Identification of EPI64 as a GTPase-activating protein specific for Rab27A

Linking molecular motors to membrane cargo

Three types of motors, myosins, kinesins, and cytoplasmic dynein, cooperate to transport intracellular membrane organelles. Transport of each cargo is determined by recruitment of specific sets of motors and their regulation. Targeting of motors to membranes often depends on the formation of large multiprotein assemblies and can be influenced by membrane lipid composition. Motor activity can be regulated by cargo-induced conformational changes such as unfolding or dimerization. The architecture and function of motor: cargo complexes can also be controlled by phosphorylation, calcium signaling, and proteolysis. The complexity of transport systems is further increased by mechanical and functional cross-talk between different types of motors on the same cargo and by participation of the same motor in the movement of different organelles.

Glucose-induced translocation of coronin 3 regulates the retrograde transport of the secretory membrane in the pancreatic beta-cells

GTP-Rab27a is known to regulate insulin exocytosis. We have recently reported that coronin 3, which paradoxically binds GDP-Rab27a, participates in endocytosis of the insulin secretory membrane. Here, we demonstrate that glucose stimulation caused redistribution of coronin 3 in the vicinity of the plasma membrane, which was mimicked by overexpression of the GDP-Rab27a mutant or the Rab27a GAP. Glucose-induced translocation of coronin 3 was inhibited by Rab27a knock-down. The internalized phogrin, an insulin granule associated protein, located near the plasma membrane by the dominant-negative coronin 3, but the protein at the outer surface of the plasma membrane was decreased. These results indicate that glucose recruits coronin 3 near the plasma membrane, and that it regulates the retrograde transport of the secretory membrane in pancreatic beta-cells.

Comprehensive screening for novel rab-binding proteins by GST pull-down assay using 60 different mammalian Rabs

The Rab family belongs to the Ras-like small GTPase superfamily and is implicated in membrane trafficking through interaction with specific effector molecules. Because of the large number of Rab isoforms in mammals, however, the effectors of most of the mammalian Rabs are yet to be identified. In this study, we systematically screened five different cell or tissue lysates for novel Rab effectors by a combination of glutathione S-transferase (GST) pull-down assay with 60 different mammalian Rabs and mass spectroscopic analysis. Three of the 21 Rab-binding proteins we identified, mKIAA1055/TBC1D2B (Rab22-binding protein), GAPCenA/TBC1D11 (Rab36-binding protein) and centaurin beta2/ACAP2 (Rab35-binding protein), are GTPase-activating proteins (GAPs) for Rab or Arf. Although it has recently been proposed that the Rab-GAP (Tre-2 /Bub2/Cdc16) domain physically interacts with its substrate Rab, these three GAPs interacted with specific Rabs via a domain other than a GAP domain, e.g. centaurin beta2 binds GTP-Rab35 via the ankyrin repeat (ANKR) domain. Although centaurin beta2 did not exhibit any Rab35-GAP activity in vitro, the Rab35-binding ANKR domain of centaurin beta2 was found to be required for its plasma membrane localization and regulation of Rab35-dependent neurite outgrowth of PC12 cells through inactivation of Arf6. These findings suggest a novel mode of interaction between Rab and GAP.

Modifiers of Prat, a de novo purine synthesis gene, in Drosophila melanogaster

Drosophila melanogaster was used to identify genes with a potential role in genetic regulation of purine biosynthesis. In this study we examine two dominant genetic modifiers of the essential gene Prat, which encodes amidophosphoribosyltransferase (EC 2.4.2.14). We found that Mod(Prat:bw)3-1 enhances Prat expression only in female heads, whereas Mod(Prat:bw)3-5 suppresses Prat in all stages and tissues examined for both sexes. For Mod-3-5, gene expression microarrays were used to identify other genes that are affected by the modifier. Three mapping approaches were used to localize these modifiers. Deficiency and meiotic mapping showed that the complex lethal complementation group previously associated with Mod-3-1 and Mod-3-5 is actually due to shared second-site lethal mutations. Using male recombination mapping, Mod-3-1 was localized to a 21 kilobase region containing nine genes, and Mod-3-5 was localized to a 53 kilobase region containing eight genes.

Structural mechanisms for regulation of membrane traffic by rab GTPases

In all eukaryotic organisms, Rab GTPases function as critical regulators of membrane traffic, organelle biogenesis and maturation, and related cellular processes. The numerous Rab proteins have distinctive yet overlapping subcellular distributions throughout the endomembrane system. Intensive investigation has clarified the underlying molecular and structural mechanisms for several ubiquitous Rab proteins that control membrane traffic between tubular-vesicular organelles in the exocytic, endocytic and recycling pathways. In this review, we focus on structural insights that inform our current understanding of the organization of the Rab family as well as the mechanisms for membrane targeting and activation, interaction with effectors, deactivation and specificity determination.

A CD317/tetherin-RICH2 complex plays a critical role in the organization of the subapical actin cytoskeleton in polarized epithelial cells

CD317/tetherin is a lipid raft–associated integral membrane protein with a novel topology. It has a short N-terminal cytosolic domain, a conventional transmembrane domain, and a C-terminal glycosyl-phosphatidylinositol anchor. We now show that CD317 is expressed at the apical surface of polarized epithelial cells, where it interacts indirectly with the underlying actin cytoskeleton. CD317 is linked to the apical actin network via the proteins RICH2, EBP50, and ezrin. Knocking down expression of either CD317 or RICH2 gives rise to the same phenotype: a loss of the apical actin network with concomitant loss of apical microvilli, an increase in actin bundles at the basal surface, and a reduction in cell height without any loss of tight junctions, transepithelial resistance, or the polarized targeting of apical and basolateral membrane proteins. Thus, CD317 provides a physical link between lipid rafts and the apical actin network in polarized epithelial cells and is crucial for the maintenance of microvilli in such cells.

Small G proteins as key regulators of pancreatic digestive enzyme secretion

Small GTP-binding (G) proteins act as molecular switches to regulate a number of cellular processes, including vesicular transport. Emerging evidence indicates that small G proteins regulate a number of steps in the secretion of pancreatic acinar cells. Diverse small G proteins have been localized at discrete compartments along the secretory pathway and particularly on the secretory granule. Rab3D, Rab27B, and Rap1 are present on the granule membrane and play a role in the steps leading up to exocytosis. Whether the function of these G proteins is simply to ensure appropriate targeting or if they are involved as regulatory molecules is discussed. Most evidence suggests that Rab3D and Rab27B play a role in tethering the secretory granule to its target membrane. Other Rabs have been identified on the secretory granule that are associated with different steps in the secretory pathway. The Rho family small G proteins RhoA and Rac1 also regulate secretion through remodeling of the actin cytoskeleton. Possible mechanisms for regulation of these G proteins and their effector molecules are considered.

Identification and characterization of a novel Tre-2/Bub2/Cdc16 (TBC) protein that possesses Rab3A-GAP activity

The Tre-2/Bub2/Cdc16 (TBC) domain is a conserved protein motif that consists of approximately 200 amino acids and is thought to function as a specific Rab-GAP domain. Although more than 40 distinct TBC domain-containing proteins have been identified in humans, the GAP activity and specificity of most TBC proteins have never been determined. In this study we developed a novel method of screening for Rab3A-GAP and identified two TBC proteins (FLJ13130 and RN-tre) whose expression in PC12 cells was associated with exclusion of endogenous Rab3A molecules from dense-core vesicles. As expression of RN-tre caused fragmentation of the Golgi, which presumably resulted in the loss of dense-core vesicles themselves, we further characterized FLJ13130 as a candidate Rab3A-GAP. The results showed that expression of FLJ13130, but not of its catalytically inactive R134K mutant, greatly reduced the amount of GTP-Rab3A in living cells and promoted the GTPase activity of Rab3A in vitro. Unexpectedly, however, FLJ13130 also promoted the GTPase activity of Rab22A, Rab27A, and Rab35, but not of Rab2A or Rab6A. Based on these results, we propose that FLJ13130 is a novel type of Rab-GAP that exhibits broad GAP specificity and inactivates several distinct Rab isoforms, including Rab3A, just near the plasma membrane.

Rab35 and its GAP EPI64C in T cells regulate receptor recycling and immunological synapse formation

Upon antigen recognition, T-cell receptor (TCR/CD3) and other signaling molecules become enriched in a specialized contact site between the T cell and antigen-presenting cell, i.e. the immunological synapse (IS). Enrichment occurs via mechanisms that include polarized secretion from recycling endosomes, but the Rabs and RabGAPs that regulate this are unknown. EPI64C (TBC1D10C) is an uncharacterized candidate RabGAP we identified by mass spectrometry as abundant in human peripheral blood T cells that is preferentially expressed in hematopoietic cells. EPI64C is a Rab35-GAP based both on in vitro Rab35-specific GAP activity and findings in transfection assays. EPI64C and Rab35 dominant negative (DN) constructs each impaired transferrin export from a recycling pathway in Jurkat T-cells and induced large vacuoles marked by transferrin receptor, TCR, and SNAREs implicated in TCR-polarized secretion. Rab35 localized to the plasma membrane and to intracellular vesicles where it substantially colocalized with TfR and with TCR. Rab35 was strongly recruited to the IS. Conjugate formation was impaired by transfection with Rab35-DN or EPI64C and by EPI64C knock down. TCR enrichment at the IS was impaired by Rab35-DN. Thus, EPI64C and Rab35 regulate a recycling pathway in T cells and contribute to IS formation, most likely by participating in TCR transport to the IS.

A pathway sensor for genome-wide screens of intracellular proteolytic cleavage

A new system based on non-conventional secretion of the luciferase from Gaussia princeps (GLUC) can be used to detect intracellular proteolysis in vivo.

Protein cleavage is a central event in many regulated biological processes. We describe a system for detecting intracellular proteolysis based on non-conventional secretion of Gaussia luciferase (GLUC). GLUC exits the cell without benefit of a secretory leader peptide, but can be anchored in the cell by fusion to β-actin. By including protease cleavage sites between GLUC and β-actin, proteolytic cleavage can be detected. Using this assay, we have identified regulators of autophagy, apoptosis and β-actin cleavage.

Rab proteins in endocytosis and Glut4 trafficking

The intracellular trafficking of numerous proteins requires a tight control to fulfil their physiological functions. It is the case of the adipocyte and muscle glucose transporter Glut4 that is retained intracellularly until insulin induces its recruitment to the plasma membrane. Rabs are evolutionarily conserved small GTPases that control intracellular traffic events from yeast to mammalian cells. In the past few decades, considerable progresses have been made in identifying the route of Glut4, the Rabs involved in controlling it, and more recently the connection between insulin signalling and Glut4 trafficking through Rab activity control.

TBC1D20 is a Rab1 GTPase-activating protein that mediates hepatitis C virus replication

Like other viruses, productive hepatitis C virus (HCV) infection depends on certain critical host factors. We have recently shown that an interaction between HCV nonstructural protein NS5A and a host protein, TBC1D20, is necessary for efficient HCV replication. TBC1D20 contains a TBC (Tre-2, Bub2, and Cdc16) domain present in most known Rab GTPase-activating proteins (GAPs). The latter are master regulators of vesicular membrane transport, as they control the activity of membrane-associated Rab proteins. To better understand the role of the NS5A-TBC1D20 interaction in the HCV life cycle, we used a biochemical screen to identify the TBC1D20 Rab substrate. TBC1D20 was found to be the first known GAP for Rab1, which is implicated in the regulation of anterograde traffic between the endoplasmic reticulum and the Golgi complex. Mutation of amino acids implicated in Rab GTPase activation by other TBC domain-containing GAPs abrogated the ability of TBC1D20 to activate Rab1 GTPase. Overexpression of TBC1D20 blocked the transport of exogenous vesicular stomatitis virus G protein from the endoplasmic reticulum, validating the involvement of TBC1D20 in this pathway. Rab1 depletion significantly decreased HCV RNA levels, suggesting a role for Rab1 in HCV replication. These results highlight a novel mechanism by which viruses can hijack host cell machinery and suggest an attractive model whereby the NS5A-TBC1D20 interaction may promote viral membrane-associated RNA replication.

Differential dynamics of Rab3A and Rab27A on secretory granules

We have assessed the dynamics of the association of Rab3A and Rab27A with secretory granules at various stages of their life in PC12 cells. Endogenous Rab3A colocalised with the secretory granule marker secretogranin II (SGII) and expressed EGFP-Rab3A and ECFP-Rab27A colocalised with one another. The extent of colocalisation between EGFP-Rab3A or EGFP-Rab27 and SGII increased after longer times post transfection suggesting that these Rab proteins are preferentially recruited to newly synthesised granules. Following the release of immature secretory granules from the trans-Golgi network, Rab3A and Rab27A became associated with the immature granules after a lag period of around 20 minutes. Rab dynamics on granules were analysed in fluorescence recovery after photobleaching (FRAP) experiments. The recovery profile of EGFP-Rab27A was comparable to that of ppANF-EGFP, whereas the recovery profile of EGFP-Rab3A was significantly faster, indicating that Rab3A but not Rab27A might be rapidly exchanged between granules and cytosol. Inhibition of heat-shock protein 90 with 10 muM geldanamycin did not affect the exchange process or regulated exocytosis. Rab dynamics during stimulation with 300 muM ATP were analysed in live cells. Loss of granular ppANF-EGFP fluorescence was seen at the cell periphery after stimulation but only limited changes in EGFP-Rab3A and EGFP-Rab27A fluorescence was observed, indicating that the Rab proteins do not immediately dissociate or disperse on stimulation. The data suggest potentially distinct roles for Rab3A and Rab27A and we suggest that the finding that young secretory granules have a higher capacity for binding Rab3A and Rab27A is functionally important for preferential exocytosis from these granules.

EPI64 regulates microvillar subdomains and structure

EPI64 is a TBC domain–containing protein that binds the PDZ domains of EBP50, which binds ezrin, a major actin-binding protein of microvilli. High-resolution light microscopy revealed that ezrin and EBP50 localize exclusively to the membrane-surrounded region of microvilli, whereas EPI64 localizes to variable regions in the structures. Overexpressing EPI64 results in its and EBP50's relocalization to the base of microvilli, including to the actin rootlet devoid of ezrin or plasma membrane. Uncoupling EPI64's binding to EBP50, expression of any construct mislocalizing its TBC domain, or knock down of EBP50 results in loss of microvilli. The TBC domain of EPI64 binds directly to Arf6-GTP. Overexpressing the TBC domain increases Arf6-GTP levels, and expressing dominant-active Arf6 results in microvillar loss. These data reveal that microvilli have distinct cytoskeletal subdomains and that EPI64 regulates microvillar structure.