Of yeast, mice, and men. Rab proteins and organelle transport

Phenylephrine increases tear cathepsin S secretion in healthy murine lacrimal gland acinar cells through an alternative secretory pathway

Little is known about the relationship between stimulation of lacrimal gland (LG) tear protein secretion by parasympathetic versus sympathetic nerves, particularly whether the spectrum of tear proteins evoked through each innervation pathway varies. We have previously shown that activity and abundance of cathepsin S (CTSS), a cysteine protease, is greatly increased in tears of Sjögren's syndrome (SS) patients and in tears from the male NOD mouse of autoimmune dacryoadenitis that recapitulates SS-associated dry eye disease. Beyond the increased synthesis of CTSS detected in the diseased NOD mouse LG, increased tear CTSS secretion in NOD mouse tears was recently linked to increased exocytosis from a novel endolysosomal secretory pathway. Here, we have compared secretion and trafficking of CTSS in healthy mouse LG acinar cells stimulated with either the parasympathetic acetylcholine receptor agonist, carbachol (CCh), or the sympathetic α₁-adrenergic agonist, phenylephrine (PE). In situ secretion studies show that PE significantly increases CTSS activity and protein in tears relative to CCh stimulation by 1.2-fold (***, p = 0.0009) and ∼5-fold (*, p-0.0319), respectively. A similar significant increase in CTSS activity with PE relative to CCh is observed when cultured LGAC are stimulated in vitro. CCh stimulation significantly elevates intracellular [Ca²⁺], an effect associated with increases in the size of Rab3D-enriched vesicles consistent with compound fusion, and subsequently decreases in their intensity of labeling consistent with their exocytosis. PE stimulation induces a lower [Ca²⁺] response and has minimal effects on Rab3D-enriched SV diameter or the intensity of Rab3D-enriched SV labeling. LG deficient in Rab3D exhibit a higher sensitivity to PE stimulation, and secrete more CTSS activity. Significant increases in the colocalization of endolysosomal vesicle markers (Lamp1, Lamp2, Rab7) with the subapical actin suggestive of fusion of endolysosomal vesicles at the apical membrane occur both with CCh and PE stimulation, but PE demonstrates increased colocalization. In conclusion, the α₁-adrenergic agonist, PE, increases CTSS secretion into tears through a pathway independent of the exocytosis of Rab3D-enriched mature SV, possibly representing an alternative endolysosomal secretory pathway.

Homozygous variant in MADD, encoding a Rab guanine nucleotide exchange factor, results in pleiotropic effects and a multisystemic disorder

Rab proteins coordinate inter-organellar vesicle-mediated transport, facilitating intracellular communication, protein recycling, and signaling processes. Dysfunction of Rab proteins or their direct interactors leads to a wide range of diseases with diverse manifestations. We describe seven individuals from four consanguineous Arab Muslim families with an infantile-lethal syndrome, including failure to thrive (FTT), chronic diarrhea, neonatal respiratory distress, variable pituitary dysfunction, and distal arthrogryposis. Exome sequencing analysis in the independent families, followed by an internal gene-matching process using a local exome database, identified a homozygous splice-site variant in MADD (c.2816 + 1 G > A) on a common haplotype. The variant segregated with the disease in all available family members. Determination of cDNA sequence verified single exon skipping, resulting in an out-of-frame deletion. MADD encodes a Rab guanine nucleotide exchange factor (GEF), which activates RAB3 and RAB27A/27B and is thus a crucial regulator of neuromuscular junctions and endocrine secretory granule release. Moreover, MADD protects cells from caspase-mediated TNF-α-induced apoptosis. The combined roles of MADD and its downstream effectors correlate with the phenotypic spectrum of disease, and call for additional studies to confirm the pathogenic mechanism and to investigate possible therapeutic avenues through modulation of TNF-α signaling.

Rab27a Contributes to Cathepsin S Secretion in Lacrimal Gland Acinar Cells

Altered lacrimal gland (LG) secretion is a feature of autoimmune dacryoadenitis in Sjögren's syndrome (SS). Cathepsin S (CTSS) is increased in tears of SS patients, which may contribute to disease. Rab3D and Rab27a/b isoforms are effectors of exocytosis in LG, but Rab27a is poorly studied. To investigate whether Rab27a mediates CTSS secretion, we utilized quantitative confocal fluorescence microscopy of LG from SS-model male NOD and control male BALB/c mice, showing that Rab27a-enriched vesicles containing CTSS were increased in NOD mouse LG. Live-cell imaging of cultured lacrimal gland acinar cells (LGAC) transduced with adenovirus encoding wild-type (WT) mCFP-Rab27a revealed carbachol-stimulated fusion and depletion of mCFP-Rab27a-enriched vesicles. LGAC transduced with dominant-negative (DN) mCFP-Rab27a exhibited significantly reduced carbachol-stimulated CTSS secretion by 0.5-fold and β-hexosaminidase by 0.3-fold, relative to stimulated LGAC transduced with WT mCFP-Rab27a. Colocalization of Rab27a and endolysosomal markers (Rab7, Lamp2) with the apical membrane was increased in both stimulated BALB/c and NOD mouse LG, but the extent of colocalization was much greater in NOD mouse LG. Following stimulation, Rab27a colocalization with endolysosomal membranes was decreased. In conclusion, Rab27a participates in CTSS secretion in LGAC though the major regulated pathway, and through a novel endolysosomal pathway that is increased in SS.

Insights into K-Ras 4B regulation by post-translational lysine acetylation

Ras is a molecular switch cycling between an active, GTP-bound and an inactive, GDP-bound state. Mutations in Ras, mostly affecting the off-switch, are found in many human tumours. Recently, it has been shown that K-Ras 4B is targeted by lysine acetylation at K104. Based on results obtained for an acetylation mimetic Ras mutant (K104Q), it was hypothesised that K104-acetylation might interfere with its oncogenicity by impairing SOS-catalysed guanine-nucleotide exchange. We prepared site-specifically K104-acetylated K-Ras 4B and the corresponding oncogenic mutant protein G12V using the genetic-code expansion concept. We found that SOS-catalysed nucleotide exchange, also of allosterically activated SOS, was neither affected by acetylation of K104 in wildtype K-Ras 4B nor in the G12V mutant, suggesting that glutamine is a poor mimetic for acetylation at this site. In vitro, the lysine-acetyltransferases CBP and p300 were able to acetylate both, wildtype and G12V K-Ras 4B. In addition to K104 we identified further acetylation sites in K-Ras 4B, including K147, within the important G5/SAK-motif. However, the intrinsic and the SOS-catalysed nucleotide exchange was not affected by K147-acetylation of K-Ras 4B. Finally, we show that Sirt2 and HDAC6 do neither deacetylate K-Ras 4B if acetylated at K104 nor if acetylated at K147 in vitro.

The cortical acto-Myosin network: from diffusion barrier to functional gateway in the transport of neurosecretory vesicles to the plasma membrane

Dysregulation of regulated exocytosis is linked to an array of pathological conditions, including neurodegenerative disorders, asthma, and diabetes. Understanding the molecular mechanisms underpinning neuroexocytosis including the processes that allow neurosecretory vesicles to access and fuse with the plasma membrane and to recycle post-fusion, is therefore critical to the design of future therapeutic drugs that will efficiently tackle these diseases. Despite considerable efforts to determine the principles of vesicular fusion, the mechanisms controlling the approach of vesicles to the plasma membrane in order to undergo tethering, docking, priming, and fusion remain poorly understood. All these steps involve the cortical actin network, a dense mesh of actin filaments localized beneath the plasma membrane. Recent work overturned the long-held belief that the cortical actin network only plays a passive constraining role in neuroexocytosis functioning as a physical barrier that partly breaks down upon entry of Ca(2+) to allow secretory vesicles to reach the plasma membrane. A multitude of new roles for the cortical actin network in regulated exocytosis have now emerged and point to highly dynamic novel functions of key myosin molecular motors. Myosins are not only believed to help bring about dynamic changes in the actin cytoskeleton, tethering and guiding vesicles to their fusion sites, but they also regulate the size and duration of the fusion pore, thereby directly contributing to the release of neurotransmitters and hormones. Here we discuss the functions of the cortical actin network, myosins, and their effectors in controlling the processes that lead to tethering, directed transport, docking, and fusion of exocytotic vesicles in regulated exocytosis.

Local termination of 3'-5'-cyclic adenosine monophosphate signals: the role of A kinase anchoring protein-tethered phosphodiesterases

A kinase anchoring proteins (AKAPs) belong to a family of functionally related proteins capable of binding protein kinase A (PKA) and tether it to relevant targets. In this way, AKAPs organize macromolecular complexes to segregate PKA activity and retain signal specificity. In the heart, AKAP-PKA interaction is central to the regulation of cardiac contractility. Phosphodiesterases belong to a large superfamily of enzymes that degrade 3'-5'-cyclic adenosine monophosphate (cAMP). They possess diverse catalytic properties and multiple regulatory mechanisms and control the duration and amplitude of the cAMP signal, including its propagation in space. AKAPs, together with PKA, can also assemble phosphodiesterases thereby providing a means to locally control cAMP dynamics at the level of single macromolecular complexes. This allows for the fine tuning of the cAMP response to the specific demands of the cell.

Rab27b regulates exocytosis of secretory vesicles in acinar epithelial cells from the lacrimal gland

Tear proteins are supplied by the regulated fusion of secretory vesicles at the apical surface of lacrimal gland acinar cells, utilizing trafficking mechanisms largely yet uncharacterized. We investigated the role of Rab27b in the terminal release of these secretory vesicles. Confocal fluorescence microscopy analysis of primary cultured rabbit lacrimal gland acinar cells revealed that Rab27b was enriched on the membrane of large subapical vesicles that were significantly colocalized with Rab3D and Myosin 5C. Stimulation of cultured acinar cells with the secretagogue carbachol resulted in apical fusion of these secretory vesicles with the plasma membrane. Evaluation of morphological changes by transmission electron microscopy of lacrimal glands from Rab27b(-/-) and Rab27(ash/ash)/Rab27b(-/-) mice, but not ashen mice deficient in Rab27a, showed changes in abundance and organization of secretory vesicles, further confirming a role for this protein in secretory vesicle exocytosis. Glands lacking Rab27b also showed increased lysosomes, damaged mitochondria, and autophagosome-like organelles. In vitro, expression of constitutively active Rab27b increased the average size but retained the subapical distribution of Rab27b-enriched secretory vesicles, whereas dominant-negative Rab27b redistributed this protein from membrane to the cytoplasm. Functional studies measuring release of a cotransduced secretory protein, syncollin-GFP, showed that constitutively active Rab27b enhanced, whereas dominant-negative Rab27b suppressed, stimulated release. Disruption of actin filaments inhibited vesicle fusion to the apical membrane but did not disrupt homotypic fusion. These data show that Rab27b participates in aspects of lacrimal gland acinar cell secretory vesicle formation and release.

Insight into the molecular switch mechanism of human Rab5a from molecular dynamics simulations

Rab5a is currently a most interesting target because it is responsible for regulating the early endosome fusion in endocytosis and possibly the budding process. We utilized longtime-scale molecular dynamics simulations to investigate the internal motion of the wild-type Rab5a and its A30P mutant. It was observed that, after binding with GTP, the global flexibility of the two proteins is increasing, while the local flexibility in their sensitive sites (P-loop, switch I and II regions) is decreasing. Also, the mutation of Ala30 to Pro30 can cause notable flexibility variations in the sensitive sites. However, this kind of variations is dramatically reduced after binding with GTP. Such a remarkable feature is mainly caused by the water network rearrangements in the sensitive sites. These findings might be of use for revealing the profound mechanism of the displacements of Rab5a switch regions, as well as the mechanism of the GDP dissociation and GTP association.

Secretory vesicle docking to the plasma membrane: molecular mechanism and functional significance

In regulated exocytic pathways, secretion occurs only in the presence of appropriate stimuli. Professional secretory cells harbour specific storage organelles that release bioactive substances with both controlled timing and quantity in response to the strength and period of stimulation. Although each secretory organelle is highly differentiated in multicellular organisms, the basic regulatory mechanism is thought to be conserved. In most instances, the secretagogue increases the intracellular Ca(2+) concentration from the resting level of approximately 100 nM to somewhere between approximately 10 and 100 microM. Although Ca(2+) sensors of the final fusion reaction, such as synaptotagmin, have been investigated intensively in synaptic vesicle exocytosis, there are other preceding rate-limiting steps influenced by Ca(2+) and other secretory signals, especially in the exocytosis of secretory granules whose time course is much slower than that of synaptic vesicles. The stable docking of secretory vesicles to the fusion site that is only seen in regulated exocytic pathways may represent one such critical step. Here, we review the molecular mechanism of docking, mainly based on recent findings on insulin granules in pancreatic beta cells, and propose a new concept for its functional significance in regulated exocytosis.

In search of neutrophil granule proteins of the tammar wallaby (Macropus eugenii)

Two approaches have been used to isolate and identify proteins of the granules of neutrophils of the tammar wallaby, Macropus eugenii. Stimulation with PMA, Ionomycin and calcium resulted in exocytosis of neutrophil granules as demonstrated with electron microscopy. However proteomic analysis using two dimensional gel electrophoresis, in-gel trypsin digestion followed by nano liquid chromatography coupled tandem mass spectrometry (LC-MS/MS) failed to identify any anticipated granule proteins in the reaction supernatants. Subsequent use of differential centrifugation and lysis followed by the application of the same proteomic analysis approach resulted in the isolation and confident identification of 39 proteins, many of which are known to be present in the granules of neutrophils of eutherian mammals or play a role in degranulation. These proteins notably consisted of the known antimicrobials, myeloperoxidase (MPO), serine proteinase, dermcidin, lysozyme and alkaline phosphatase. A number of important known antimicrobials, however, were not detected and these include defensins and cathelicidins. This is the first report of the neutrophil granule proteins of any marsupial and complements previous reports on the cytosolic proteins.

Loss of RAB25 expression in breast cancer

A novel breast cancer cell line (RAO-3) was established by transduction of the Q61L mutant RAS into human mammary epithelial cells that were immortalized with catalytic subunit of telomerase (hTERT). The cells displayed anchorage-independent growth and proliferation, and formed human mammary spindle cell carcinoma when injected into nude mice. Chromosome locus 1q22-23 was partially duplicated and inverted on one of the 3 chromosomes present in the cell line. We report here that mutations of chromosome 1q22-23 locus have resulted in the loss of RAB25 expression in the breast cancer cell line. Transduction of RAB25 into the breast cancer cell line arrests anchorage-independent growth. We have also demonstrated loss of RAB25 in human breast tumor tissue. These data suggest that loss of RAB25 might contribute to tumorigenesis of breast cancer, and RAB25 is likely to be an important factor in the development of breast cancer. RAB25 could be used as biological marker of breast cancer and provides a target for gene replacement therapy.

Proteomic Analysis of Human Neutrophil Granules

Stimulated exocytosis of intracellular granules plays a critical role in conversion of inactive, circulating neutrophils to fully activated cells capable of chemotaxis, phagocytosis, and bacterial killing. The functional changes induced by exocytosis of each of the granule subsets, gelatinase (tertiary) granules, specific (secondary) granules, and azurophil (primary) granules, are poorly defined. To improve the understanding of the role of exocytosis of these granule subsets, a proteomic analysis of the azurophil, specific, and gelatinase granules from human neutrophils was performed. Two different methods for granule protein identification were applied. First, two-dimensional (2D) gel electrophoresis followed by MALDI-TOF MS analysis of peptides obtained by in-gel trypsin digestion of proteins was performed. Second, peptides from tryptic digests of granule membrane proteins were separated by two-dimensional microcapillary chromatography using strong cation exchange and reverse phase microcapillary high pressure liquid chromatography and analyzed with electrospray ionization tandem mass spectrometry (2D HLPC ESI-MS/MS). Our analysis identified 286 proteins on the three granule subsets, 87 of which were identified by MALDI MS and 247 were identified by 2D HPLC ESI-MS/MS. The increased sensitivity of 2D HPLC ESI-MS/MS, however, resulted in identification of over 500 proteins from subcellular organelles contaminating isolated granules. Defining the proteome of neutrophil granule subsets provides a basis for understanding the role of exocytosis in neutrophil biology. Additionally, the described methods may be applied to mobilizable compartments of other secretory cells.

Identification of rab12 as a vesicle-associated small GTPase highly expressed in Sertoli cells of rat testis

We examined the expression and the localization of a small GTPase, rab12, in rat testis. Northern blot analysis showed that 2.3 kb transcript of rab12 was expressed in rat testis. RT-PCR analysis indicated constant expression of rab12 throughout testis development. Immunohistochemical studies revealed that rab12 protein was highly expressed in Sertoli cells in the seminiferous tubules, while both spermatogenic germ cells and interstitial cells exhibited faint or no immunosignal for rab12. The expression pattern of rab12 in Sertoli cells varied between the tubules: its immunostaining appeared as a wheel-like pattern at stage I approximately III and as a luminal staining pattern at stage IV approximately VI, whereas the immunostaining signals were only rudimentary detected at stage VIII and thereafter (approximately stage XIV). The diversified staining pattern of rab12 in the tubules seemed to reflect either the different shape of Sertoli cells during the cycle of the seminiferous epithelium or the variant expression levels of rab12 in Sertoli cells at each stage of the tubules. In cultured rat Sertoli cells and normal rat kidney (NRK) cells, rab12 was found to be associated with small vesicles distributed throughout the cytoplasm, but not with the Golgi apparatus. When overexpressed in NRK cells, rab12-associated small vesicles were not only distributed throughout the cytoplasm but also accumulated in the perinuclear cytoplasm around centrosome. We interrupt these data as a potential role of rab12 in acceleration of vesicular transport from the cell periphery to the perinuclear centrosome region.

Myosin Va transports dense core secretory vesicles in pancreatic MIN6 beta-cells

The role of unconventional myosins in neuroendocrine cells is not fully understood, with involvement suggested in the movement of both secretory vesicles and mitochondria. Here, we demonstrate colocalization of myosin Va (MyoVa) with insulin in pancreatic beta-cells and show that MyoVa copurifies with insulin in density gradients and with the vesicle marker phogrin-enhanced green fluorescent protein upon fluorescence-activated sorting of vesicles. By contrast, MyoVa immunoreactivity was poorly colocalized with mitochondrial or other markers. Demonstrating an important role for MyoVa in the recruitment of secretory vesicles to the cell surface, a reduction of MyoVa protein levels achieved by RNA interference caused a significant decrease in glucose- or depolarization-stimulated insulin secretion. Similarly, expression of the dominant-negative-acting globular tail domain of MyoVa decreased by approximately 50% the number of vesicles docked at the plasma membrane and by 87% the number of depolarization-stimulated exocytotic events detected by total internal reflection fluorescence microscopy. We conclude that MyoVa-driven movements of vesicles along the cortical actin network are essential for the terminal stages of regulated exocytosis in beta-cells.

Griscelli syndrome: characterization of a new mutation and rescue of T-cytotoxic activity by retroviral transfer of RAB27A gene

Griscelli syndrome (GS) is caused by mutations in the MYO5A (GS1), RAB27A (GS2), or MLPH (GS3) genes, all of which lead to a similar pigmentary dilution. In addition, GS1 patients show primary neurological impairment, whereas GS2 patients present immunodeficiency and periods of lymphocyte proliferation and activation, leading to their infiltration in many organs, such as the nervous system, causing secondary neurological damage. We report the diagnosis of GS2 in a 4-year-old child with haemophagocytic syndrome, immunodeficiency, and secondary neurological disorders. Typical melanosome accumulation was found in skin melanocytes and pigment clumps were observed in hair shafts. Two heterozygous mutant alleles of the RAB27A gene were found, a C-T transition (C352T) that leads to Q118stop and a G-C transversion on the exon 5 splicing donor site (G467+1C). Functional assays showed increased cellular activation and decreased cytotoxic activity of NK and CD8+ T cells, associated with defective lytic granules release. Myosin-Va expression and localization in the patient lymphocytes were also analyzed. Most importantly, we show that cytotoxic activity of the patient's CD8+ T lymphocytes can be rescued in vitro by RAB27A gene transfer mediated by a recombinant retroviral vector, a first step towards a potential treatment of the acute phase of GS2 by RAB27A transduced lymphocytes.

A Rho-signaling pathway mediates cortical granule translocation in the sea urchin oocyte

Cortical granules are secretory vesicles of the egg that play a fundamental role in preventing polyspermy at fertilization. In the sea urchin egg, they localize directly beneath the plasma membrane forming a compact monolayer and, upon fertilization, undergo a Ca(2+)-dependent exocytosis. Cortical granules form during early oogenesis and, during maturation, translocate from the cytosol to the oocyte cortex in a microfilament-mediated process. We tested the hypothesis that these cortical granule dynamics were regulated by Rho, a GTPase of the Ras superfamily. We observed that Rho is synthesized early in oogenesis, mainly in a soluble form. At the end of maturation, however, Rho associates with cortical granules. Inhibition of Rho with the C3 transferase from C. botulinum blocks cortical granule translocation and microfilaments undergo a significant disorganization. A similar effect is observed by GGTI-286, a geranylgeranyl transferase inhibitor, suggesting that the association of Rho with the cortical granules is indispensable for its function. In contrast, the anchorage of the cortical granules in the cortex, as well as their fusion at fertilization, are Rho-independent processes. We conclude that Rho association with the cortical granules is a critical regulatory step in their translocation to the egg cortex.

Reconstitution of vesicular transport to Rab11-positive recycling endosomes in vitro

Rab GTPases are key regulators of vesicular protein transport in both the endocytic and exocytic pathways. In endocytosis and recycling, Rab11 plays a role in receptor recycling to plasma membrane via the pericentriolar recycling compartment. However, little is known about the molecular requirements and partners that promote transport through Rab11-positive recycling endosomes. Here, we report a novel approach to reconstitute transport to immunoabsorbed recycling endosomes in vitro. We show that transport is temperature-, energy-, and time-dependent and requires the presence of Rab proteins, as it is inhibited by the Rab-interacting protein Rab GDP-dissociation inhibitor that removes Rab proteins from the membrane. Cytochalasin D, a drug that blocks actin polymerization, inhibits the in vitro assay, suggesting that transport to recycling endosomes depends on an intact actin cytoskeleton. Using an affinity chromatography approach we show the identification of Rab11-interacting proteins including actin that stimulate transport to recycling endosomes in vitro.

Secretory granule exocytosis

Regulated exocytosis of secretory granules or dense-core granules has been examined in many well-characterized cell types including neurons, neuroendocrine, endocrine, exocrine, and hemopoietic cells and also in other less well-studied cell types. Secretory granule exocytosis occurs through mechanisms with many aspects in common with synaptic vesicle exocytosis and most likely uses the same basic protein components. Despite the widespread expression and conservation of a core exocytotic machinery, many variations occur in the control of secretory granule exocytosis that are related to the specialized physiological role of particular cell types. In this review we describe the wide range of cell types in which regulated secretory granule exocytosis occurs and assess the evidence for the expression of the conserved fusion machinery in these cells. The signals that trigger and regulate exocytosis are reviewed. Aspects of the control of exocytosis that are specific for secretory granules compared with synaptic vesicles or for particular cell types are described and compared to define the range of accessory control mechanisms that exert their effects on the core exocytotic machinery.

Regulation of lymphocyte-mediated killing by GTP-binding proteins

Exocytosis of granules containing apoptosis-inducing proteins is one mechanism of target cell killing by cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells. Granules containing perforin and granzymes are redistributed to the area of cell contact initiated by specific interactions between surface ligands on a target cell and receptors on an effector lymphocyte. The formation of a stable conjugate between a cytotoxic lymphocyte and its potential target cell, followed by the directed delivery of granule components to the target cell are prerequisites of lymphocyte-mediated killing. Critical to understanding the development of cytotoxic function by CTLs and NK cells is the delineation of the second messenger pathways that specifically control the reorganization of the actin cytoskeleton during cell-mediated cytotoxicity. The low molecular weight guanosine 5'-triphosphate-binding proteins of the Rho family play a central role in these regulatory events controlling cytotoxic lymphocyte activation.

High resolution crystal structures of human Rab5a and five mutants with substitutions in the catalytically important phosphate-binding loop

GTPase domain crystal structures of Rab5a wild type and five variants with mutations in the phosphate-binding loop are reported here at resolutions up to 1.5 A. Of particular interest, the A30P mutant was crystallized in complexes with GDP, GDP+AlF(3), and authentic GTP, respectively. The other variant crystals were obtained in complexes with a non-hydrolyzable GTP analog, GppNHp. All structures were solved in the same crystal form, providing an unusual opportunity to compare structures of small GTPases with different catalytic rates. The A30P mutant exhibits dramatically reduced GTPase activity and forms a GTP-bound complex stable enough for crystallographic analysis. Importantly, the A30P structure with bound GDP plus AlF(3) has been solved in the absence of a GTPase-activating protein, and it may resemble that of a transition state intermediate. Conformational changes are observed between the GTP-bound form and the transition state intermediate, mainly in the switch II region containing the catalytic Gln(79) residue and independent of A30P mutation-induced local alterations in the P-loop. The structures suggest an important catalytic role for a P-loop backbone amide group, which is eliminated in the A30P mutant, and support the notion that the transition state of GTPase-mediated GTP hydrolysis is of considerable dissociative character.

Proteomic analysis of early melanosomes: identification of novel melanosomal proteins

Melanin is a heterogeneous biopolymer produced only by specific cells termed melanocytes, which synthesize and deposit the pigment in specialized membrane-bound organelles known as melanosomes. Although melanosomes have been suspected of being closely related to lysosomes and platelets, the total number of melanosomal proteins is still unknown. Thus far, six melanosome-specific proteins have been identified, and the challenge is to characterize the complete proteome of the melanosome to further understand its mechanism of biogenesis. In this report, we used mass spectrometry and subcellular fractionation to identify protein components of early melanosomes. Using this approach, we have identified all 6 of the known melanosome-specific proteins, 56 proteins that are shared with other organelles, and confirmed the presence of 6 novel melanosomal proteins using western blotting and by immunohistochemistry.

Complex formation with Ypt11p, a rab-type small GTPase, is essential to facilitate the function of Myo2p, a class V myosin, in mitochondrial distribution in Saccharomyces cerevisiae

We identified Ypt11p, a rab-type small GTPase, by its functional and two-hybrid interaction with Myo2p, a class V myosin of the budding yeast Saccharomyces cerevisiae. The tail domain of Myo2p was coimmunoprecipitated with Ypt11p, suggesting that Ypt11p forms a complex with Myo2p at its tail domain in vivo. Mutational analysis of YPT11 suggests that Myo2p is a putative effector of Ypt11p. Deletion of YPT11 induced partial delay of mitochondrial transmission to the bud, and overexpression of YPT11 resulted in mitochondrial accumulation in the bud, indicating that Ypt11p acts positively on mitochondrial distribution toward the bud. We isolated two myo2 mutants, myo2-338 and myo2-573, which showed genetic interactions with YPT11. The myo2-573 mutation, identified by a synthetic lethal interaction with ypt11-null, induced a defect in mitochondrial distribution toward the bud, indicating that Myo2p plays a crucial role in polarized distribution of mitochondria. The myo2-338 mutation was identified as the mutation that abolished the effect of overexpressed YPT11, such as the Ypt11p-dependent accumulation of mitochondria in the bud, and the affinity of Myo2p for Ypt11p was reduced. These results indicate that complex formation of Ypt11p with Myo2p accelerates the function of Myo2p for mitochondrial distribution toward the bud.

Cortical granule translocation is microfilament mediated and linked to meiotic maturation in the sea urchin oocyte

Cortical granules exocytose after the fusion of egg and sperm in most animals, and their contents function in the block to polyspermy by creating an impenetrable extracellular matrix. Cortical granules are synthesized throughout oogenesis and translocate en masse to the cell surface during meiosis where they remain until fertilization. As the mature oocyte is approximately 125 μm in diameter (Lytechinus variegatus), many of the cortical granules translocate upwards of 60 μm to reach the cortex within a 4 hour time window. We have investigated the mechanism of this coordinated vesicular translocation event. Although the stimulus to reinitiate meiosis in sea urchin oocytes is not known, we found many different ways to reversibly inhibit germinal vesicle breakdown, and used these findings to discover that meiotic maturation and cortical granule translocation are inseparable. We also learned that cortical granule translocation requires association with microfilaments but not microtubules. It is clear from endocytosis assays that microfilament motors are functional prior to meiosis, even though cortical granules do not use them. However, just after GVBD, cortical granules attach to microfilaments and translocate to the cell surface. This latter conclusion is based on organelle stratification within the oocyte followed by positional quantitation of the cortical granules. We conclude from these studies that maturation promoting factor (MPF) activation stimulates vesicle association with microfilaments, and is a key regulatory step in the coordinated translocation of cortical granules to the egg cortex.

Identification of the novel proteins Yip4p and Yip5p as Rab GTPase interacting factors

The Rab GTPases are key regulators of membrane traffic. Yip1p is a membrane protein of unknown function that has been reported to interact with the Rabs Ypt1p and Ypt31p. In this study we identify Yif1p, and two unknown open reading frames, Ygl198p and Ygl161p, which we term Yip4p and Yip5p, as Yip1p-related sequences. We demonstrate that the Yip1p-related proteins possess several features: (i) they have a common overall domain topology, (ii) they are capable of biochemical interaction with a variety of Rab proteins in a manner dependent on C-terminal prenylation, and (iii) they share an ability to physically associate with other members of the YIP1 family.

Determinants of the broad recognition of exocytic Rab GTPases by Mss4

Rab GTPases function as essential regulators of vesicle transport between subcellular compartments of eukaryotic cells. Mss4, an evolutionarily conserved Rab accessory factor, facilitates nucleotide release and binds tightly to the nucleotide-free form of exocytic but not endocytic Rab GTPases. A structure-based mutational analysis of residues that are conserved only in exocytic Rab GTPases reveals three residues that are critical determinants of the broad specificity recognition of exocytic Rab GTPases by Mss4. One of these residues is located at the N-terminus of the switch I region near the nucleotide binding site whereas the other two flank an exposed hydrophobic triad previously implicated in effector recognition. The spatial disposition of these residues with respect to the structure of Rab3A correlates with the dimensions of the elongated Rab interaction epitope in Mss4 and supports a mode of interaction similar to that of other exchange factor-GTPase complexes. The complementarity of the corresponding interaction surfaces suggests a hypothetical structural model for the complex between Mss4 and Rab GTPases.

A novel plant kinesin-related protein specifically associates with the phragmoplast organelles

In higher plants, the formation of the cell plate during cytokinesis requires coordinated microtubule (MT) reorganization and vesicle transport in the phragmoplast. MT-based kinesin motors are important players in both processes. To understand the mechanisms underlying plant cytokinesis, we have identified AtPAKRP2 (for Arabidopsis thaliana phragmoplast-associated kinesin-related protein 2). AtPAKRP2 is an ungrouped N-terminal motor kinesin. It first appeared in a punctate pattern among interzonal MTs during late anaphase. When the phragmoplast MT array appeared in a mirror pair, AtPAKRP2 became more concentrated near the division site, and additional signal could be detected elsewhere in the phragmoplast. In contrast, the previously identified AtPAKRP1 protein is associated specifically with bundles of MTs in the phragmoplast at or near their plus ends. Localization of the tobacco homolog(s) of AtPAKRP2 was altered by treatment of brefeldin A in BY-2 cells. We discuss the possibility that AtPAKRP1 plays a role in establishing and/or maintaining the phragmoplast MT array, and AtPAKRP2 may contribute to the transport of Golgi-derived vesicles in the phragmoplast.

A novel plant kinesin-related protein specifically associates with the phragmoplast organelles

In higher plants, the formation of the cell plate during cytokinesis requires coordinated microtubule (MT) reorganization and vesicle transport in the phragmoplast. MT-based kinesin motors are important players in both processes. To understand the mechanisms underlying plant cytokinesis, we have identified AtPAKRP2 (for Arabidopsis thaliana phragmoplast-associated kinesin-related protein 2). AtPAKRP2 is an ungrouped N-terminal motor kinesin. It first appeared in a punctate pattern among interzonal MTs during late anaphase. When the phragmoplast MT array appeared in a mirror pair, AtPAKRP2 became more concentrated near the division site, and additional signal could be detected elsewhere in the phragmoplast. In contrast, the previously identified AtPAKRP1 protein is associated specifically with bundles of MTs in the phragmoplast at or near their plus ends. Localization of the tobacco homolog(s) of AtPAKRP2 was altered by treatment of brefeldin A in BY-2 cells. We discuss the possibility that AtPAKRP1 plays a role in establishing and/or maintaining the phragmoplast MT array, and AtPAKRP2 may contribute to the transport of Golgi-derived vesicles in the phragmoplast.

Ypt/rab gtpases: regulators of protein trafficking

Ypt/Rab guanosine triphosphatases (GTPases) have emerged in the last decade as key regulators of protein transport in all eukaryotic cells. They seem to be involved in all aspects of vesicle trafficking: vesicle formation, motility, and docking, and membrane remodeling and fusion. The functions of Ypt/Rabs are themselves controlled by upstream regulators that stimulate both their nucleotide cycling and their cycling between membranes. Ypt/Rabs transmit signals to downstream effectors in a guanosine triphosphate (GTP)-dependent manner. The identity of upstream regulators and downstream effectors is known for a number of Ypt/Rabs, and models for their mechanisms of action are emerging. In at least two cases, Ypt/Rab upstream regulators and downstream effectors are found together in a single complex. In agreement with the idea that Ypt/Rabs function in all aspects of vesicular transport, their diverse effectors have recently been shown to function in all identified aspects of vesicle transport. Activators and effectors for individual Ypt/Rabs share no similarity, but are conserved between yeast and mammalian cells. Finally, cross talk demonstrated among the various Ypt/Rabs, and between Ypt/Rabs and other signaling factors, suggests possible coordination among secretory steps, as well as between protein transport and other cellular processes.

Organization of the Rab-GDI/CHM superfamily: the functional basis for choroideremia disease

Choroideremia is an X-chromosome-linked disease that leads to the degeneration of the choriocapillaris, the retinal pigment epithelium and the photoreceptor layer in the eye. The gene product defective in choroideremia, CHM, is identical to Rab escort protein 1 (REP1). CHM/REP1 is an essential component of the catalytic geranylgeranyltransferase II complex (GGTrII) that delivers newly synthesized small GTPases belonging to the RAB gene family to the catalytic complex for post-translational modification. CHM/REP family members are evolutionarily related to members of the guanine nucleotide dissociation inhibitor (GDI) family, proteins involved in the recycling of Rab proteins required for vesicular membrane trafficking through the exocytic and endocytic pathways, forming the GDI/CHM superfamily. Biochemical and structural analyses have now revealed a striking parallel in the organization and function of these two families allowing us to generate a general model for GDI/CHM superfamily function in health and disease.

Ypt and Rab GTPases: insight into functions through novel interactions

Ypt/Rab GTPases are key regulators of vesicular transport in eukaryotic cells. During the past two years, a number of new Ypt/Rab-interacting proteins have been identified and shown to serve as either upstream regulators or downstream effectors. Proteins that interact with these regulators and effectors of Ypt/Rabs have also been identified, and together they provide new insights into Ypt/Rab mechanisms of action. The picture that emerges from these studies suggests that Ypt/Rabs function in multiple and diverse aspects of vesicular transport. In addition, not only are Ypt/Rabs highly conserved, but their functions and interactions are as well. Interestingly, crosstalk among Ypt/Rabs and between Ypt/Rabs and other signaling factors, suggest the possibility of coordination of the individual vesicular transport steps and of the protein transport machinery with other cellular processes.

Identification of rab7 as a melanosome-associated protein involved in the intracellular transport of tyrosinase-related protein 1

The melanosome is a unique secretory granule of the melanocyte in which melanin pigments are synthesized by tyrosinase gene family glycoproteins. Melanogenesis is a highly regulated process because of its inherent toxicity. An understanding of the various regulatory mechanisms is important in delineating the pathophysiology involved in pigmentary disorders and melanoma. We have purified and analyzed the total melanosomal proteins from B16 mouse melanoma tumors in order to identify new proteins that may be involved in the control of the melanogenesis process. Melanosomal proteins were resolved by two-dimensional sodium dodecyl sulfate polyacrylamide gel electrophoresis, a predominant spot (27 kDa with isoelectric point 5.8-6.4) was excised and digested with cyanogen bromide, and the fragments were sequenced. Synthetic oligonucleotide primers were synthesized corresponding to the peptide sequences, and reverse transcriptase polymerase chain reaction amplification of total RNA from B16 cells was carried out. Sequencing of one of the polymerase-chain-reaction-mediated clones demonstrated 80%-97% sequence homology of 200 bp nucleotide with GTP-binding proteins at the 3'-untranslated region. GTP-binding assay on two-dimensional gels of melanosomal proteins showed the presence of several (five to six) small GTP-binding proteins, suggesting that small GTP-binding proteins are associated with the melanosome. Among the known GTP-binding proteins with similar molecular weight and isoelectric point ranges, rab3, rab7, and rab8 were found to be present in the melanosomal fraction by immunoblotting. Confocal immunofluorescence microscopy showed that rab7 is colocalized with the tyrosinase-related protein 1 around the perinuclear area as well as, in part, in the perikaryon, thereby suggesting that rab7 might be involved in the intracellular transport of tyrosinase-related protein 1. Tyrosinase-related protein 1 transport was blocked by the treatment of B16 cells with antisense oligonucleotide to rab7. We suggest (i) that rab7 is a melanosome-associated molecule, (ii) that tyrosinase-related protein 1 is present in late-endosome delineated granules, and (iii) that rab7 is involved in the transport of tyrosinase-related protein 1 from the late-endosome delineated granule to the melanosome.

Prenylation of Rab GTPases: molecular mechanisms and involvement in genetic disease

Small GTPases of the Rab family regulate membrane transport pathways. More than 50 mammalian Rab proteins are known, many with transport step-specific localisation. Rabs must associate with cellular membranes for activity and membrane attachment is mediated by prenyl (geranylgeranyl) post-translational modification. Mutations in genes encoding proteins essential for the geranylgeranylation reaction, Rab escort protein and Rab geranylgeranyl transferase, underlie genetic diseases. Choroideremia patients have loss of function mutations in REP1 and the murine Hermansky-Pudlak syndrome model gunmetal possesses a splice-site mutation in the alpha-subunit of RGGT. Here we discuss recent insights into Rab prenylation and advances towards our understanding of both diseases.

The Rab GTPase family

SUMMARY

The Rab family is part of the Ras superfamily of small GTPases. There are at least 60 Rab genes in the human genome, and a number of Rab GTPases are conserved from yeast to humans. The different Rab GTPases are localized to the cytosolic face of specific intracellular membranes, where they function as regulators of distinct steps in membrane traffic pathways. In the GTP-bound form, the Rab GTPases recruit specific sets of effector proteins onto membranes. Through their effectors, Rab GTPases regulate vesicle formation, actin- and tubulin-dependent vesicle movement, and membrane fusion.