Large scale screening for novel rab effectors reveals unexpected broad Rab binding specificity

OCRL localizes to the primary cilium: a new role for cilia in Lowe syndrome

Oculocerebral renal syndrome of Lowe (OCRL or Lowe syndrome), a severe X-linked congenital disorder characterized by congenital cataracts and glaucoma, mental retardation and kidney dysfunction, is caused by mutations in the OCRL gene. OCRL is a phosphoinositide 5-phosphatase that interacts with small GTPases and is involved in intracellular trafficking. Despite extensive studies, it is unclear how OCRL mutations result in a myriad of phenotypes found in Lowe syndrome. Our results show that OCRL localizes to the primary cilium of retinal pigment epithelial cells, fibroblasts and kidney tubular cells. Lowe syndrome-associated mutations in OCRL result in shortened cilia and this phenotype can be rescued by the introduction of wild-type OCRL; in vivo, knockdown of ocrl in zebrafish embryos results in defective cilia formation in Kupffer vesicles and cilia-dependent phenotypes. Cumulatively, our data provide evidence for a role of OCRL in cilia maintenance and suggest the involvement of ciliary dysfunction in the manifestation of Lowe syndrome.

GTPase networks in membrane traffic

Members of the Rab or ARF/Sar branches of the Ras GTPase superfamily regulate almost every step of intracellular membrane traffic. A rapidly growing body of evidence indicates that these GTPases do not act as lone agents but are networked to one another through a variety of mechanisms to coordinate the individual events of one stage of transport and to link together the different stages of an entire transport pathway. These mechanisms include guanine nucleotide exchange factor (GEF) cascades, GTPase-activating protein (GAP) cascades, effectors that bind to multiple GTPases, and positive-feedback loops generated by exchange factor-effector interactions. Together these mechanisms can lead to an ordered series of transitions from one GTPase to the next. As each GTPase recruits a unique set of effectors, these transitions help to define changes in the functionality of the membrane compartments with which they are associated.

Rabs and EHDs: alternate modes for traffic control

Endocytic trafficking is a highly organized process regulated by a network of proteins, including the Rab family of small GTP-binding proteins and the C-terminal EHDs (Eps15 homology-domain-containing proteins). Central roles for Rab proteins have been described in vesicle budding, delivery, tethering and fusion, whereas little is known about the functions of EHDs in membrane transport. Common effectors for these two protein families have been identified, and they facilitate regulation of sequential steps in transport. By comparing and contrasting key aspects in their modes of function, we shall promote a better understanding of how Rab proteins and EHDs regulate endocytic trafficking.

Inositol 5-phosphatases: insights from the Lowe syndrome protein OCRL

The precise regulation of phosphoinositide lipids in cellular membranes is crucial for cellular survival and function. Inositol 5-phosphatases have been implicated in a variety of disorders, including various cancers, obesity, type 2 diabetes, neurodegenerative diseases and rare genetic conditions. Despite the obvious impact on human health, relatively little structural and biochemical information is available for this family. Here, we review recent structural and mechanistic work on the 5-phosphatases with a focus on OCRL, whose loss of function results in oculocerebrorenal syndrome of Lowe and Dent 2 disease. Studies of OCRL emphasize how the actions of 5-phosphatases rely on both intrinsic and extrinsic membrane recognition properties for full catalytic function. Additionally, structural analysis of missense mutations in the catalytic domain of OCRL provides insight into the phenotypic heterogeneity observed in Lowe syndrome and Dent disease.

Rab35 regulates Arf6 activity through centaurin-β2 (ACAP2) during neurite outgrowth

Two small GTPases, Rab and Arf, are well-known molecular switches that function in diverse membrane-trafficking routes in a coordinated manner; however, very little is known about the direct crosstalk between Rab and Arf. Although Rab35 and Arf6 were independently reported to regulate the same cellular events, including endocytic recycling, phagocytosis, cytokinesis and neurite outgrowth, the molecular basis that links them remains largely unknown. Here we show that centaurin-β2 (also known as ACAP2) functions both as a Rab35 effector and as an Arf6-GTPase-activating protein (GAP) during neurite outgrowth of PC12 cells. We found that Rab35 accumulates at Arf6-positive endosomes in response to nerve growth factor (NGF) stimulation and that centaurin-β2 is recruited to the same compartment in a Rab35-dependent manner. We further showed by knockdown and rescue experiments that after the Rab35-dependent recruitment of centaurin-β2, the Arf6-GAP activity of centaurin-β2 at the Arf6-positive endosomes was indispensable for NGF-induced neurite outgrowth. These findings suggest a novel mode of crosstalk between Rab and Arf: a Rab effector and Arf-GAP coupling mechanism, in which Arf-GAP is recruited to a specific membrane compartment by its Rab effector function.

Chlamydia trachomatis hijacks intra-Golgi COG complex-dependent vesicle trafficking pathway

Chlamydia spp. are obligate intracellular bacteria that replicate inside the host cell in a bacterial modified unique compartment called the inclusion. As other intracellular pathogens, chlamydiae exploit host membrane trafficking pathways to prevent lysosomal fusion and to acquire energy and nutrients essential for their survival and replication. The Conserved Oligomeric Golgi (COG) complex is a ubiquitously expressed membrane-associated protein complex that functions in a retrograde intra-Golgi trafficking through associations with coiled-coil tethers, SNAREs, Rabs and COPI proteins. Several COG complex-interacting proteins, including Rab1, Rab6, Rab14 and Syntaxin 6 are implicated in chlamydial development. In this study, we analysed the recruitment of the COG complex and GS15-positive COG complex-dependent vesicles to Chlamydia trachomatis inclusion and their participation in chlamydial growth. Immunofluorescent analysis revealed that both GFP-tagged and endogenous COG complex subunits associated with inclusions in a serovar-independent manner by 8 h post infection and were maintained throughout the entire developmental cycle. Golgi v-SNARE GS15 was associated with inclusions 24 h post infection, but was absent on the mid-cycle (8 h) inclusions, indicating that this Golgi SNARE is directed to inclusions after COG complex recruitment. Silencing of COG8 and GS15 by siRNA significantly decreased infectious yield of chlamydiae. Further, membranous structures likely derived from lysed bacteria were observed inside inclusions by electron microscopy in cells depleted of COG8 or GS15. Our results showed that C. trachomatis hijacks the COG complex to redirect the population of Golgi-derived retrograde vesicles to inclusions. These vesicles likely deliver nutrients that are required for bacterial development and replication.

Melanoregulin regulates retrograde melanosome transport through interaction with the RILP-p150Glued complex in melanocytes

Melanoregulin (Mreg), a product of the dilute suppressor gene, has been implicated in the regulation of melanosome transport in mammalian epidermal melanocytes, given that Mreg deficiency was found to restore peripheral melanosome distribution from perinuclear melanosome aggregation in Rab27A-deficient melanocytes. However, the function of Mreg in melanosome transport has remained unclear. Here, we show that Mreg regulates microtubule-dependent retrograde melanosome transport through the dynein-dynactin motor complex. Mreg interacted with the C-terminal domain of Rab-interacting lysosomal protein (RILP) and formed a complex with RILP and p150(Glued) (also known as dynactin subunit 1, DCTN1), a component of the dynein-dynactin motor complex, in cultured cells. Overexpression of Mreg, RILP or both, in normal melanocytes induced perinuclear melanosome aggregation, whereas knockdown of Mreg or functional disruption of the dynein-dynactin motor complex restored peripheral melanosome distribution in Rab27A-deficient melanocytes. These findings reveal a new mechanism by which the dynein-dynactin motor complex recognizes Mreg on mature melanosomes through interaction with RILP and is involved in the centripetal movement of melanosomes.

EPI64 interacts with Slp1/JFC1 to coordinate Rab8a and Arf6 membrane trafficking

ETOC: EPI64 is a TBC domain containing a microvillar protein that binds to Arf6 and induces actin-coated vacuole accumulation when overexpressed. EPI64 does this by stabilizing Arf6-GTP levels to induce clathrin-independent endocytosis and by preventing endosomes from maturing to the tubular endosome by lowering Rab8a GTP levels via association with JFC1.

Cell function requires the integration of cytoskeletal organization and membrane trafficking. Small GTP-binding proteins are key regulators of these processes. We find that EPI64, an apical microvillar protein with a Tre-2/Bub2/Cdc16 (TBC) domain that stabilizes active Arf6 and has RabGAP activity, regulates Arf6-dependent membrane trafficking. Expression of EPI64 in HeLa cells induces the accumulation of actin-coated vacuoles, a distinctive phenotype seen in cells expressing constitutively active Arf6. Expression of EPI64 with defective RabGAP activity does not induce vacuole formation. Coexpression of Rab8a suppresses the vacuole phenotype induced by EPI64, and EPI64 expression lowers the level of Rab8-GTP in cells, strongly suggesting that EPI64 has GAP activity toward Rab8a. JFC1, an effector for Rab8a, colocalizes with and binds directly to a C-terminal region of EPI64. Together this region and the N-terminal TBC domain of EPI64 are required for the accumulation of vacuoles. Through analysis of mutants that uncouple JFC1 from either EPI64 or from Rab8-GTP, our data suggest a model in which EPI64 binds JFC1 to recruit Rab8a-GTP for deactivation by the RabGAP activity of EPI64. We propose that EPI64 regulates membrane trafficking both by stabilizing Arf6-GTP and by inhibiting the recycling of membrane through the tubular endosome by decreasing Rab8a-GTP levels.

Phosphoinositides in Golgi complex function

The Golgi complex is a ribbon-like organelle composed of stacks of flat cisternae interconnected by tubular junctions. It occupies a central position in the endomembrane system as proteins and lipids that are synthesized in the endoplasmic reticulum (ER) pass through the Golgi complex to undergo biosynthetic modification (mainly glycosylation) and to be sorted to their final destinations. In addition the Golgi complex possesses a number of activities, apparently not directly connected with its main role in trafficking and sorting, which have been recently reviewed in Wilson et al. 2011. In spite of the constant massive flux of material the Golgi complex maintains its identity and phosphoinositides (PIs), among other factors, play a central role in this process. The active metabolism of PIs at the Golgi is necessary for the proper functioning of the organelle both in terms of membrane trafficking/sorting and its manifold metabolic and signalling activities. Phosphatidylinositol 4-phosphate (PtdIns4P), in particular, is responsible for the recruitment of numerous cytosolic proteins that recognise and bind PtdIns4P via specific lipid-binding domains. In this chapter we will summarize the findings that have contributed to our current understanding of the role of PIs in the biology of the Golgi complex in terms of the regulation of PI metabolism and the functional roles and regulation of PtdIns4P effectors.

Impaired neural development in a zebrafish model for Lowe syndrome

Lowe syndrome, which is characterized by defects in the central nervous system, eyes and kidneys, is caused by mutation of the phosphoinositide 5-phosphatase OCRL1. The mechanisms by which loss of OCRL1 leads to the phenotypic manifestations of Lowe syndrome are currently unclear, in part, owing to the lack of an animal model that recapitulates the disease phenotype. Here, we describe a zebrafish model for Lowe syndrome using stable and transient suppression of OCRL1 expression. Deficiency of OCRL1, which is enriched in the brain, leads to neurological defects similar to those reported in Lowe syndrome patients, namely increased susceptibility to heat-induced seizures and cystic brain lesions. In OCRL1-deficient embryos, Akt signalling is reduced and there is both increased apoptosis and reduced proliferation, most strikingly in the neural tissue. Rescue experiments indicate that catalytic activity and binding to the vesicle coat protein clathrin are essential for OCRL1 function in these processes. Our results indicate a novel role for OCRL1 in neural development, and support a model whereby dysregulation of phosphoinositide metabolism and clathrin-mediated membrane traffic leads to the neurological symptoms of Lowe syndrome.

The Rab21-GEF activity of Varp, but not its Rab32/38 effector function, is required for dendrite formation in melanocytes

ETOC: Varp contains two Rab-signaling domains—VPS9/Rab21-GEF domain and ANKR1/Rab32/38 effector domain—whose functional relationship had not previously been determined. It is shown that the Rab21-GEF activity of Varp, but not its Rab32/38 effector function, is required for forskolin-induced dendrite formation in melanocytes.

Vacuolar protein sorting 9 (VPS9)–ankyrin-repeat protein (Varp) has recently been identified as an effector molecule for two small GTPases—Rab32 and Rab38—in the transport of a melanogenic enzyme tyrosinase-related protein 1 (Tyrp1) to melanosomes in melanocytes. Although Varp contains a Rab21–guanine nucleotide exchange factor (GEF) domain (i.e., VPS9 domain), since Rab21-GEF activity is not required for Tyrp1 transport, nothing is known about the physiological significance of the Rab21-GEF activity in melanocytes. Here we show by knockdown-rescue experiments that the Rab21-GEF activity of Varp, but not its Rab32/38 effector function, is required for forskolin-induced dendrite formation of cultured melanocytes. We found that Varp-deficient cells are unable to extend dendrites in response to forskolin stimulation and that reexpression of wild-type Varp or a Rab32/38-binding–deficient mutant Varp(Q509A/Y550A) in Varp-deficient cells completely restores their ability to form dendrites. By contrast, VPS9 mutants (D310A and Y350A) and a vesicle-associated membrane protein 7 (VAMP7)-binding–deficient mutant were unable to support forskolin-induced dendrite formation in Varp-deficient cells. These findings indicate that the Rab21-GEF activity and Rab32/38 binding activity of Varp are required for different melanocyte functions, that is, Rab21 activation by the VPS9 domain is required for dendrite formation, and the Rab32/38 effector function of the ankyrin repeat 1 domain is required for Tyrp1 transport to melanosomes, although VAMP7-binding ability is required for both functions.

MICALs in control of the cytoskeleton, exocytosis, and cell death

MICALs form an evolutionary conserved family of multidomain signal transduction proteins characterized by a flavoprotein monooxygenase domain. MICALs are being implicated in the regulation of an increasing number of molecular and cellular processes including cytoskeletal dynamics and intracellular trafficking. Intriguingly, some of these effects are dependent on the MICAL monooxygenase enzyme and redox signaling, while other functions rely on other parts of the MICAL protein. Recent breakthroughs in our understanding of MICAL signaling identify the ability of MICALs to bind and directly modify the actin cytoskeleton, link MICALs to the docking and fusion of exocytotic vesicles, and uncover MICALs as anti-apoptotic proteins. These discoveries could lead to therapeutic advances in neural regeneration, cancer, and other diseases.

MICAL-L1 is a tubular endosomal membrane hub that connects Rab35 and Arf6 with Rab8a

Endocytosis is a conserved process across species in which cell surface receptors and lipids are internalized from the plasma membrane. Once internalized, receptors can either be degraded or be recycled back to the plasma membrane. A variety of small GTP-binding proteins regulate receptor recycling. Despite our familiarity with many of the key regulatory proteins involved in this process, our understanding of the mode by which these proteins co-operate and the sequential manner in which they function remains limited. In this study, we identify two GTP-binding proteins as interaction partners of the endocytic regulatory protein molecule interacting with casl-like protein 1 (MICAL)-L1. First, we demonstrate that Rab35 is a MICAL-L1-binding partner in vivo. Over-expression of active Rab35 impairs the recruitment of MICAL-L1 to tubular recycling endosomes, whereas Rab35 depletion promotes enhanced MICAL-L1 localization to these structures. Moreover, we demonstrate that Arf6 forms a complex with MICAL-L1 and plays a role in its recruitment to tubular endosomes. Overall, our data suggest a model in which Rab35 is a critical upstream regulator of MICAL-L1 and Arf6, while both MICAL-L1 and Arf6 regulate Rab8a function.

OCRL controls trafficking through early endosomes via PtdIns4,5P₂-dependent regulation of endosomal actin

Mutations in the phosphatidylinositol 4,5-bisphosphate (PtdIns4,5P(2)) 5-phosphatase OCRL cause Lowe syndrome, which is characterised by congenital cataracts, central hypotonia, and renal proximal tubular dysfunction. Previous studies have shown that OCRL interacts with components of the endosomal machinery; however, its role in endocytosis, and thus the pathogenic mechanisms of Lowe syndrome, have remained elusive. Here, we show that via its 5-phosphatase activity, OCRL controls early endosome (EE) function. OCRL depletion impairs the recycling of multiple classes of receptors, including megalin (which mediates protein reabsorption in the kidney) that are retained in engorged EEs. These trafficking defects are caused by ectopic accumulation of PtdIns4,5P(2) in EEs, which in turn induces an N-WASP-dependent increase in endosomal F-actin. Our data provide a molecular explanation for renal proximal tubular dysfunction in Lowe syndrome and highlight that tight control of PtdIns4,5P(2) and F-actin at the EEs is essential for exporting cargoes that transit this compartment.

Organization of SNAREs within the Golgi stack

Antero- and retrograde cargo transport through the Golgi requires a series of membrane fusion events. Fusion occurs at the cis- and trans-side and along the rims of the Golgi stack. Four functional SNARE complexes have been identified mediating lipid bilayer merger in the Golgi. Their function is tightly controlled by a series of reactions involving vesicle tethering and SM proteins. This network of protein interactions spatially and temporally determines the specificity of transport vesicle targeting and fusion within the Golgi.

Golgi glycosylation and human inherited diseases

The Golgi factory receives custom glycosylates and dispatches its cargo to the correct cellular locations. The process requires importing donor substrates, moving the cargo, and recycling machinery. Correctly glycosylated cargo reflects the Golgi's quality and efficiency. Genetic disorders in the specific equipment (enzymes), donors (nucleotide sugar transporters), or equipment recycling/reorganization components (COG, SEC, golgins) can all affect glycosylation. Dozens of human glycosylation disorders fit these categories. Many other genes, with or without familiar names, well-annotated pedigrees, or likely homologies will join the ranks of glycosylation disorders. Their broad and unpredictable case-by-case phenotypes cross the traditional medical specialty boundaries. The gene functions in patients may be elusive, but their common feature may include altered glycosylation that provide clues to Golgi function. This article focuses on a group of human disorders that affect protein or lipid glycosylation. Readers may find it useful to generalize some of these patient-based, translational observations to their own research.

Rab11 function in Trypanosoma brucei: identification of conserved and novel interaction partners

The Ras-like GTPase Rab11 is implicated in multiple aspects of intracellular transport, including maintenance of plasma membrane composition and cytokinesis. In metazoans, these functions are mediated in part via coiled-coil Rab11-interacting proteins (FIPs) acting as Rab11 effectors. Additional interaction between Rab11 and the exocyst subunit Sec15 connects Rab11 with exocytosis. We find that FIPs are metazoan specific, suggesting that other factors mediate Rab11 functions in nonmetazoans. We examined Rab11 interactions in Trypanosoma brucei, where endocytosis is well studied and the role of Rab11 in recycling well documented. TbSec15 and TbRab11 interact, demonstrating evolutionary conservation. By yeast two-hybrid screening, we identified additional Rab11 interaction partners. Tb927.5.1640 (designated RBP74) interacted with both Rab11 and Rab5. RBP74 shares a coiled-coil architecture with metazoan FIPs but is unrelated by sequence and appears to play a role in coordinating endocytosis and recycling. A second coiled-coil protein, Tb09.211.4830 (TbAZI1), orthologous to AZI1 in Homo sapiens, interacts exclusively with Rab11. AZI1 is restricted to taxa with motile cilia/flagella. These data suggest that Rab11 functions are mediated by evolutionarily conserved (i.e., AZI1 and Sec15) and potentially lineage-specific (RBP74) interactions essential for the integration of the endomembrane system.

Extracellular inhibitors, repellents, and semaphorin/plexin/MICAL-mediated actin filament disassembly

Multiple extracellular signals have been identified that regulate actin dynamics within motile cells, but how these instructive cues present on the cell surface exert their precise effects on the internal actin cytoskeleton is still poorly understood. One particularly interesting class of these cues is a group of extracellular proteins that negatively alter the movement of cells and their processes. Over the years, these types of events have been described using a variety of terms and herein we provide an overview of inhibitory/repulsive cellular phenomena and highlight the largest known protein family of repulsive extracellular cues, the Semaphorins. Specifically, the Semaphorins (Semas) utilize Plexin cell-surface receptors to dramatically collapse the actin cytoskeleton and we summarize what is known of the direct molecular and biochemical mechanisms of Sema-triggered actin filament (F-actin) disassembly. We also discuss new observations from our lab that reveal that the multidomain oxidoreductase (Redox) enzyme Molecule Interacting with CasL (MICAL), an important mediator of Sema/Plexin repulsion, is a novel F-actin disassembly factor. Our results indicate that MICAL triggers Sema/Plexin-mediated reorganization of the F-actin cytoskeleton and suggest a role for specific Redox signaling events in regulating actin dynamics.

Small GTPase Rab12 regulates constitutive degradation of transferrin receptor

Transferrin receptor (TfR) is a well-characterized plasma membrane protein that travels between the plasma membrane and intracellular membrane compartments. Although TfR itself should undergo degradation, the same as other intracellular proteins, whether a specific TfR degradation pathway exists has never been investigated. In this study, we screened small GTPase Rab proteins, common regulators of membrane traffic in all eukaryotes, for proteins that are specifically involved in TfR degradation. We performed the screening by three sequential methods, i.e. colocalization of Rab with TfR, colocalization with lysosomes, and knockdown of Rab by specific small interfering RNA (siRNA), and succeeded in identifying Rab12, a previously uncharacterized Rab isoform, as a prime candidate among the 60 human or mouse Rabs screened. We showed that expression of a constitutive active mutant of Rab12 reduced the amount of TfR protein, whereas functional ablation of Rab12 by knockdown of either Rab12 itself or its upstream activator Dennd3 increased the amount of TfR protein. Interestingly, however, knockdown of Rab12 had no effect on the degradation of epidermal growth factor receptor (EGFR) protein, i.e. on a conventional degradation pathway. Our findings indicated that TfR is constitutively degraded by a Rab12-dependent pathway (presumably from recycling endosomes to lysosomes), which is independent of the conventional degradation pathway.

MICAL-L1: An unusual Rab effector that links EHD1 to tubular recycling endosomes

A key regulator of the slow recycling of receptors and lipids that occurs from the endocytic recycling compartment (ERC) back to the cell surface is EHD1. We have recently identified the Rab8a-interacting protein, MICAL-L1, as a novel binding partner for EHD1 that both recruits and interacts with EHD1 on tubular recycling endosomes. MICAL-L1 belongs to the MICALfamily of proteins that are highly expressed in neurons and involved in plexin-mediated repulsive axon guidance. Interestingly, MICAL-L1 contains a coiled coil region in its C-terminus that is both necessary and sufficient for its localization to the EHD1-containing long tubular membranes of the ERC. Furthermore, MICAL-L1-depletion also impaired recycling of both transferrin and integrin receptors from the ERC back to the plasma membrane. In conclusion, our studies implicate MICAL-L1 as a novel regulator of endocytic recycling, and raises the possibility that additional neuronal-expressed proteins may mediate endocytic events in non-neuronal cells.

Genome-wide investigation of the Rab binding activity of RUN domains: development of a novel tool that specifically traps GTP-Rab35

The RUN domain is a less conserved protein motif that consists of approximately 70 amino acids, and because RUN domains are often found in proteins involved in the regulation of Rab small GTPases, the RUN domain has been suggested to be involved in Rab-mediated membrane trafficking, possibly as a Rab-binding site. However, since the Rab binding activity of most RUN domains has never been investigated, in this study we performed a genome-wide analysis of the Rab binding activity of the RUN domains of 19 different RUN domain-containing proteins by yeast two-hybrid assays with 60 different Rabs as bait. The results showed that only six of them interact with specific Rab isoforms with different Rab binding specificity, i.e., DENND5A/B with Rab6A/B, PLEKHM2 with Rab1A, RUFY2/3 with Rab33, and RUSC2 with Rab1/Rab35/Rab41. We also identified the minimal functional Rab35-binding site of RUSC2 (amino acid residues 982-1199) and succeeded in developing a novel GTP-Rab35-specific trapper, which we named RBD35 (Rab-binding domain specific for Rab35). Recombinant RBD35 was found to trap GTP-Rab35 specifically both in vitro and in PC12 cells, and overexpression of fluorescently tagged RBD35 in PC12 cells strongly inhibited nerve growth factor-dependent neurite outgrowth.

Recognition of the F&H motif by the Lowe syndrome protein OCRL

Lowe syndrome and type 2 Dent disease are caused by defects in the inositol 5-phosphatase OCRL. Most missense mutations in the OCRL ASH-RhoGAP domain that are found in affected individuals abolish interactions with the endocytic adaptors APPL1 and Ses (both Ses1 and Ses2), which bind OCRL through a short phenylalanine and histidine (F&H) motif. Using X-ray crystallography, we have identified the F&H motif binding site on the RhoGAP domain of OCRL. Missense mutations associated with disease affected F&H binding indirectly by destabilizing the RhoGAP fold. By contrast, a disease-associated mutation that does not perturb F&H binding and ASH-RhoGAP stability disrupted the interaction of OCRL with Rab5. The F&H binding site of OCRL is conserved even in species that do not have an identified homolog for APPL or Ses. Our study predicts the existence of other OCRL binding partners and shows that the perturbation of OCRL interactions has a crucial role in disease.

Rab6, Rab8, and MICAL3 cooperate in controlling docking and fusion of exocytotic carriers

Rab6 is a conserved small GTPase that localizes to the Golgi apparatus and cytoplasmic vesicles and controls transport and fusion of secretory carriers [1]. Another Rab implicated in trafficking from the trans-Golgi to the plasma membrane is Rab8 [2-5]. Here we show that Rab8A stably associates with exocytotic vesicles in a Rab6-dependent manner. Rab8A function is not needed for budding or motility of exocytotic carriers but is required for their docking and fusion. These processes also depend on the Rab6-interacting cortical factor ELKS [1], suggesting that Rab8A and ELKS act in the same pathway. We show that Rab8A and ELKS can be linked by MICAL3, a member of the MICAL family of flavoprotein monooxygenases [6]. Expression of a MICAL3 mutant with an inactive monooxygenase domain resulted in a strong accumulation of secretory vesicles that were docked at the cell cortex but failed to fuse with the plasma membrane, an effect that correlated with the strongly reduced mobility of MICAL3. We propose that the monooxygenase activity of MICAL3 is required to regulate its own turnover and the concomitant remodeling of vesicle-docking protein complexes in which it is engaged. Taken together, the results of our study illustrate cooperation of two Rab proteins in constitutive exocytosis and implicates a redox enzyme in this process.

Crystallization of an engineered RUN domain of Rab6-interacting protein 1/DENND5

Effectors of the Rab small GTPases are large multi-domain proteins which have proved difficult to express in soluble form in Escherichia coli. Generally, effectors are recruited to a distinct subcellular compartment by active (GTP-bound) Rabs, which are linked to membranes by one or two prenylated Cys residues at their C-termini. Following recruitment via their Rab-binding domain (RBD), effectors carry out various aspects of vesicle formation, transport, tethering and fusion through their other domains. Previously, successful purification of the RUN-PLAT tandem domains (residues 683-1061) of the 1263-residue Rab6-interacting protein 1 (R6IP1) required co-expression with Rab6, as attempts to solubly express the effector alone were unsuccessful. R6IP1 is also known as DENN domain-containing protein 5 (DENND5) and is expressed as two isoforms, R6IP1A/B (DENND5A/B), which differ by 24 amino acids at the N-terminus. Here, a deletion in R6IP1 was engineered to enable soluble expression and to improve the quality of the crystals grown in complex with Rab6. A large 23-residue loop linking two α-helices in the RUN1 domain was removed and replaced with a short linker. This loop resides on the opposite face to the Rab6-binding site and is not conserved in the RUN-domain family. In contrast to wild-type R6IP1-Rab6 crystals, which took several weeks to grow to full size, the engineered R6IP1 (RPdel)-Rab6 crystals could be grown in a matter of days.

A structural basis for Lowe syndrome caused by mutations in the Rab-binding domain of OCRL1

The oculocerebrorenal syndrome of Lowe (OCRL), also called Lowe syndrome, is characterized by defects of the nervous system, the eye and the kidney. Lowe syndrome is a monogenetic X-linked disease caused by mutations of the inositol-5-phosphatase OCRL1. OCRL1 is a membrane-bound protein recruited to membranes via interaction with a variety of Rab proteins. The structural and kinetic basis of OCRL1 for the recognition of several Rab proteins is unknown. In this study, we report the crystal structure of the Rab-binding domain (RBD) of OCRL1 in complex with Rab8a and the kinetic binding analysis of OCRL1 with several Rab GTPases (Rab1b, Rab5a, Rab6a and Rab8a). In contrast to other effectors that bind their respective Rab predominantly via α-helical structure elements, the Rab-binding interface of OCRL1 consists mainly of the IgG-like β-strand structure of the ASPM-SPD-2-Hydin domain as well as one α-helix. Our results give a deeper structural understanding of disease-causing mutations of OCRL1 affecting Rab binding.

Proteomic analysis of endocytic vesicles: Rab1a regulates motility of early endocytic vesicles

Texas-Red-asialoorosomucoid (ASOR) fluorescence-sorted early and late endocytic vesicles from rat liver were subjected to proteomic analysis with the aim of identifying functionally important proteins. Several Rab GTPases, including Rab1a, were found. The present study immunolocalized Rab1a to early and late endocytic vesicles and examined its potential role in endocytosis. Huh7 cells with stable knockdown of Rab1a exhibited reduced endocytic processing of ASOR. This correlated with the finding that Rab1a antibody reduced microtubule-based motility of rat-liver-derived early but not late endocytic vesicles in vitro. The inhibitory effect of Rab1a antibody was observed to be specifically towards minus-end-directed motility. Total and minus-end-directed motility was also reduced in early endocytic vesicles prepared from Rab1a-knockdown cells. These results corresponded with virtual absence of the minus-end-directed kinesin Kifc1 from early endocytic vesicles in Rab1a knockdown cells and imply that Rab1a regulates minus-end-directed motility largely by recruiting Kifc1 to early endocytic vesicles.

Role of Rab GTPases in membrane traffic and cell physiology

Intracellular membrane traffic defines a complex network of pathways that connects many of the membrane-bound organelles of eukaryotic cells. Although each pathway is governed by its own set of factors, they all contain Rab GTPases that serve as master regulators. In this review, we discuss how Rabs can regulate virtually all steps of membrane traffic from the formation of the transport vesicle at the donor membrane to its fusion at the target membrane. Some of the many regulatory functions performed by Rabs include interacting with diverse effector proteins that select cargo, promoting vesicle movement, and verifying the correct site of fusion. We describe cascade mechanisms that may define directionality in traffic and ensure that different Rabs do not overlap in the pathways that they regulate. Throughout this review we highlight how Rab dysfunction leads to a variety of disease states ranging from infectious diseases to cancer.

Rab GTPases regulating phagosome maturation are differentially recruited to mycobacterial phagosomes

Mycobacterium tuberculosis (M. tb) is an intracellular pathogen that can replicate within infected macrophages. The ability of M. tb to arrest phagosome maturation is believed to facilitate its intracellular multiplication. Rab GTPases regulate membrane trafficking, but details of how Rab GTPases regulate phagosome maturation and how M. tb modulates their localization during inhibiting phagolysosome biogenesis remain elusive. We compared the localization of 42 distinct Rab GTPases to phagosomes containing either Staphylococcus aureus or M. tb. The phagosomes containing S. aureus were associated with 22 Rab GTPases, but only 5 of these showed similar localization kinetics as the phagosomes containing M. tb. The Rab GTPases responsible for phagosome maturation, phagosomal acidification and recruitment of cathepsin D were examined in macrophages expressing the dominant-negative form of each Rab GTPase. LysoTracker staining and immunofluorescence microscopy revealed that Rab7, Rab20 and Rab39 regulated phagosomal acidification and Rab7, Rab20, Rab22b, Rab32, Rab34, Rab38 and Rab43 controlled the recruitment of cathepsin D to the phagosome. These results suggest that phagosome maturation is achieved by a series of interactions between Rab GTPases and phagosomes and that differential recruitment of these Rab GTPases, except for Rab22b and Rab43, to M. tb-containing phagosomes is involved in arresting phagosome maturation and inhibiting phagolysosome biogenesis.

Tctex-1, a novel interaction partner of Rab3D, is required for osteoclastic bone resorption

Vesicular transport along microtubules must be strictly regulated to sustain the unique structural and functional polarization of bone-resorbing osteoclasts. However, the molecular mechanisms bridging these vesicle-microtubule interactions remain largely obscure. Rab3D, a member of the Rab3 subfamily (Rab3A/B/C/D) of small exocytotic GTPases, represents a core component of the osteoclastic vesicle transport machinery. Here, we identify a new Rab3D-interacting partner, Tctex-1, a light chain of the cytoplasmic dynein microtubule motor complex, by a yeast two-hybrid screen. We demonstrate that Tctex-1 binds specifically to Rab3D in a GTP-dependent manner and co-occupies Rab3D-bearing vesicles in bone-resorbing osteoclasts. Furthermore, we provide evidence that Tctex-1 and Rab3D intimately associate with the dynein motor complex and microtubules in osteoclasts. Finally, targeted disruption of Tctex-1 by RNA interference significantly impairs bone resorption capacity and mislocalizes Rab3D vesicles in osteoclasts, attesting to the notion that components of the Rab3D-trafficking pathway contribute to the maintenance of osteoclastic resorptive function.

TBC proteins: GAPs for mammalian small GTPase Rab?

The TBC (Tre-2/Bub2/Cdc16) domain was originally identified as a conserved domain among the tre-2 oncogene product and the yeast cell cycle regulators Bub2 and Cdc16, and it is now widely recognized as a conserved protein motif that consists of approx. 200 amino acids in all eukaryotes. Since the TBC domain of yeast Gyps [GAP (GTPase-activating protein) for Ypt proteins] has been shown to function as a GAP domain for small GTPase Ypt/Rab, TBC domain-containing proteins (TBC proteins) in other species are also expected to function as a certain Rab-GAP. More than 40 different TBC proteins are present in humans and mice, and recent accumulating evidence has indicated that certain mammalian TBC proteins actually function as a specific Rab-GAP. Some mammalian TBC proteins {e.g. TBC1D1 [TBC (Tre-2/Bub2/Cdc16) domain family, member 1] and TBC1D4/AS160 (Akt substrate of 160 kDa)} play an important role in homoeostasis in mammals, and defects in them are directly associated with mouse and human diseases (e.g. leanness in mice and insulin resistance in humans). The present study reviews the structure and function of mammalian TBC proteins, especially in relation to Rab small GTPases.

The PH domain proteins IPIP27A and B link OCRL1 to receptor recycling in the endocytic pathway

We identify two new binding partners for the OCRL1 protein that is mutated in Lowe syndrome and type 2 Dent disease, which we call IPIP27A and B. The IPIPs are required for receptor recycling in the endocytic pathway, suggesting that defects in this process lead to the aforementioned disorders.

Mutation of the inositol polyphosphate 5-phosphatase OCRL1 results in two disorders in humans, namely Lowe syndrome (characterized by ocular, nervous system, and renal defects) and type 2 Dent disease (in which only the renal symptoms are evident). The disease mechanisms of these syndromes are poorly understood. Here we identify two novel OCRL1-binding proteins, termed inositol polyphosphate phosphatase interacting protein of 27 kDa (IPIP27)A and B (also known as Ses1 and 2), that also bind the related 5-phosphatase Inpp5b. The IPIPs bind to the C-terminal region of these phosphatases via a conserved motif similar to that found in the signaling protein APPL1. IPIP27A and B, which form homo- and heterodimers, localize to early and recycling endosomes and the trans-Golgi network (TGN). The IPIPs are required for receptor recycling from endosomes, both to the TGN and to the plasma membrane. Our results identify IPIP27A and B as key players in endocytic trafficking and strongly suggest that defects in this process are responsible for the pathology of Lowe syndrome and Dent disease.

Structure-function analysis of VPS9-ankyrin-repeat protein (Varp) in the trafficking of tyrosinase-related protein 1 in melanocytes

Because Varp (VPS9-ankyrin-repeat protein)/Ankrd27 specifically binds two small GTPases, Rab32 and Rab38, which redundantly regulate the trafficking of melanogenic enzymes in mammalian epidermal melanocytes, it has recently been implicated in the regulation of trafficking of a melanogenic enzyme tyrosinase-related protein 1 (Tyrp1) to melanosomes. However, the functional interaction between Rab32/38 and Varp and the involvement of the VPS9 domain (i.e. Rab21-GEF domain) in Tyrp1 trafficking have never been elucidated. In this study, we succeeded in identifying critical residues of Rab32/38 and Varp that are critical for the formation of the Rab32/38·Varp complex by performing Ala-based site-directed mutagenesis, and we discovered that a conserved Val residue in the switch II region of Rab32(Val-92) and Rab38(Val-78) is required for Varp binding activity and that its point mutant, Rab38(V78A), does not support Tyrp1 trafficking in Rab32/38-deficient melanocytes. We also identified two critical residues for Rab32/38 binding in the Varp ANKR1 domain and demonstrated that their point mutants, Varp(Q509A) and Varp(Y550A), do not support peripheral melanosomal distribution of Tyrp1 in Varp-deficient cells. Interestingly, the VPS9 domain point mutants, Varp(D310A) and Varp(Y350A), did support Tyrp1 trafficking in Varp-deficient cells, and knockdown of Rab21 had no effect on Tyrp1 distribution. We also found evidence for the functional interaction between a vesicle SNARE VAMP7/TI-VAMP and Varp in Tyrp1 trafficking. These results collectively indicated that both the Rab32/38 binding activity and VAMP7 binding activity of Varp are essential for trafficking of Tyrp1 in melanocytes but that activation of Rab21 by the VPS9 domain is not necessary for Tyrp1 trafficking.

Important relationships between Rab and MICAL proteins in endocytic trafficking

The internalization of essential nutrients, lipids and receptors is a crucial process for all eukaryotic cells. Accordingly, endocytosis is highly conserved across cell types and species. Once internalized, small cargo-containing vesicles fuse with early endosomes (also known as sorting endosomes), where they undergo segregation to distinct membrane regions and are sorted and transported on through the endocytic pathway. Although the mechanisms that regulate this sorting are still poorly understood, some receptors are directed to late endosomes and lysosomes for degradation, whereas other receptors are recycled back to the plasma membrane; either directly or through recycling endosomes. The Rab family of small GTP-binding proteins plays crucial roles in regulating these trafficking pathways. Rabs cycle from inactive GDP-bound cytoplasmic proteins to active GTP-bound membrane-associated proteins, as a consequence of the activity of multiple specific GTPase-activating proteins (GAPs) and GTP exchange factors (GEFs). Once bound to GTP, Rabs interact with a multitude of effector proteins that carry out Rab-specific functions. Recent studies have shown that some of these effectors are also interaction partners for the C-terminal Eps15 homology (EHD) proteins, which are also intimately involved in endocytic regulation. A particularly interesting example of common Rab-EHD interaction partners is the MICAL-like protein, MICAL-L1. MICAL-L1 and its homolog, MICAL-L2, belong to the larger MICAL family of proteins, and both have been directly implicated in regulating endocytic recycling of cell surface receptors and junctional proteins, as well as controlling cytoskeletal rearrangement and neurite outgrowth. In this review, we summarize the functional roles of MICAL and Rab proteins, and focus on the significance of their interactions and the implications for endocytic transport.

AATYK1A phosphorylation by Cdk5 regulates the recycling endosome pathway

Trafficking of recycling endosomes (REs) is regulated by the small GTPase, Rab11A; however, the regulatory mechanism remains elusive. Apoptosis-associated tyrosine kinase 1A (AATYK1A) is a Ser/Thr kinase expressed highly in brain. We have recently shown that AATYK1A localizes to Rab11A-positive RE and is phosphorylated at Ser34 by cyclin-dependent kinase 5 (Cdk5). Here, we have investigated a role of AATYK1A and its phosphorylation in recycling endosomal trafficking using Chinese hamster ovary-K1 (CHO-K1) cells. AATYK1A localizes predominantly to Rab11A-positive pericentrosomal endocytic recycling compartment (ERC). Phosphorylation at Ser34 of AATYK1A disrupts its accumulation in the pericentrosomal ERC. Consistently, phosphorylation-mimic mutant (AATYK1A-S34D) did not accumulate in the ERC and additionally attenuated ERC formation. ERC formation suppression can be reversed by constitutively active Rab11A-Q70L, suggesting a functional link between AATYK1A phosphorylation and Rab11A activity. Although no direct interaction between AATYK1A and Rab11A could be detected, the exchange of guanine nucleotides bound to Rab11A was significantly reduced in the presence of the phosphorylation-mimic AATYK1A-S34D. Together, our results reveal a regulatory role for AATYK1A in the formation of pericentrosomal ERC. They furthermore indicate that Cdk5 can disrupt ERC formation via Ser34 phosphorylation of AATYK1A. Finally, our data suggest a mechanism by which AATYK1A signaling couples Cdk5 to Rab11A activity.

Structural basis for Rab GTPase recognition and endosome tethering by the C2H2 zinc finger of Early Endosomal Autoantigen 1 (EEA1)

Regulation of endosomal trafficking by Rab GTPases depends on selective interactions with multivalent effectors, including EEA1 and Rabenosyn-5, which facilitate endosome tethering, sorting, and fusion. Both EEA1 and Rabenosyn-5 contain a distinctive N-terminal C(2)H(2) zinc finger that binds Rab5. How these C(2)H(2) zinc fingers recognize Rab GTPases remains unknown. Here, we report the crystal structure of Rab5A in complex with the EEA1 C(2)H(2) zinc finger. The binding interface involves all elements of the zinc finger as well as a short N-terminal extension but is restricted to the switch and interswitch regions of Rab5. High selectivity for Rab5 and, to a lesser extent Rab22, is observed in quantitative profiles of binding to Rab family GTPases. Although critical determinants are identified in both switch regions, Rab4-to-Rab5 conversion-of-specificity mutants reveal an essential requirement for additional substitutions in the proximal protein core that are predicted to indirectly influence recognition through affects on the structure and conformational stability of the switch regions.

Multiple host proteins that function in phosphatidylinositol-4-phosphate metabolism are recruited to the chlamydial inclusion

Chlamydiae replicate within a nonacidified vacuole, termed an inclusion. As obligate intracellular bacteria, chlamydiae actively modify their vacuole to exploit host signaling and trafficking pathways. Recently, we demonstrated that several Rab GTPases are actively targeted to the inclusion. To define the biological roles of inclusion localized Rab GTPases, we have begun to identify inclusion-localized Rab effectors. Here we demonstrate that oculocerebrorenal syndrome of Lowe protein 1 (OCRL1), a Golgi complex-localized phosphatidylinositol (PI)-5-phosphatase that binds to multiple Rab GTPases, localizes to chlamydial inclusions. By examining the intracellular localization of green fluorescent protein (GFP) fusion proteins that bind to unique phosphoinositide species, we also demonstrate that phosphatidylinositol-4-phosphate (PI4P), the product of OCRL1, is present at the inclusion membrane. Furthermore, two additional host proteins, Arf1, which together with PI4P mediates the recruitment of PI4P-binding proteins to the Golgi complex, and PI4KII alpha, a major producer of Golgi complex-localized PI4P, also localize to chlamydial inclusions. Depletion of OCRL1, Arf1, or PI4KII alpha by small interfering RNA (siRNA) decreases inclusion formation and the production of infectious progeny. Infectivity is further decreased in cells simultaneously depleted for all three host proteins, suggesting partially overlapping functions in infected cells. Collectively, these data demonstrate that Chlamydia species create a unique replication-competent vacuolar environment by modulating both the Rab GTPase and the PI composition of the chlamydial inclusion.

MICAL-L1 links EHD1 to tubular recycling endosomes and regulates receptor recycling

Endocytic recycling of receptors and lipids occurs via a complex network of tubular and vesicular membranes. EHD1 is a key regulator of endocytosis and associates with tubular membranes to facilitate recycling. Although EHD proteins tubulate membranes in vitro, EHD1 primarily associates with preexisting tubules in vivo. How EHD1 is recruited to these tubular endosomes remains unclear. We have determined that the Rab8-interacting protein, MICAL-L1, associates with EHD1, with both proteins colocalizing to long tubular membranes, in vitro and in live cells. MICAL-L1 is a largely uncharacterized member of the MICAL-family of proteins that uniquely contains two asparagine-proline-phenylalanine motifs, sequences that typically interact with EH-domains. Our data show that the MICAL-L1 C-terminal coiled-coil region is necessary and sufficient for its localization to tubular membranes. Moreover, we provide unexpected evidence that endogenous MICAL-L1 can link both EHD1 and Rab8a to these structures, as its depletion leads to loss of the EHD1-Rab8a interaction and the absence of both of these proteins from the membrane tubules. Finally, we demonstrate that MICAL-L1 is essential for efficient endocytic recycling. These data implicate MICAL-L1 as an unusual type of Rab effector that regulates endocytic recycling by recruiting and linking EHD1 and Rab8a on membrane tubules.

Two closely related endocytic proteins that share a common OCRL-binding motif with APPL1

Mutations of the inositol 5' phosphatase oculocerebrorenal syndrome of Lowe (OCRL) give rise to the congenital X-linked disorders oculocerebrorenal syndrome of Lowe and Dent disease, two conditions giving rise to abnormal kidney proximal tubule reabsorption, and additional nervous system and ocular defects in the case of Lowe syndrome. Here, we identify two closely related endocytic proteins, Ses1 and Ses2, which interact with the ASH-RhoGAP-like (ASPM-SPD-2-Hydin homology and Rho-GTPase Activating Domain-like) domain of OCRL. The interaction is mediated by a short amino acid motif similar to that used by the rab-5 effector APPL1 (Adaptor Protein containing pleckstrin homology [PH] domain, PTB domain and Leucine zipper motif 1) APPL1 for OCRL binding. Ses binding is mutually exclusive with APPL1 binding, and is disrupted by the same missense mutations in the ASH-RhoGAP-like domain that also disrupt APPL1 binding. Like APPL1, Ses1 and -2 are localized on endosomes but reside on different endosomal subpopulations. These findings define a consensus motif (which we have called a phenylalanine and histidine [F&H] motif) for OCRL binding and are consistent with a scenario in which Lowe syndrome and Dent disease result from perturbations at multiple sites within the endocytic pathway.

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.

RAB6C is a retrogene that encodes a centrosomal protein involved in cell cycle progression

Rab-GTPases are key regulators of membrane transport, and growing evidence indicates that their expression levels are altered in certain human malignancies, including cancer. Rab6C, a newly identified Rab6 subfamily member, has attracted recent attention because its reduced expression might confer a selective advantage to drug-resistant breast cancer cells. Here, we report that RAB6C is a primate-specific retrogene derived from a RAB6A' transcript. RAB6C is transcribed in a limited number of human tissues including brain, testis, prostate, and breast. Endogenous Rab6C is considerably less abundant and has a much shorter half-life than Rab6A'. Comparison of the GTP-binding motifs of Rab6C and Rab6A', homology modeling, and GTP-blot overlay assays indicate that amino acid changes in Rab6C have greatly reduced its GTP-binding affinity. Instead, the noncanonical GTP-binding domain of Rab6C mediates localization of the protein to the centrosome. Overexpression of Rab6C results in G1 arrest, and its specific depletion generates tetraploid cells with supernumerary centrosomes, revealing a role of Rab6C in events related to the centrosome and cell cycle progression. Thus, RAB6C is a rare example of a recently emerged retrogene that has acquired the status of a new gene, encoding a functional protein with altered characteristics compared to Rab6A'.

Role of vesicle tethering factors in the ER-Golgi membrane traffic

Tethers are a diverse group of loosely related proteins and protein complexes grouped into three families based on structural and functional similarities. A well-accepted role for tethering factors is the initial attachment of transport carriers to acceptor membranes prior to fusion. However, accumulating evidence indicates that tethers are more than static bridges. Tethers have been shown to interact with components of the fusion machinery and with components involved in vesicle formation. Tethers belonging to the three families act at the same stage of traffic, suggesting that they mediate distinct events during vesicle tethering. Thus, multiple tether-facilitated events are required to provide selectivity to vesicle fusion. In this review, we highlight findings that support this model.

Class I Rab11-family interacting proteins are binding targets for the Rab14 GTPase

BACKGROUND INFORMATION

Rab11 and Rab14 are two related Rab GTPases that are believed to function in endosomal recycling and Golgi/endosome transport processes. We, and others, have identified a group of proteins that interact with Rab11 and function as Rab11 effectors, known as the Rab11-FIPs (family interacting proteins). This protein family has been sub-classified into two groups - class I FIPs [FIP2, RCP (Rab coupling protein) and Rip11 (Rab11-interacting protein)] and class II FIPs (FIP3 and FIP4).

RESULTS

In the present study we identify the class I FIPs as dual Rab-binding proteins by demonstrating that they also interact with Rab14 in a GTP-dependent manner. We show that these interactions are specific for the class I FIPs and that they occur via their C-terminal regions, which encompass the previously described RBD (Rab11-binding domain). Furthermore, we show that Rab14 significantly co-localizes with the TfnR (transferrin receptor) and that Rab14 Q70L co-localizes with Rab11a and with the class I FIPs on the ERC (endosomal recycling compartment) during interphase. Additionally, we show that during cytokinesis Rab14 localizes to the cleavage furrow/midbody.

CONCLUSIONS

The data presented in the present study, which identifies the class I FIPs as the first putative effector proteins for the Rab14 GTPase, indicates greater complexity in the Rab-binding specificity of the class I FIP proteins.

Structure and function of regulator of G protein signaling homology domains

All regulator of G protein signaling (RGS) proteins contain a conserved domain of approximately 130 amino acids that binds to activated heterotrimeric G protein α subunits (Gα) and accelerates their rate of GTP hydrolysis. Homologous domains are found in at least six other protein families, including a family of Rho guanine nucleotide exchange factors (RhoGEFs) and the G protein-coupled receptor kinases (GRKs). Although some of the RhoGEF and GRK RGS-like domains can also bind to activated Gα subunits, they do so in distinct ways and with much lower levels of GTPase activation. In other protein families, the domains have as of yet no obvious relationship to heterotrimeric G protein signaling. These RGS homology (RH) domains are now recognized as mediators of extraordinarily diverse protein-protein interactions. Through these interactions, they play roles that range from enzyme to molecular scaffold to signal transducing module. In this review, the atomic structures of RH domains from RGS proteins, Axins, RhoGEFs, and GRKs are compared in light of what is currently known about their functional roles.

D-AKAP2 interacts with Rab4 and Rab11 through its RGS domains and regulates transferrin receptor recycling

Dual-specific A-kinase-anchoring protein 2 (D-AKAP2/AKAP10), which interacts at its carboxyl terminus with protein kinase A and PDZ domain proteins, contains two tandem regulator of G-protein signaling (RGS) domains for which the binding partners have remained unknown. We show here that these RGS domains interact with Rab11 and GTP-bound Rab4, the first demonstration of RGS domains binding small GTPases. Rab4 and Rab11 help regulate membrane trafficking through the endocytic recycling pathways by recruiting effector proteins to specific membrane domains. Although D-AKAP2 is primarily cytosolic in HeLa cells, a fraction of the protein localizes to endosomes and can be recruited there to a greater extent by overexpression of Rab4 or Rab11. D-AKAP2 also regulates the morphology of the Rab11-containing compartment, with co-expression causing accumulation of both proteins on enlarged endosomes. Knockdown of D-AKAP2 by RNA interference caused a redistribution of both Rab11 and the constitutively recycling transferrin receptor to the periphery of cells. Knockdown also caused an increase in the rate of transferrin recycling, suggesting that D-AKAP2 promotes accumulation of recycling proteins in the Rab4/Rab11-positive endocytic recycling compartment.

Lowe syndrome patient fibroblasts display Ocrl1-specific cell migration defects that cannot be rescued by the homologous Inpp5b phosphatase

The Lowe syndrome (LS) is a life-threatening, developmental disease characterized by mental retardation, cataracts and renal failure. Although this human illness has been linked to defective function of the phosphatidylinositol 5-phosphatase, Ocrl1 (Oculo-Cerebro-Renal syndrome ofLowe protein1), the mechanism by which this enzyme deficiency triggers the disease is not clear. Ocrl1 is known to localize mainly to the Golgi apparatus and endosomes, however it translocates to plasma membrane ruffles upon cell stimulation with growth factors. The functional implications of this inducible translocation to the plasma membrane are presently unknown. Here we show that Ocrl1 is required for proper cell migration, spreading and fluid-phase uptake in both established cell lines and human dermal fibroblasts. We found that primary fibroblasts from two patients diagnosed with LS displayed defects in these cellular processes. Importantly, these abnormalities were suppressed by expressing wild-type Ocrl1 but not by a phosphatase-deficient mutant. Interestingly, the homologous human PI-5-phosphatase, Inpp5b, was unable to complement the Ocrl1-dependent cell migration defect. Further, Ocrl1 variants that cannot bind the endocytic adaptor AP2 or clathrin, like Inpp5b, were less apt to rescue the migration phenotype. However, no defect in membrane recruitment of AP2/clathrin or in transferrin endocytosis by patient cells was detected. Collectively, our results suggest that Ocrl1, but not Inpp5b, is involved in ruffle-mediated membrane remodeling. Our results provide new elements for understanding how Ocrl1 deficiency leads to the abnormalities associated with the LS.