhanced endosomal retention of miRNPs. Interestingly, depletion of P-body components partly rescued the miRNA function in glial cells exposed to amyloid-β and restricted the excess cytokine expression. This article has an associated First Person interview with the first author of the paper.

Interactome Profile of Visceral Adipose Tissue in Obesity Links Key Genes to Cancer Pathogenesis

Obesity increases the risk of the development of several malignancies. The visceral adipose tissue (VAT) depot is one of the pivotal contributors behind the obesity-related pathogenetic mechanisms. In this study, we analyzed the differential gene expression profile in the VAT of obese children using two Gene Expression Omnibus datasets. GSE29718 and GSE9624 were sorted and 68 common differentially expressed genes (DEG) with fold change 1.5 upregulation or downregulation (cutoff |logFC|≥0.58496) were obtained. Gene ontology and functional enrichment and protein-protein interaction (PPI) network for the DEG were analyzed in Search Tool for the Retrieval of Interacting Genes (STRING), which revealed 37 biological processes, 3 cellular components, and 1 molecular function to be significantly associated. Reactome pathway analysis showed the DEG to be involved in- one carbon pool by folate, glycine degradation, transcriptional regulation by TP53, ERK inactivation, G1/S-specific transcription, Fanconi anemia pathway, beta-catenin phosphorylation cascade, RAF activation, and negative regulation of the MAPK pathway. The PPI network was set with a minimum interaction score of 0.400 and a maximum of 10 interactions, and it was significantly enriched (p-value 0.047) with 66 nodes and 46 edges. Target prediction was performed using miRNet. Several miRNA, including hsa-miR-1-3p, hsa-let-7b-5p, hsa-miR-16-5p, hsa-miR-27a-3p and hsa-miR-34a-5p were part of the mRNA-miRNA interaction network. Using the CytoHubba plugin in Cytoscape, the top 10 hub genes from the PPI network were discovered. Thymidine phosphorylase (TYMP) and dihydrofolate reductase (DHFR), essential components of nucleic acid metabolism, have been shown to be involved in angiogenesis and endothelial cell growth, and correlated to p53 mutations, respectively. Protein phosphatase 2, regulatory subunit A & regulatory subunit B (PPP2R1A and PPP2R1B) mutations are involved in ovarian, endometrial, lung and colorectal cancers. HLA-DQA1 mutation is involved cervical cancer, and it is involved in increased immune sensitivity and liver damage in breast cancer patients. The RAB7Ab and RAB7-interacting lysosomal protein (RILP) are regulators of endo-lysosomal trafficking and suppresses breast cancer cell invasion. To conclude, this study identifies several genes and their regulatory pathways in VAT which may contribute to the increased risk of cancer pathogenesis in obese individuals.

Methylation of RILP in lung cancer promotes tumor cell proliferation and invasion.

Rab-interacting lysosomal protein (RILP) has been suggested to perform as a tumor suppressor in breast and prostate cancer cell lines. However, its expression profile and functional role in lung cancer have never been investigated. We applied the well-established cancer genomic database-The Cancer Genome Atlas to compare the RILP expression and methylation between lung cancer tissues and normal tissues. The potential correlation of RILP with clinical characteristics of lung cancer patients (e.g., stages, smoking, TP53, and methylation) was also be explored. Our results showed that the downregulation of RILP and upregulation of RILP methylation were identified in lung cancer tissues compared to normal healthy tissues. Downregulation of RILP was positively associated with lung cancer later stage (N3), smoking history, TP53 mutation, and poor prognosis, as well as inversely correlated with DNA (cytosine-5)-methyltransferase 1 (DNMT1) expression. Demethylation treatment enhanced RILP expression in lung cancer cells, suggesting hypermethylation is responsible for RILP silencing in lung cancer. We further found that RILP depletion promoted lung cancer cell proliferation, migration, and invasion.We concluded that RILP acts as a tumor suppressor in lung cancer cells. Our results provided the theoretical basis for developing RILP-targeting or demethylating agents for lung cancer treatment.

Inflammasomes: Exosomal miRNAs loaded for action.

Inflammasomes are multiprotein complexes of the innate immune system. Their activation leads to robust secretion of exosomes. In this issue, Wozniak et al. (2020. J. Cell Biol. https://doi.org/10.1083/jcb.201912074) reveal a connection between inflammasome-mediated cleavage of Rab-interacting lysosomal protein (RILP) and sequence-specific loading of miRNAs into exosomes.

Cln1-mutations suppress Rab7-RILP interaction and impair autophagy contributing to neuropathology in a mouse model of infantile neuronal ceroid lipofuscinosis.

Infantile neuronal ceroid lipofuscinosis (INCL) is a devastating neurodegenerative lysosomal storage disease (LSD) caused by inactivating mutations in the CLN1 gene. CLN1 encodes palmitoyl-protein thioesterase-1 (PPT1), a lysosomal enzyme that catalyzes the deacylation of S-palmitoylated proteins to facilitate their degradation and clearance by lysosomal hydrolases. Despite the discovery more than two decades ago that CLN1 mutations causing PPT1-deficiency underlies INCL, the precise molecular mechanism(s) of pathogenesis has remained elusive. Here, we report that autophagy is dysregulated in Cln1-/- mice, which mimic INCL and in postmortem brain tissues as well as cultured fibroblasts from INCL patients. Moreover, Rab7, a small GTPase, critical for autophagosome-lysosome fusion, requires S-palmitoylation for trafficking to the late endosomal/lysosomal membrane where it interacts with Rab-interacting lysosomal protein (RILP), essential for autophagosome-lysosome fusion. Notably, PPT1-deficiency in Cln1-/- mice, dysregulated Rab7-RILP interaction and preventing autophagosome-lysosome fusion, which impaired degradative functions of the autolysosome leading to INCL pathogenesis. Importantly, treatment of Cln1-/- mice with a brain-penetrant, PPT1-mimetic, small molecule, N-tert (butyl)hydroxylamine (NtBuHA), ameliorated this defect. Our findings reveal a previously unrecognized role of CLN1/PPT1 in autophagy and suggest that small molecules functionally mimicking PPT1 may have therapeutic implications.

Structural characterization of the RH1-LZI tandem of JIP3/4 highlights RH1 domains as a cytoskeletal motor-binding motif

JIP3 and JIP4 (JNK-interacting proteins 3 and 4) are adaptors for cargo recruitment by dynein/dynactin and kinesin1 motors. Both are dimers that are stabilised by two sections of leucine zipper coiled coils. The N-terminal Leucine Zipper I (LZI) belongs to a section that binds dynein-DLIC and kinesin1-KHC, whilst the medial Leucine Zipper II (LZII) binds dynactin-p150glued and kinesin1-KLC. Structural data is available for the LZII, but the LZI section is still uncharacterized. Here we characterize the N-terminal part of JIP3/4 which consists of an RH1 (RILP homology 1) domain followed by the LZI coiled coil using bioinformatical, biophysical and structural approaches. The RH1-LZI tandem of JIP3 associates as a high affinity homodimer exhibiting elongated alpha-helical fold. 3D homology modelling of the RH1-LZI tandem reveals that the kinesin1-KHC binding site mainly overlaps with the RH1 domain. A sequence comparison search indicates that only one other protein family has RH1 domains similar to those of JIP3/4, the RILP (Rab-interacting lysosomal protein) family which consists of adaptor proteins linking Rab GTPases to cytoskeletal motors. RILPL2 is recruited through its RH1 domain by the myosin 5a motor. Here, we showed that the RH1 domain of JIP3 also interacts with myosin 5 A in vitro, highlighting JIP3/4 as possible myosin 5a adaptors. Finally, we propose that JIP3/4 and RILP family members define a unique RH1/RH2-architecture adaptor superfamily linking cytoskeletal motors and Rab GTPases.

RILP Restricts Insulin Secretion Through Mediating Lysosomal Degradation of Proinsulin.

Insulin secretion is tightly regulated by membrane trafficking. RILP (Rab7 interacting lysosomal protein) regulates the endocytic trafficking, but its role in insulin secretion has not been investigated. In this study, we found that overexpression of RILP inhibited insulin secretion in both the β-cell lines and freshly isolated islets. Consequently, the expression of RILP in islets suppressed the ability to recover the glucose homeostasis in type 1 diabetes mice upon transplantation. Of physiological relevance is that RILP expression was upregulated in the diabetic mouse islets. Mechanistically, overexpression of RILP induced insulin granule clustering, decreased the number of proinsulin-containing granules in β-cells, and significantly promoted proinsulin degradation. Conversely, RILP depletion sustained proinsulin and increased insulin secretion. The proinsulin degradation induced by RILP expression was inhibited by lysosomal inhibitors and was Rab7-dependent. Finally, we showed that RILP interacts with insulin granule-associated Rab26 to restrict insulin secretion. This study presents a new pathway regulating insulin secretion and mechanically demonstrates a novel function of RILP in modulating insulin secretion through mediating the lysosomal degradation of proinsulin.

Enhancing the retrograde axonal transport by curcumin promotes autophagic flux in N2a/APP695swe cells.

The accumulation of autophagosomes and dysfunction at the axonal terminal of neurons play crucial roles in the genesis and development of Alzheimer’s disease (AD). Abnormalities in neuron axonal transport-related proteins prevent autophagosome maturation in AD. Curcumin, a polyphenol plant compound, has been shown to exert neuroprotective effects by increasing autophagy in AD, but the underlying mechanism of its effect on autophagy axon transport remains elusive. This study investigated the effects of curcumin on autophagosome formation and axonal transport in N2a/APP695swe cells (AD cell model) as well as the mechanism underlying those effects. Curcumin treatment significantly increased the expression of Beclin1, Atg5, and Atg16L1, induced the formation of autophagosomes, and promoted autophagosome–lysosome fusion in N2a/APP695swe cells. At the same time, curcumin promoted the expression of dynein, dynactin, and BICD2 as well as their binding to form the retrograde axonal transport molecular motor complex. Moreover, curcumin also increased the expression of the scaffolding proteins Rab7- interacting lysosomal protein (RILP) and huntingtin in N2a/APP695swe cells. Taken together, our findings indicate that curcumin increases autophagic flux by promoting interactions among autophagic axonal transport-related proteins and inducing lysosome–autophagosome fusion. This study provides evidence suggesting the potential use of curcumin as a novel treatment for AD.

Molecular cloning of the Rab7 effector RILP (Rab-interacting lysosomal protein) in Litopenaeus vannamei and preliminary analysis of its role in white spot syndrome virus infection

To investigate the role of the Rab7 effector RILP (Rab-interacting lysosomal protein) in white spot syndrome virus (WSSV) infection, the full-length cDNA of RILP (LvRILP) was cloned in Litopenaeus vannamei, which consists of 1595 bp and encodes a polypeptide of 411 amino acids. Sequence analysis and multiple sequence alignment displayed that LvRILP contained a conserved RILP region from 277 amino acid to 325 amino acid. Both the LvRILP and Rab7 mRNA were most highly expressed in stomach and most lowly expressed in hemocyte, which were significantly up-regulated and exhibited similar kinetics post WSSV infection. The interaction of Rab7 with LvRILP was verified by both GST Pull-down and ELISA. Meanwhile, the results of Pull-down assays showed that the GST-tagged VP28 (GST-VP28), His-tagged Rab7 (His-Rab7) and His-RILP formed a tripartite complex. After silencing by specific LvRILP dsRNA, the LvRILP mRNA level exhibited a significant reduction, and the expression levels of three WSSV genes ie1, wsv477 and vp28 all exhibited decreases at 24, 36 and 48 h post WSSV infection. These results suggested that the Rab7 effector RILP was involved in WSSV infection.

Structural basis of human ORP1-Rab7 interaction for the late-endosome and lysosome targeting.

Oxysterol-binding protein (OSBP) and OSBP-related proteins (ORPs) constitute a family of lipid transfer proteins conserved in eukaryotes. ORP1 transports cholesterol at the interface between the late endosomes/lysosomes (LELs) and the endoplasmic reticulum (ER). ORP1 is targeted to the endosomal membranes by forming a tripartite complex with the LE GTPase Rab7 and its effector RILP (Rab7-interacting lysosomal protein). Here, we determined the crystal structure of human ORP1 ANK domain in complex with the GTP-bound form of Rab7. ORP1 ANK binds to the helix α3 of Rab7 located away from the switching regions, which makes the interaction independent of the nucleotide-binding state of Rab7. Thus, the effector-interacting switch regions of Rab7 are accessible for RILP binding, allowing formation of the ORP1-Rab7-RILP complex. ORP1 ANK binds to Rab7 and the Rab7-RILP complex with similar micro-molar affinities, which is consistent with the independence binding of ORP1 and RILP to Rab7. The structural model of the ORP1-Rab7-RILP complex correlates with the recruitment of ORP1 at the LEL-ER interface and the role in lipid transport and regulation.

A transient helix in the disordered region of dynein light intermediate chain links the motor to structurally diverse adaptors for cargo transport.

All animal cells use the motor cytoplasmic dynein 1 (dynein) to transport diverse cargo toward microtubule minus ends and to organize and position microtubule arrays such as the mitotic spindle. Cargo-specific adaptors engage with dynein to recruit and activate the motor, but the molecular mechanisms remain incompletely understood. Here, we use structural and dynamic nuclear magnetic resonance (NMR) analysis to demonstrate that the C-terminal region of human dynein light intermediate chain 1 (LIC1) is intrinsically disordered and contains two short conserved segments with helical propensity. NMR titration experiments reveal that the first helical segment (helix 1) constitutes the main interaction site for the adaptors Spindly (SPDL1), bicaudal D homolog 2 (BICD2), and Hook homolog 3 (HOOK3). In vitro binding assays show that helix 1, but not helix 2, is essential in both LIC1 and LIC2 for binding to SPDL1, BICD2, HOOK3, RAB-interacting lysosomal protein (RILP), RAB11 family-interacting protein 3 (RAB11FIP3), ninein (NIN), and trafficking kinesin-binding protein 1 (TRAK1). Helix 1 is sufficient to bind RILP, whereas other adaptors require additional segments preceding helix 1 for efficient binding. Point mutations in the C-terminal helix 1 of Caenorhabditis elegans LIC, introduced by genome editing, severely affect development, locomotion, and life span of the animal and disrupt the distribution and transport kinetics of membrane cargo in axons of mechanosensory neurons, identical to what is observed when the entire LIC C-terminal region is deleted. Deletion of the C-terminal helix 2 delays dynein-dependent spindle positioning in the one-cell embryo but overall does not significantly perturb dynein function. We conclude that helix 1 in the intrinsically disordered region of LIC provides a conserved link between dynein and structurally diverse cargo adaptor families that is critical for dynein function in vivo.

This Is the End: Regulation of Rab7 Nucleotide Binding in Endolysosomal Trafficking and Autophagy.

Rab7 – or in yeast, Ypt7p – governs membrane trafficking in the late endocytic and autophagic pathways. Rab7 also regulates mitochondrion-lysosome contacts, the sites of mitochondrial fission. Like all Rab GTPases, Rab7 cycles between an “active” GTP-bound form that binds downstream effectors – e.g., the HOPS and retromer complexes and the dynactin-binding Rab-interacting lysosomal protein (RILP) – and an “inactive” GDP-bound form that cannot bind effectors. Accessory proteins regulate the nucleotide binding state of Rab7: guanine nucleotide exchange factors (GEFs) stimulate exchange of bound GDP for GTP, resulting in Rab7 activation, whereas GTPase activating proteins (GAPs) boost Rab7’s GTP hydrolysis activity, thereby inactivating Rab7. This review will discuss the GEF and GAPs that control Rab7 nucleotide binding, and thus regulate Rab7’s activity in endolysosomal trafficking and autophagy. It will also consider how bacterial pathogens manipulate Rab7 nucleotide binding to support intracellular invasion and immune evasion.

A non-canonical GTPase interaction enables ORP1L-Rab7-RILP complex formation and late endosome positioning

Endosomal transport represents the primary mode for intracellular trafficking and signaling transduction and thus has to be tightly controlled. The molecular processes controlling the endosomal positioning utilize several large protein complexes, one of which contains the small GTPase Rab7, Rab-interacting lysosomal protein (RILP), and oxysterol-binding protein-related protein 1 (ORP1L). Rab7 is known to interact with RILP through a canonical binding site termed the effector-interacting switch region, but it is not clear how Rab7 interacts with ORP1L, limiting our understanding of the overall process. Here, we report structural and biochemical investigation of the Rab7-ORP1L interaction. We found that, contrary to prior studies, the interaction between Rab7 and the N-terminal ankyrin repeat domain (ARDN) of ORP1L is independent of Rab7's GTP- or GDP-binding state. Moreover, we show that Rab7 interacts with ORP1L ARDN via a unique region consisting of helix3 (α3) and 310-helix 2 (η2). This architecture leaves the canonical effector-interacting switch regions available for RILP binding and thus allows formation of the ORP1L-Rab7-RILP tripartite complex. Mutational disruption of the interacting interface between ORP1L and Rab7 compromised the ability of ORP1L-Rab7-RILP to regulate the late endosome positioning. Collectively, our results again manifested the versatility in the interaction between GTPase and its effector.