Rab-protein 11

Rab11 and vesicle transport

The rab11 GTPase has been localized to both the Golgi and recycling endosomes; however, its Golgi-associated function has remained obscure. In this study, rab11 function in exocytic transport was analyzed by using two independent means to perturb its activity. First, expression of the dominant interfering rab11S25N mutant protein led to a significant inhibition of the cell surface transport of vesicular stomatitis virus (VSV) G protein and caused VSV G protein to accumulate in the Golgi. Second, the expression of wild-type rab11 or the activating rab11Q70L mutant, in contrast, had no adverse effect on VSV G transport. Next, the membrane association of rab11, which is crucial for its function, was perturbed by modest increases in GDP dissociation inhibitor (GDI) levels. This led to selective inhibition of the trans-Golgi network to cell surface delivery, whereas endoplasmic reticulum-to-Golgi and intra-Golgi transport were largely unaffected. The transport inhibition was reversed specifically by coexpression of wild-type rab11 with GDI. Under the same conditions two other exocytic rab proteins, rab2 and rab8, remained membrane bound, and the transport steps regulated by these rab proteins were unaffected. Neither mutant rab11S25N nor GDI overexpression had any impact on the cell surface delivery of influenza hemagglutinin. These data show that functional rab11 is critical for the export of a basolateral marker but not an apical marker from the trans-Golgi network and pinpoint rab11 as a sensitive target for inhibition by excess GDI (Chen, 1998).

Multivesicular bodies (MVBs) are membranous structures within which vesicles of 60-100 nm diameter accumulate. MVBs are generated after invagination and pinching off of the endosomal membrane in the lumen of the vacuole. In certain cell types, fusion of MVBs with the plasma membrane results in the release of the internal vesicles called exosomes. This report examines how an increase in cytosolic calcium affects the development of MVBs and exosome release in K562 cells overexpressing GFP-Rab11 wt or its mutants. In cells overexpressing the Rab11Q70 L mutant or Rab11 wt, an increase in the cytosolic calcium concentration induced by monensin causes a marked enlargement of the MVBs. This effect is abrogated by the membrane permeant calcium chelator BAPTA-AM. The behavior of MVBs was examined in living cells by time lapse confocal microscopy. Many MVBs, decorated by wt or Q70L mutant GFP-Rab11, were docked and ready to fuse in the presence of a calcium chelator. This observation suggests that Rab11 is acting in the tethering/docking of MVBs to promote homotypic fusion, but that the final fusion reaction requires the presence of calcium. Additionally, a rise in intracellular calcium concentration enhances exosome secretion in Rab11 wt overexpressing cells and reverses the inhibition of the mutants. The results suggest that both Rab11 and calcium are involved in the homotypic fusion of MVBs (Savina, 2005).

Rab11 regulates recycling of receptors

The pericentriolar recycling endosome (RE) may be an alternative compartment through which some beta2-adrenergic receptors (beta2ARs) recycle from early endosomes to the cell surface during prolonged exposure to agonist. For the transferrin receptor, CXCR2, and the M4-muscarinic acetylcholine receptor, trafficking through the RE and receptor recycling is regulated by the small GTPase rab11. The precise role of the RE and rab11 in regulating the cellular trafficking of the beta2AR is not understood. Trafficking of beta2ARs in HEK293 cells was therefore monitored following the modulation of rab11 activity. Expression of a rab11 mutant deficient in GTP binding (as a fusion between enhanced green fluorescent protein (EGFP) and the rab11S25N mutant) significantly slows receptor recycling to the cell surface from dispersed transferrin-positive peripheral vesicles following a brief exposure to agonist. The agonist was applied at a time when receptors had undergone only one or two rounds of endocytosis and recycling. In cells overexpressing wild-type rab11, beta2ARs localize to a rab11-positive compartment and the rate of beta2AR recycling to the cell surface is reduced, but only after prolonged exposure to agonist and multiple rounds of receptor endocytosis and recycling. This effect is associated with impaired beta2AR trafficking to lysosomes and receptor proteolysis, whereas the sorting of low-density lipoprotein from transferrin-positive vesicles to late endosomes and lysosomes is not affected. These data highlight a pivotal role for rab11 in regulating the traffic of a G protein-coupled receptor at the level of the RE, where modulation of rab11 activity dictates the balance between receptor recycling and downregulation during prolonged exposure to agonist (Moore, 2004).

Proteins in the transforming growth factor-beta family recognize transmembrane serine/threonine kinases known as type I and type II receptors. Binding of TGF-beta to receptors results in receptor down-regulation and signaling. Whereas previous work has focused on activities controlling TGF-beta signaling, more recent studies have begun to address the trafficking properties of TGF-beta receptors. In this report, it is shown that receptors undergo recycling both in the presence and absence of ligand activation, with the rates of internalization and recycling being unaffected by ligand binding. Recycling occurs as receptors are most likely internalized through clathrin-coated pits, and then returned to the plasma membrane via a rab11-dependent, rab4-independent mechanism. Together, the results suggest a mechanism wherein activated TGF-beta receptors are directed to a distinct endocytic pathway for down-regulation and clathrin-dependent degradation after one or more rounds of recycling (Mitchell, 2004).

E-cadherin plays an essential role in cell polarity and cell-cell adhesion; however, the pathway for delivery of E-cadherin to the basolateral membrane of epithelial cells has not been fully characterized. The post-Golgi, exocytic transport of GFP-tagged E-cadherin (Ecad-GFP) in unpolarized cells was traced. In live cells, Ecad-GFP is found to exit the Golgi complex in pleiomorphic tubulovesicular carriers, which, instead of moving directly to the cell surface, most frequently fuse with an intermediate compartment, subsequently identified as a Rab11-positive recycling endosome. In MDCK cells, basolateral targeting of E-cadherin relies on a dileucine motif. Both E-cadherin and a targeting mutant, DeltaS1-E-cadherin, colocalized with Rab11 and fused with the recycling endosome before diverging to basolateral or apical membranes, respectively. In polarized and unpolarized cells, coexpression of Rab11 mutants disrupts the cell surface delivery of E-cadherin and causes its mistargeting to the apical membrane, whereas apical DeltaS1-E-cadherin is unaffected. Thus, a novel pathway has been demonstrated for Rab11 dependent, dileucine-mediated, mu1B-independent sorting and basolateral trafficking, exemplified by E-cadherin. The recycling endosome is identified as an intermediate compartment for the post-Golgi trafficking and exocytosis of E-cadherin, with a potentially important role in establishing and maintaining cadherin-based adhesion (Lock, 2005).

Rab11 structure

The small GTP-binding protein Rab11 is an essential regulator of the dynamics of recycling endosomes. This study reports the crystallographic analysis of the GDP/GTP cycle of human Rab11a, and a structure-based mutagenesis study that identifies a novel mutant phenotype. The crystal structures show that the nucleotide-sensitive switch 1 and 2 regions differ from those of other Rab proteins. In Rab11-GDP, they contribute to a close packed symmetrical dimer, which may associate to membranes in the cell and allow Rab11 to undergo GDP/GTP cycles without recycling to the cytosol. The structure of active Rab11 delineates a three-dimensional site that includes switch 1 and is separate from the site defined by the Rab3/Rabphilin interface. It is proposed to form a novel interface for a Rab11 partner compatible with the simultaneous binding of another partner at the Rabphilin interface. Mutation of Ser(29) to Phe in this epitope result in morphological modifications of the recycling compartment that are distinct from those induced by the classical dominant-negative and constitutively active Rab11 mutants. Recycling endosomes condense in the perinuclear region where they retain recycling transferrin, and they cluster Rab11- and EEA1-positive membranes. Altogether, this study suggests that mutation of Ser(29 to Phe) impairs a specific subset of Rab11 interactions, possibly those involved in cytoskeleton-based movements driving the slow recycling pathway (Pasqualato, 2004).

Signaling upstream of Rab11

Phosphatidylinositol 4-kinasebeta (PI4Kbeta) plays an essential role in maintaining the structural integrity of the Golgi complex. In a search for PI4Kbeta-interacting proteins, PI4Kbeta was found to specifically interact with the GTP-bound form of the small GTPase rab11. The PI4Kbeta-rab11 interaction is of functional significance because inhibition of rab11 binding to PI4Kbeta abolishes the localization of rab11 to the Golgi complex and significantly inhibits transport of vesicular stomatitis virus G protein from the Golgi complex to the plasma membrane. It is proposed that a novel function of PI4Kbeta is to act as a docking protein for rab11 in the Golgi complex, which is important for biosynthetic membrane transport from the Golgi complex to the plasma membrane (de Graaf, 2004).

Rab11 effectors

Rab11a is a small GTP-binding protein enriched in the pericentriolar plasma membrane recycling system. It has been hypothesized that Rab11a-binding proteins exist as downstream effectors of its action. A family of four Rab11-interacting proteins is defined in this study: Rab11-Family Interacting Protein 1 (Rab11-FIP1), Rab11-Family Interacting Protein 2 (Rab11-FIP2), Rab11-Family Interacting Protein 3 (Rab11-FIP3), and pp75/Rip11. All four interacting proteins associate with wild type Rab11a and dominant active Rab11a (Rab11aS20V) as well as Rab11b and Rab25. Rab11-FIP2 also interacts with dominant negative Rab11a (Rab11aS25N) and the tail of myosin Vb. The binding of Rab11-FIP1, Rab11-FIP2, and Rab11-FIP3 to Rab11a is dependent upon a conserved carboxyl-terminal amphipathic alpha-helix. Rab11-FIP1, Rab11-FIP2, and pp75/Rip11 colocalize with Rab11a in plasma membrane recycling systems in both non-polarized HeLa cells and polarized Madin-Darby canine kidney cells. GFP-Rab11-FIP3 also colocalizes with Rab11a in HeLa cells. Rab11-FIP1, Rab11-FIP2, and pp75/Rip11 also co-enrich with Rab11a and H(+)K(+)-ATPase on parietal cell tubulovesicles, and Rab11-FIP1 and Rab11-FIP2 translocate with Rab11a and the H(+)K(+)-ATPase upon stimulating parietal cells with histamine. The results suggest that the function of Rab11a in plasma membrane recycling systems is dependent upon a compendium of protein effectors (Hales, 2001).

Rab11, a low molecular weight GTP-binding protein, has been shown to play a key role in a variety of cellular processes, including endosomal recycling, phagocytosis, and transport of secretory proteins from the trans-Golgi network. A novel Rab11 effector, EF-hands-containing Rab11-interacting protein (Eferin), is described in this study. In addition, a 20-amino acid domain has been defined that is present at the C terminus of Eferin and other Rab11/25-interacting proteins, such as Rip11 and nRip11. Using biochemical techniques, this domain is shown to be necessary and sufficient for Rab11 binding in vitro and it is required for localization of Rab11 effector proteins in vivo. The data suggest that various Rab effectors compete with each other for binding to Rab11/25 possibly accounting for the diversity of Rab11 functions (Prekeris, 2001).

Rab11-FIP2 is a member of a newly identified family of Rab11-binding proteins that have been implicated in the function of recycling endosomes. Rab11-FIP2 may also be involved with the process of receptor-mediated endocytosis. Rab11-FIP2 contains an NPF motif that allows it to bind Reps1, a member of a family of EH domain proteins involved in endocytosis. Rab11-FIP2 associates with the alpha-adaptin subunit of AP-2 complexes, which are known to recruit receptors into clathrin-coated vesicles. Overexpression of Rab11-FIP2 suppresses the internalization of epidermal growth factor receptors, but not transferrin receptors, through binding sites that promote complex formation with Rab11, Reps1, and alpha-adaptin. These findings suggest that Rab11-FIP2 may participate in the coupling of receptor-mediated endocytosis to the subsequent sorting of receptor-containing vesicles in endosomes (Cullis, 2002).

The Rab11-FIP/Rip/RCP proteins are a recently described novel protein family, whose members interact with Rab GTPases that function in endosomal recycling. To date, five such proteins have been described in humans, all of which interact with Rab11, and one (RCP) also interacts with Rab4. Several of these proteins have been characterized with respect to their ability to interact with Rab4, as well as their ability to self-interact, and to interact with each other. Two of the family members-pp75/Rip11 and Rab11-FIP3 do not bind Rab4; several members of the family can self-interact and interact with each other. These interactions primarily involve their C-terminal end that includes the Rab binding domain (RBD) that is contained within a predicted coiled-coil, or ERM motif. A new (sixth) member of the protein family has been identified, that has been named Rab11-FIP4; the family evolutionary complexity and chromosomal distribution is reported. Furthermore, it is proposed that the ability of these proteins to bind each other are important in effecting membrane trafficking events by forming protein 'platforms,' regulated by Rab11 and/or Rab4 activity (Wallace, 2002a).

Rab11-FIP4 interacts with Rab11 in a GTP-dependent manner and its C-terminal region allows the protein to self-interact and interact with pp75/Rip11, Rab11-FIP2, and Rab11-FIP3. However, Rab11-FIP4 does not appear to interact directly with Rab coupling protein (RCP). The subcellular localization of Rab11-FIP4 in HeLa cells was investigated; it colocalizes extensively with transferrin and with Rab11. Furthermore, when overexpressed, it causes a condensation of the Rab11 compartment in the perinuclear region. The carboxy-terminal region of Rab11-FIP4 [Rab11-FIP4(C-ter)] is necessary and sufficient for its endosomal membrane association. Expression of Rab11-FIP4(C-ter) causes a dispersal of the Rab11 compartment towards the cell periphery and does not inhibit transferrin recycling in HeLa cells. It is likely that Rab11-FIP4 serves as a Rab11 effector in a Rab11 mediated function other than transferrin recycling (Wallace, 2002b).

Rab4 and Rab11 are small GTPases belonging to the Ras superfamily. They both function as regulators along the receptor recycling pathway. A novel 80-kDa protein has been identified that interacts specifically with the GTP-bound conformation of Rab4, and it also interacts strongly with Rab11. This protein has been named Rab coupling protein (RCP). RCP is predominantly membrane-bound and is expressed in all cell lines and tissues tested. It colocalizes with early endosomal markers including Rab4 and Rab11 as well as with the transferrin receptor. Overexpression of the carboxyl-terminal region of RCP, which contains the Rab4- and Rab11-interacting domain, results in a dramatic tubulation of the transferrin compartment. Furthermore, expression of this mutant causes a significant reduction in endosomal recycling without affecting ligand uptake or degradation in quantitative assays. RCP is a homolog of Rip11 and therefore belongs to the recently described Rab11-FIP family (Lindsay, 2002a).

Rab11-FIP2 is a recently described member of the Rip11/Rab11-FIP/Rab coupling protein family of Rab11 interacting proteins. Rab11-FIP2 interacts with both Rab11 and myosin Vb and co-localizes with Rab11 in both HeLa and Madin-Darby canine kidney cells. The specificity of the interaction between Rab11-FIP2 and Rab11 has been characterized; it does not interact with Rab4, Rab3, Rab5, Rab6, or Rab7. The COOH-terminal region of Rab11-FIP2, which contains the Rab11 binding domain (RBD), is necessary and sufficient for its early endosomal membrane association. In contrast, the amino-terminal region, which contains a phospholipid binding C2-domain, by itself is insufficient for membrane binding. Expression of a deletion mutant of Rab11-FIP2, containing the RBD, causes tubulation of a transferrin receptor-positive early endosomal compartment in HeLa cells. Endogenous Rab11 is also associated with this compartment. This phenotype cannot be reversed by excess wild-type Rab11, or dominant-positive Rab11 (Rab11Q70L), suggesting that Rab11-FIP2 functions downstream of Rab11 in endosomal trafficking (Lindsay, 2002b).

The Rab11 family of interacting proteins (Rab11-FIP) is a recently identified protein family composed of, to date, six members that interact with Rab11. They all share a highly homologous Rab11-binding domain (RBD) at their C-termini. However, apart from the RBD, they vary in their domain organization. Rab11-FIP3 and Rab11-FIP4 possess an ezrin-radixin-moesin (ERM) domain in their C-terminal half and EF hands in their N-terminal region. They have been termed class II Rab11-FIPs. The class I Rab11-FIPs, Rab coupling protein (RCP), Rip11 and Rab11-FIP2, each have a C2 phospholipid-binding domain near their N-termini. Although they are still membrane associated, truncation mutants of the class I Rab11-FIPs that lack their C2 domains display an altered subcellular distribution in vivo, indicating that this domain plays an important role in specifying their correct intracellular localization. To determine the phospholipids to which they bind, a protein phospholipid overlay assay was performed. The results indicate that the class-I Rab11-FIPs bind preferentially to phosphatidylinositol-(3,4,5)-trisphosphate [PtdIns(3,4,5)P3] and the second messenger phosphatidic acid. Stimulation of PtdIns(3,4,5)P3 or phosphatidic acid synthesis results in the translocation of the Rab11-FIPs from a perinuclear location to the periphery of the cell. By contrast, the transferrin receptor does not translocate to the plasma membrane under these conditions. This translocation is dependent on the presence of the C2 domain, because class I Rab11-FIP green-fluorescent-protein fusions that lack the C2 domain cannot translocate to the plasma membrane. It is proposed that the C2 domains of the class I Rab11-FIPs function to target these proteins to 'docking sites' in the plasma membrane that are enriched in PtdIns(3,4,5)P3 and phosphatidic acid (Lindsay, 2004).

Arfophilin is an ADP ribosylation factor (Arf) binding protein of unknown function. It is identical to the Rab11 binding protein eferin/Rab11-FIP3, and it binds both Arf5 and Rab11. A related protein, arfophilin-2, is described that interacts with Arf5 in a nucleotide-dependent manner, but not Arf1, 4, or 6 and also binds Rab11. Arfophilin-2 localizes to a perinuclear compartment, the centrosomal area, and focal adhesions. The localization of arfophilin-2 to the perinuclear compartment is selectively blocked by overexpression of Arf5-T31N. In contrast, a green fluorescent protein-arfophilin-2 chimera or arfophilin-2 deletions are localized around the centrosome in a region that is also enriched for transferrin receptors and Rab11 but not early endosome markers, suggesting that the distribution of the endosomal recycling compartment is altered. The arfophilins belong to a conserved family that includes Drosophila Nuclear fallout, a centrosomal protein required for cellularization. Expression of green fluorescent protein-Nuclear fallout in HeLa cells results in a similar phenotype, indicative of functional homology and thus implicating the arfophilins in mitosis/cytokinesis. It is suggested that the novel dual GTPase-binding capacity of the arfophilins could serve as an interface of signals from Rab and Arf GTPases to regulate membrane traffic and integrate distinct signals in the late endosomal recycling compartment (Hickson, 2003).

The observation that GFP-Nuf overexpression in HeLa cells results in largely the same phenotype as that of GFP-arfophilin-2 overexpression suggests that these proteins are functionally related. This has several important implications. It suggests a potential role for the arfophilins in cell division, particularly cytokinesis, because Nuf is required for the formation of cellularization furrows in the Drosophila embryo. Because testis is a tissue undergoing a high degree of cell division, such a role may help to explain the high level of expression observed in this tissue. The findings also implicate the endosomal recycling compartment in cytokinesis. This is consistent with recent studies in C. elegans where RNA interference-induced suppression of Rab 11 leads to specific regression of the cleavage furrow at the final stage of abscission (Skop, 2001). The significance of the role of Rab 11 should not be overlooked, particularly given the interaction of arfophilins with this GTPase. Arfs have also been implicated in cellularization in Drosophila and cytokinesis in C. elegans. Such observations suggest the hypothesis that arfophilin-2, and by extension the endosomal recycling compartment, may be involved in traffic to the midbody during cytokinesis. Moreover, the identification of arfophilin-2 as a Rab 11 and Arf binding protein offers the tantalizing suggestion that this protein may integrate the Rab 11 and Arf signals to membrane traffic during cytokinesis (Hickson, 2003 and references therein).

Rab/Ypt GTPases play key roles in the regulation of vesicular trafficking. They perform most of their functions in a GTP-bound form by interacting with specific downstream effectors. The exocyst is a complex of eight polypeptides involved in constitutive secretion and functions as an effector for multiple Ras-related small GTPases, including the Rab protein Sec4p in yeast. In this study, the localization and function of the Sec15 exocyst subunit was examined in mammalian cells. Overexpressed Sec15 associates with clusters of tubular/vesicular elements that are concentrated in the perinuclear region. The tubular/vesicular clusters are dispersed throughout the cytoplasm upon treatment with the microtubule-depolymerizing agent nocodazole and are accessible to endocytosed transferrin, but not exocytic cargo (vesicular stomatitis virus glycoprotein). Consistent with these observations, Sec15 colocalizes selectively with the recycling endosome marker Rab11 and exhibits a GTP-dependent interaction with the Rab11 GTPase, but not with Rab4, Rab6, or Rab7. These findings provide the first evidence that the exocyst functions as a Rab effector complex in mammalian cells (Zhang, 2004).

The Rab11 subfamily of GTPases plays an important role in vesicle trafficking from endosomes to the plasma membrane. At least six Rab11 effectors (family of Rab11-interacting proteins (FIPs)) have been shown to interact with Rab11 and are hypothesized to regulate various membrane trafficking pathways such as transferrin recycling, cytokinesis, and epidermal growth factor trafficking. In this study, interactions of FIPs with the Rab11 GTPase were characterized using isothermal titration calorimetric studies and mutational analysis. The data suggest that FIPs cannot differentiate between GTP-bound Rab11a and Rab11b in vitro (50-100 nm affinity) and in vivo. Although FIPs interact with the GDP-bound form of Rab11 in vitro, the binding affinity (>1000 nm) is not sufficient for FIP and GDP-bound Rab11 interactions to occur in vivo. Mutational analysis revealed that both the conserved hydrophobic patch and Tyr628 are important for the GTP-dependent binding of Rab11 to FIPs. The entropy and enthalpy analyses suggest that binding to Rab11a/b may induce conformational changes in FIPs (Junutula, 2004).

Rab 11 GTPase is an important regulator of endocytic membrane traffic. A novel family of Rab11 binding proteins has been identified, known as Rab11-family interacting proteins (FIPs). One of the family members, Rab coupling protein (RCP), was identified as a protein binding to both Rab4 and Rab11 GTPases. RCP was therefore suggested to serve a dual function as Rab4 and Rab11 binding protein. In this study, the cellular functions of RCP was studied and its interactions with Rab4 and Rab11 were examined. The data show that RCP interacts only weakly with Rab4 in vitro and does not play the role of coupling Rab11 and Rab4 in vivo. Furthermore, the data indicate that the RCP-Rab11 complex regulates the sorting of transferrin receptors from the degradative to the recycling pathway. It is therefore proposed that RCP functions primarily as a Rab11 binding protein that regulates protein sorting in tubular endosomes (Peden, 2004).

An integral part of cell division is the separation of daughter cells via cytokinesis. There is now good evidence that the completion of cytokinesis requires coordinated membrane trafficking to deliver new membrane to the tip of the furrow and to complete the abscission. Membrane traffic in cytokinesis was examined in this study and several novel observations are described. (1) Rab11- and FIP3-containing recycling endosomes were found to accumulate near the cleavage furrow and are required for successful completion of cytokinesis. (2) The Rab11-FIP3 protein complex was demonstrated to be intimately involved in the delivery of endosomes to the cleavage furrow. Significantly, although FIP3 recruitment to endosomes is Rab11 dependent, the targeting of FIP3 to the midbody was found to be independent of Rab11. (3) It was shown that the Rab11-FIP3 complex is required for a late stage of cytokinesis, possibly abscission. (4) It was demonstrated that localization of FIP3 is subject to substantial spatial and temporal regulation. These data provide the first detailed analysis of recycling endosomes in cell division and provide a new model for membrane traffic to the furrow. It is proposed that the dynamic Rab11-FIP3 interaction controls the delivery, targeting, and fusion of recycling endosomes with furrow during late cytokinesis and abscission (Wilson, 2005).

Rab11 endosomes contribute to mitotic spindle organization and orientation

During interphase, Rab11-GTPase-containing endosomes recycle endocytic cargo. However, little is known about Rab11 endosomes in mitosis. This paper show that Rab11 localizes to the mitotic spindle and regulates dynein-dependent endosome localization at poles. Mitotic recycling endosomes were found to bind gamma-TuRC components and associate with tubulin in vitro. Rab11 depletion or dominant-negative Rab11 expression disrupts astral microtubules, delays mitosis, and redistributes spindle pole proteins. Reciprocally, constitutively active Rab11 increases astral microtubules, restores gamma-tubulin spindle pole localization, and generates robust spindles. This suggests a role for Rab11 activity in spindle pole maturation during mitosis. Rab11 depletion causes misorientation of the mitotic spindle and the plane of cell division. These findings suggest a molecular mechanism for the organization of astral microtubules and the mitotic spindle through Rab11-dependent control of spindle pole assembly and function. It is proposes that Rab11 and its associated endosomes cocontribute to these processes through retrograde transport to poles by dynein (Hehnly, 2014).

A paired RNAi and RabGAP overexpression screen identifies Rab11 as a regulator of β-Amyloid production

Alzheimer's disease (AD) is characterized by cerebral deposition of β-amyloid (Aβ) peptides, which are generated from amyloid precursor protein (APP) by β- and γ-secretases. APP and the secretases are membrane associated, but whether membrane trafficking controls Aβ levels is unclear. An RNAi screen of all human Rab-GTPases, which regulate membrane trafficking, was performed complemented with a Rab-GTPase-activating protein screen, and a road map is presented of the membrane-trafficking events regulating Aβ production. Rab11 and Rab3 were identified as key players. Although retromers and retromer-associated proteins control APP recycling, Rab11 controls β-secretase endosomal recycling to the plasma membrane and thus affects Aβ production. Exome sequencing revealed a significant genetic association of Rab11A with late-onset AD, and network analysis identified Rab11A and Rab11B as components of the late-onset AD risk network, suggesting a causal link between Rab11 and AD. These results reveal trafficking pathways that regulate Aβ levels and show how systems biology approaches can unravel the molecular complexity underlying AD (Udayar, 2013).

Coordinated recruitment of Spir actin nucleators and myosin V motors to Rab11 vesicle membranes

There is growing evidence for a coupling of actin assembly and myosin motor activity in cells. However, mechanisms for recruitment of actin nucleators and motors on specific membrane compartments remain unclear. This study reports how Spir actin nucleators (see Drosophila Spire) and myosin V (see Drosophila Didum) motors coordinate their specific membrane recruitment. The myosin V globular tail domain (MyoV-GTD) interacts directly with an evolutionarily conserved Spir sequence motif. Crystal structures of MyoVa-GTD bound either to the Spir-2 motif or to Rab11 was determined, and it was shown that a Spir-2:MyoVa:Rab11 complex can form. The ternary complex architecture explains how Rab11 vesicles support coordinated F-actin nucleation and myosin force generation for vesicle transport and tethering. New insights are also provided into how myosin activation can be coupled with the generation of actin tracks. Since MyoV binds several Rab GTPases, synchronized nucleator and motor targeting could provide a common mechanism to control force generation and motility in different cellular processes (Pylypenko, 2016).

Rab-protein 11: Biological Overview | Evolutionary Homologs | Regulation | Developmental Biology | Effects of Mutation | References

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