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Localized translation is vital to polarized cells and requires precise and robust distribution of different mRNAs and ribosomes across the cell. However, the underlying molecular mechanisms are poorly understood and important players are lacking. Here we show that the novel Rab5 effector Five-subunit Endosomal Rab5 and RNA/ribosome intermediarY, FERRY complex recruits mRNAs and ribosomes to early endosomes, through direct mRNA interaction. FERRY displays preferential binding to certain groups of transcripts, including mRNAs encoding mitochondrial proteins. Deletion of FERRY subunits reduces the endosomal localization of transcripts in cells and has a significant impact on mRNA and protein levels. Clinical studies show that genetic disruption of FERRY causes severe brain damage. We found that, in neurons, FERRY co-localizes with mRNA on early endosomes and mRNA loaded FERRY-positive endosomes are in close proximity of mitochondria. FERRY thus transforms endosomes into mRNA carriers and plays a key role in regulating mRNA distribution and transport.

Ypt51p, a small GTPase of Saccharomyces cerevisiae, has been previously identified as a structural homolog of mammalian Rab5. Although disruption analysis revealed that the protein is required for endocytic transport and for vacuolar protein sorting, the precise step controlled by Ypt51p was not determined. In this work we show that by heterologous expression in animal cells Ypt51p was targeted to Rab5-positive early endosomes and stimulated endocytosis. Furthermore, two Ypt51p mutants induced similar morphological alterations as the corresponding Rab5 mutants. Also in yeast cells Ypt51p was found to be required at an early step in endocytic membrane traffic, since alpha-factor accumulated in an early endocytic intermediate in the absence of Ypt51p. Cell fractionation analysis revealed cofractionation of Ypt51p with endocytic intermediates, while no association with the late Golgi compartment could be detected. Indirect immunofluorescence microscopy allowed us to morphologically identify the Ypt51p-containing compartment. Similar to the mammalian system larger Ypt51p-positive structures were revealed upon expression of Ypt51p Q66L. These structures were also positive for alpha-factor receptor and for carboxypeptidase Y, thus providing direct evidence for their endocytic nature and for the convergence of the vacuolar biosynthetic and endocytic pathways.

Rab5 is a small GTPase associated with the plasma membrane and the early endosomes. Expression of wild type rab5 and a mutant protein defective in GTP-binding in BHK cells led to alterations in the rate of endocytosis and in the morphology of endocytic organelles. The results obtained suggest that rab5 is a rate limiting GTPase that regulate the kinetics of both lateral fusion of early endosomes and fusion of plasma membrane-derived endocytic vesicles with early endosomes.

The activities of three Rab-specific factors with GDP/GTP exchange activity, Vps9p, Rabex-5 and DSS4, with their cognate GTPases, Ypt51p, Rab5 and Ypt1p, have been analysed quantitatively. In contrast to other exchange factors examined and to DSS4, Vps9p, and by analogy probably Rabex-5, have considerably lower affinity than GDP to the respective GTPases. In keeping with this, they are relatively weak exchangers, with a maximal rate constant for GDP release from the ternary complex between exchange factor, GTPase and GDP of ca 0.01 s(-1), which is several orders of magnitude lower than for other exchange factors examined. If interaction with these proteins is a mandatory aspect of the Rab cycle, this suggests that the overall rate of cycling might be controlled at this point of the cycle. Surprisingly, DSS4, which has the thermodynamic potential to displace GDP effectively from Ypt1p, also does this very slowly, again with a maximal rate constant of ca 0.01 s(-1). An additional, and based on present knowledge, unique, feature of the Ypt1p.DSS4 complex, is that the association of GTP (or GDP) is more than 10(3)-fold slower than to Ypt1p, thus leading to a long life-time of the binary complex between the two proteins, even at the high nucleotide concentrations that prevail in the cell. This leads to the conclusion that the protein-protein complex is likely to have an important biological significance in addition to its probable role in GTP/GDP exchange.

Epithelial cell lines can be grown on filter supports and form polarized monolayers with distinct basolateral and apical plasma membrane domains. This property has been extensively used in cell biology to investigate epithelial cell function. To date, a major limitation of this approach has been the difficulty of obtaining transient gene expression in polarized epithelia. Here we present an approach to overcome this problem using gene transfer into polarized epithelial cells grown on filters using a herpes virus-based vector. Recombinant genes are inserted into a defective HSV-1 plasmid and packaged with a replication-incompetent HSV-1 helper virus into virus particles which are used to infect the polarized epithelial cells grown on filters. The transepithelial resistance of the cells is not affected by the addition of virus, and there are no detectable cytopathic effects.

Rab proteins are a family of approximately 25kD ras-related GTPases which are associated with distinct intracellular membranes where they control vesicle traffic between intracellular compartments. The late-endosomal rab protein rab7(1-207), (lacking only the C-terminal lipids of the native molecule) and three C-terminal truncated constructs rab7(1-202), rab7(1-197) and rab7(1-182) were purified using an E. coli expression system. The C-terminal tail region of rab proteins is of special interest because it is thought to target rab proteins to particular intracellular membranes. A comparison of TOCSY-NMR spectra from intact rab7(1-207) and the tail-less construct rab7(1-182) suggested that much of the C-terminal tail is flexible in solution. The GTP hydrolysis, and GDP association and dissociation rates for all the truncated and intact constructs were similar, showing that the tail region of rab7(1-207) has little influence on the hydrolysis and exchange rates of the nucleotide.

We investigated whether Rab5, a small guanosine triphosphatase that regulates early endocytic transport in different cell types is involved in the insulin-regulated endocytic pathways in adipocytes. Rab5 was detected in freshly isolated adipocytes and 3T3-L1 adipocytes, but its expression level was not markedly increased with adipocyte differentiation. After subcellular fractionation of adipocytes incubated in the absence of insulin, Rab5 was found to be abundant in plasma membrane and cytosol, but was also present in high and low density microsomes. This subcellular distribution was compatible with a role in early endocytosis. When cells were incubated with insulin, the concentration of Rab5 decreased by about 50% in the internal compartments. In contrast to Rab4, which also leaves the low density microsomes in response to insulin, Rab5 was not found in Glut4-containing vesicles purified by immunoadsorption on antibodies to Glut4. When adipocytes were treated with wortmannin, an inhibitor of phosphatidylinositol 3-kinase, the effect of insulin on Rab5 movement was not affected, whereas the insulin-induced movements of Rab4 and Glut4 were abolished. In parallel, wortmannin inhibited the increase in horseradish peroxidase uptake induced by insulin, an index of fluid phase endocytosis, but did not prevent the endocytosis of the glucose transporters. As a whole, our results suggest that Rab5 is not involved in insulin-stimulated Glut4 exocytosis. These results are compatible with the postulated role of Rab5 in the endocytotic pathway, at a step that does not require phosphatidyl-inositol 3-kinase activation.

Overlapping cDNAs encoding the entire human ERGIC-53, a 53 kDa membrane protein of the ER-Golgi intermediate compartment, have been isolated and their nucleotide sequence determined. The isolated cDNA is about 2.7 kb in length. The deduced polypeptide chain for ERGIC-53 consists of 510 amino acids (M(r) 54217) including an N-terminal signal sequence of 30 amino acids, a single putative transmembrane segment of 18 amino acids, and a short cytoplasmic domain of 12 amino acids. Surprisingly, the cytoplasmic segment contains two lysines positioned three and four residues from the C-terminus. Such a double lysine motif is known to function as a retention signal for a group of membrane proteins associated with the ER. Expression of a full-length cDNA of ERGIC-53 in Vero cells revealed intracellular localization similar but not always identical to the endogenously expressed ERGIC-53. The presence of an ER retention motif in a protein of the ER-Golgi intermediate compartment has important implications for the retention mechanism mediated by this signal.

Classically, membrane-bound compartments have been viewed as comprising physically distinct entities each having a specific protein composition and characteristic functions. By this criterion, compartment boundaries should be easily delineated by ultrastructural localization of resident proteins. This simple view, however, is complicated by several factors. For example, even the localization of two proteins to different regions might not unequivocally demonstrate that they are present in physically discontinuous compartments. Such a distribution of proteins could reflect the existence of subdomains in an otherwise continuous membrane array. An example is the lateral heterogeneity found between the rough and the smooth endoplasmic reticulum (ER). Conversely, markers may be segregated into physically distinct membranes which nevertheless remain functionally continuous by repeated membrane fissions and fusions or transient tubular interconnections. The existence of such interactions among “like” elements would play an important role in preserving continuity among equivalent, but physically separated organelles. Furthermore, the definition of a compartment becomes more complicated if one considers organelles involved in membrane traffic. Given that membrane material is continuously transported and recycled between two organelles, precise biochemical definition of the compartments may be difficult. Some components are, of course, known to be resident, due perhaps to interactions with compartment-specific structural frameworks, and these can be used for compartment characterization. However, precise definition of the subcellular compartments requires detailed knowledge of the machinery regulating the entry and the exit of material at compartment boundaries, as well as the retention of molecules within the compartment (for further discussion see Mellman and Simons 1992). One family of proteins that may play an important role in defining compartment boundaries are the rab proteins, small GTPases that will be discussed in this chapter.

We have investigated the role of phosphorylation in the endocytosis of the human transferrin receptor (TR) by replacing its phosphorylation site, Ser24, with Ala through site-directed mutagenesis of the TR cDNA. The TR Ala24 mutant expressed in mouse 3T3 cells was not phosphorylated, even following stimulation of protein kinase C by phorbol ester. However, in spite of this defect the mutant was efficiently endocytosed and recycled back to the plasma membrane with kinetics similar to those of TR and a control mutant TR Ala63. Thus, these results confirm earlier results by Davis et al. (1986, J. Biol. Chem., 261-9034-9041) that Ser24 of human TR is the phosphorylation site for protein kinase C but do not support a role of this modification as a signal for TR endocytosis and recycling.

The human transferrin receptor (TR) is a protein comprising 760 amino acid residues that spans the membrane once with its N terminus towards the cytoplasm. It is synthesized without a cleavable signal peptide. We have tested whether the signal responsible for its membrane insertion is present within its transmembrane peptide using a combined recombinant DNA/in vitro translation approach. The complete TR coding region was first reconstructed from overlapping TR cDNA clones and then engineered into an SP6-based transcription vector. In vitro transcription and subsequent translation in the presence of rough microsomes yielded TR molecules that were glycosylated and correctly inserted into the membrane. Two kinds of experiments demonstrated that the spanning region of the TR polypeptide contained the signal for translocation across the membrane of the rough endoplasmic reticulum. First, we deleted the spanning region of TR and showed that this deletion mutant could not be inserted. Second, we showed that two cytoplasmic proteins (the mouse dihydrofolate reductase and the chimpanzee alpha-globin) could be inserted into the microsomal membrane in the expected orientation when the TR transmembrane segment was added to their N termini. Thus, the spanning peptide was shown to be both necessary and sufficient for chain translocation. Further analyses demonstrated that the translocation event was dependent on the signal recognition particle.

Unspecific immunostimulation by bacterial vaccines of a patient with mucopolysaccharidosis IIIA (Sanfilippo A) syndrome induces a marked increase in the urinary excretion of heparan sulfate and of uronatecontaining oligosaccharides. This event is presumably linked to an increased vascular permeability and exocytosis of storage material, elicited by mediators of inflammation, as well as to enhanced degradation of stored polymers in activated macrophages and surrounding tissue.

Studies of the mechanism of cytokinesis, the process by which a mother cell undergoes cleavage to form two separated daughter cells, often focus on the action of the contractile actomyosin ring at the cell equator. Ewa Paluch and colleagues instead investigate the mechanics of the actomyosin cortex found at the cell poles during cytokinesis. They find that the presence of a contractile polar cortex makes cytokinesis an inherently unstable process that can result in misalignment of the constriction ring. They propose that the membrane blebs forming at the poles of dividing cells stabilize the position by releasing cortical contractility. These findings reveal an inherent instability in the shape of a dividing cell and demonstrate a novel mechanism that helps to limit shape instability. Cytokinesis, the physical separation of daughter cells at the end of mitosis, requires precise regulation of the mechanical properties of the cell periphery1,2. Although studies of cytokinetic mechanics mostly focus on the equatorial constriction ring3, a contractile actomyosin cortex is also present at the poles of dividing cells2,4. Whether polar forces influence cytokinetic cell shape and furrow positioning remains an open question. Here we demonstrate that the polar cortex makes cytokinesis inherently unstable. We show that limited asymmetric polar contractions occur during cytokinesis, and that perturbing the polar cortex leads to cell shape oscillations, resulting in furrow displacement and aneuploidy. A theoretical model based on a competition between cortex turnover and contraction dynamics accurately accounts for the oscillations. We further propose that membrane blebs, which commonly form at the poles of dividing cells5 and whose role in cytokinesis has long been enigmatic, stabilize cell shape by acting as valves releasing cortical contractility. Our findings reveal an inherent instability in the shape of the dividing cell and unveil a novel, spindle-independent mechanism ensuring the stability of cleavage furrow positioning.

In addition to apicobasal polarization, some epithelia also display polarity within the plane of the epithelium. To what extent polarized endocytosis plays a role in the establishment and maintenance of planar cell polarity (PCP) is at present unclear. Here, we investigated the role of Rabenosyn-5 (Rbsn-5), an evolutionarily conserved effector of the small GTPase Rab5, in the development of Drosophila wing epithelium. We found that Rbsn-5 regulates endocytosis at the apical side of the wing epithelium and, surprisingly, further uncovered a novel function of this protein in PCP. At early stages of pupal wing development, the PCP protein Fmi redistributes between the cortex and Rab5- and Rbsn-5-positive early endosomes. During planar polarization, Rbsn-5 is recruited at the apical cell boundaries and redistributes along the proximodistal axis in an Fmi-dependent manner. At pre-hair formation, Rbsn-5 accumulates at the bottom of emerging hairs. Loss of Rbsn-5 causes intracellular accumulation of Fmi and typical PCP alterations such as defects in cell packing, in the polarized distribution of PCP proteins, and in hair orientation and formation. Our results suggest that establishment of planar polarity requires the activity of Rbsn-5 in regulating both the endocytic trafficking of Fmi at the apical cell boundaries and hair morphology.

International audience; Rab5 regulates endocytic membrane traffic by specifically recruiting cytosolic effector proteins to their site of action on early endosomal membranes. We have characterized a new Rab5 effector complex involved in endosomal fusion events. This complex includes a novel protein, Rabenosyn-5, which, like the previously characterized Rab5 effector early endosome antigen 1 (EEA1), contains an FYVE finger domain and is recruited in a phosphatidylinositol-3-kinase–dependent fashion to early endosomes. Rabenosyn-5 is complexed to the Sec1-like protein hVPS45. hVPS45 does not interact directly with Rab5, therefore Rabenosyn-5 serves as a molecular link between hVPS45 and the Rab5 GTPase. This property suggests that Rabenosyn-5 is a closer mammalian functional homologue of yeast Vac1p than EEA1. Furthermore, although both EEA1 and Rabenosyn-5 are required for early endosomal fusion, only overexpression of Rabenosyn-5 inhibits cathepsin D processing, suggesting that the two proteins play distinct roles in endosomal trafficking. We propose that Rab5-dependent formation of membrane domains enriched in phosphatidylinositol-3-phosphate has evolved as a mechanism for the recruitment of multiple effector proteins to mammalian early endosomes, and that these domains are multifunctional, depending on the differing activities of the effector proteins recruited.