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.     

Ultrastructural localization of dipeptidyl peptidase IV in rat salivary glands by immunocytochemistry

Summary The ultrastructural localization of dipeptidyl peptidase IV (DPP IV) (EC 3.4.14.5) in rat submandibular and parotid glands was studied immunocytochemically by the peroxidase-antiperoxidase (PAP) method, using a monospecific antiserum against rat kidney DPP IV. There were no differences in the immunocytochemical localization of DPP IV between submandibular and parotid glands. In these glands, DPP IV was primarily found to be associated with the luminal and intercellular canalicular plasma membranes of acinar cells and with the luminal plasma membranes of intercalated and striated duct cells. Occasionally, immunoreaction of DPP IV was detected in cytoplasmic vesicles (vacuoles), lysosomes, and multivesicular bodies in some acinar cells as well as in ductal epithelial cells. Furthermore, the reaction product was also found within the lumina of peri-acinar and peri-ductal capillaries and in the cytoplasm of some fibroblasts in the interstitial connective tissue. These data suggest that DPP IV in the submandibular and parotid glands may play some role in the secretion or reabsorption processes of secretory proteins and peptides in these glands.     

Exocytosis in human salivary glands visualized by high-resolution scanning electron microscopy

The luminal membrane of salivary acinar cells creates a specialized cell surface area that accepts exocytosis and undergoes dynamic changes during secretion. These changes were visualized three-dimensionally from both the inside and outside of the cell in human parotid and submandibular glands, by application of in vitro secretory stimulation and then of OsO₄ maceration to remove cytoplasmic organelles by varying degrees. In control glands treated without secretagogues, the luminal surface of serous acinar cells bore well-developed microvilli with only an occasional incidence of exocytotic profiles. Following treatment with the β-adrenergic agonist, isoproterenol, considerable shortening and loss of microvilli occurred along the luminal membrane where, on its cytoplasmic side, many protuberances of sizes similar to or smaller than those of single secretory granules (～1 μm in diameter) appeared. The cytoplasmic surface of these protuberances exhibited small vesicles (～100–150 nm in diameter) that, by transmission electron microscopy, were shown to be coated pits or vesicles present on or around the exocytosed granule membranes. Treatment of tissues with the muscarinic agonist carbachol also caused a decrease of microvilli and the appearance of protrusions at the luminal membrane. However, unlike isoproterenol treatment, many of these protrusions were devoid of small pits or vesicles and were much larger than a single secretory granule. These results indicate that (1) secretory stimulation causes the dynamic transformation of microvilli at the luminal membrane, where granule docking and membrane fusion take place, and (2) after fusion, the exocytosed membranes are processed differently, by coated pit/vesicle mediated or non-mediated mechanisms, according to the autonomic receptor control.     

Immunofluorescence-microscopic demonstration of myosin and actin in salivary glands and exocrine pancreas of the rat

Summary Actin and myosin were localized in various salivary glands (parotid, submandibular, sublingual, lingual and Harderian gland) and the exocrine pancreas of rats by indirect immunofluorescence microscopy using specific rabbit antibodies against chicken gizzard myosin and actin. A bright immunofluorescent staining with both antibodies was observed at three main sites: (1) In myoepithelial cells of all salivary glands, (2) in secretory gland cells underneath the cell membrane bordering the acinar lumen (except Harderian and mucous lingual gland), and (3) in epithelial cells of the various secretory ducts (of all glands) in similar distribution as in acinar cells. The present immunohistochemical findings in acinar cells could lend further support to a concept suggesting that myosin and actin are involved in the process of transport and exocytosis of secretory granules.Supported by grants form Deutsche Forschungsgemeinschaft (Dr. 91/1, Ste. 105/19 and U. 34/4). We thank Mrs. Ursula König, Mrs. Christine Mahlmeister and Miss Renate Steffens for excellent technical assistance.     

Localization and function of soluble N-ethylmaleimide-sensitive factor attachment protein-25 and vesicle-associated membrane protein-2 in functioning gastric parietal cells

The soluble N-ethylmaleimide-sensitive factor attachment protein of 25 kDa (SNAP-25) plays an important role in vesicle trafficking. Together with vesicle-associated membrane protein-2 (VAMP-2) and syntaxin, SNAP-25 forms a ternary complex implicated in docking and fusion of secretory vesicles with the plasma membrane during exocytosis. These so-called SNARE proteins are believed to regulate tubulovesicle trafficking and fusion during the secretory cycle of the gastric parietal cell. Here we examined the cellular localization and functional importance of SNAP-25 in parietal cell cultures. Adenoviral constructs were used to express SNAP-25 tagged with cyan fluorescent protein, VAMP-2 tagged with yellow fluorescent protein, and SNAP-25 in which the C-terminal 25 amino acids were deleted (SNAP-25 Delta181-206). Membrane fractionation experiments and fluorescent imaging showed that SNAP-25 is localized to the apical plasma membrane. The expression of the mutant SNAP-25 Delta181-226 inhibited the acid secretory response of parietal cells. Also, SNAP Delta181-226 bound poorly in vitro with recombinant syntaxin-1 compared with wild type SNAP-25, indicating that pairing between syntaxin-1 and SNAP-25 is required for parietal cell activation. Dual expression of SNAP-25 tagged with cyan fluorescent protein and VAMP-2 tagged with yellow fluorescent protein revealed a dynamic change in distribution associated with acid secretion. In resting cells, SNAP-25 is at the apical plasma membrane and VAMP-2 is associated with cytoplasmic H,K-ATPase-rich tubulovesicles. After stimulation, the two proteins co-localize on the apical plasma membrane. These data demonstrate the functional significance of SNAP-25 as a SNARE protein in the parietal cell and show the dynamic stimulation-associated redistribution of VAMP-2 from H,K-ATPase-rich tubulovesicles to co-localize with SNAP-25 on the apical plasma membrane.     

Septin Dynamics Are Essential for Exocytosis

Septins are a family of 14 cytoskeletal proteins that dynamically form hetero-oligomers and organize membrane microdomains for protein complexes. The previously reported interactions with SNARE proteins suggested the involvement of septins in exocytosis. However, the contradictory results of up- or down-regulation of septin-5 in various cells and mouse models or septin-4 in mice suggested either an inhibitory or a stimulatory role for these septins in exocytosis. The involvement of the ubiquitously expressed septin-2 or general septin polymerization in exocytosis has not been explored to date. Here, by nano-LC with tandem MS and immunoblot analyses of the septin-2 interactome in mouse brain, we identified not only SNARE proteins but also Munc-18-1 (stabilizes assembled SNARE complexes), N-ethylmaleimide-sensitive factor (NSF) (disassembles SNARE complexes after each membrane fusion event), and the chaperones Hsc70 and synucleins (maintain functional conformation of SNARE proteins after complex disassembly). Importantly, α-soluble NSF attachment protein (SNAP), the adaptor protein that mediates NSF binding to the SNARE complex, did not interact with septin-2, indicating that septins undergo reorganization during each exocytosis cycle. Partial depletion of septin-2 by siRNA or impairment of septin dynamics by forchlorfenuron inhibited constitutive and stimulated exocytosis of secreted and transmembrane proteins in various cell types. Forchlorfenuron impaired the interaction between SNAP-25 and its chaperone Hsc70, decreasing SNAP-25 levels in cultured neuroendocrine cells, and inhibited both spontaneous and stimulated acetylcholine secretion in mouse motor neurons. The results demonstrate a stimulatory role of septin-2 and the dynamic reorganization of septin oligomers in exocytosis.     

Trafficking of green fluorescent protein-tagged SNARE proteins in HSY cells

SNARE proteins are widely accepted to be involved in the docking and fusion process of intracellular vesicle trafficking. VAMP-2, syntaxin-4, and SNAP-23 are plausible candidate SNARE proteins for non-neuronal exocytosis. Thus, we examined the localization, protein-protein interaction, and intracellular trafficking of these proteins by expressing them as green fluorescent protein (GFP)- and FLAG-tagged fusion proteins in various cells, including HSY cells, a human parotid epithelial cell line. GFP-VAMP-2 was ex-pressed strongly in the Golgi area and weakly on the plasma membrane. Although GFP-SNAP-23 seemed to be expressed universally in the cytosol, the GFP signal was clearly seen on the plasma membrane, when soluble GFP-SNAP-23 was removed by treatment with saponin. GFP-syntaxin-4 was undetectable on the plasma membrane but was strongly expressed on unidentified unusually large vesicles. GFP-syntaxin-4 without its transmembrane domain was still incompletely soluble and observed as aggregates. When syntaxin-4 and munc18c were coexpressed, syntaxin-4 was translocated at least in part to the plasma membrane. The protein-protein interaction between syntaxin-4 and VAMP-2 with their transmembrane domains was markedly inhibited on coexpression of munc18c. These results suggest that munc18c plays an important role in the trafficking of syntaxin-4 to its proper destination by preventing premature interactions with other proteins, including SNARE proteins.     

The number of secretory vesicles remains unchanged following exocytosis.

Earlier studies using electron microscopy demonstrate that there is no loss of secretory vesicles following exocytosis. Depletion however, of vesicular contents resulting in the formation of empty or partially empty vesicles is seen in electron micrographs, post exocytosis, in a variety of cells. Our studies using atomic force microscopy (AFM) reveal that following stimulation of secretion, live pancreatic acinar cells having 100-180 nm in diameter fusion pores located at the apical plasma membrane, dilate only 25-35% during exocytosis. Since secretory vesicles in pancreatic acinar cells range in size from 200 nm to 1200 nm in diameter, their total incorporation at the fusion pore, would distend the structure much more then what is observed. These earlier results prompted the current study to determine secretory vesicle dynamics in live pancreatic acinar cells following exocytosis. AFM studies on live acinar cells reveal no loss of secretory vesicle number following exocytosis. Parallel studies using electron microscopy, further confirmed our AFM results. These studies demonstrate that following stimulation of secretion, membrane-bound secretory vesicles transiently dock and fuse to release vesicular contents.     

Aquaporin-5 (AQP5), a water channel protein, in the rat salivary and lacrimal glands: immunolocalization and effect of secretory stimulation

Aquaporin-5 (AQP5) is a water channel protein and is considered to play an important role in water movement across the plasma membrane. We raised anti-AQP5 antibody and examined the localization of AQP5 protein in rat salivary and lacrimal glands by immunofluorescence microscopy. AQP5 was found in secretory acinar cells of submandibular, parotid, and sublingual glands, where it was restricted to apical membranes including intercellular secretory canaliculi. In the submandibular gland, abundant AQP5 was also found additionally at the apical membrane of intercalated duct cells. Upon stimulation by isoproterenol, apical staining for AQP5 in parotid acinar cells tended to appear as clusters of dots. These results suggest that AQP5 is one of the candidate molecules responsible for the water movement in the salivary glands.     

The Slp4-a linker domain controls exocytosis through interaction with Munc18-1.syntaxin-1a complex

Synaptotagmin-like protein 4-a (Slp4-a)/granuphilin-a is specifically localized on dense-core vesicles in certain neuroendocrine cells and negatively controls dense-core vesicle exocytosis through specific interaction with Rab27A. However, the precise molecular mechanism of its inhibitory effect on exocytosis has never been elucidated and is still a matter of controversy. Here we show by deletion and chimeric analyses that the linker domain of Slp4-a interacts with the Munc18-1.syntaxin-1a complex by directly binding to Munc18-1 and that this interaction promotes docking of dense-core vesicles to the plasma membrane in PC12 cells. Despite increasing the number of plasma membrane docked vesicles, expression of Slp4-a strongly inhibited high-KCl-induced dense-core vesicle exocytosis. The inhibitory effect by Slp4-a is absolutely dependent on the linker domain of Slp4-a, because substitution of the linker domain of Slp4-a by that of Slp5 (the closest isoform of Slp4-a that cannot bind the Munc18-1.syntaxin-1a complex) completely abrogated the inhibitory effect. Our findings reveal a novel docking machinery for dense-core vesicle exocytosis: Slp4-a simultaneously interacts with Rab27A and Munc18-1 on the dense-core vesicle and with syntaxin-1a in the plasma membrane.     

Roles of microfilaments in exocytosis: a new hypothesis

We observed the dynamic changes in the localization of microfilaments during the exocytic secretion of rat parotid and submandibular gland acinar cells, and obtained results which led us to propose a new concept of microfilament function in exocytosis. With the electron microscopy, NBD-Phallacidin (NBD-PL) fluorescence technique and immunohistochemistry for myosin, microfilaments consisting of F-actin and myosin were localized mainly underneath the luminal plasma membrane. Microfilaments were not detectable around the secretory granules which were stored in the cytoplasm, but were clearly observed around them whose membranes were continuous with the luminal plasma membrane. When viewed with NBD-PL and myosin fluorescence, the area of fused granule membranes revealed bright fluorescence in association with the luminal border, so that the luminal membrane undergoing exocytosis appeared like a 'bunch of grapes'. When excess exocytosis was stimulated by isoproterenol (IPR), the number of individual 'grapes' increased dramatically, indicating that the secretory granules are surrounded by microfilaments after the fusion with the luminal membrane. Microfilaments thus continuously undercoat the luminal membrane during exocytosis although the exocytic process involves the dilation and subsequent reduction of the luminal membrane due to the addition and removal of secretory granule membranes. This reduction of the dilated luminal membrane following exocytosis was, however, inhibited when the microfilaments were disrupted by cytochalasin D. Following this treatment, the lumina was expanded extraordinarily and the secretory products remained in the enlarged lumina, showing that the release of secretory products is inhibited when the microfilament function is disturbed. These results indicate that 1) microfilaments are localized mainly underneath the luminal plasma membrane and act as an obstacle to exocytosis when cells are at the resting phase and 2) at the secretory phase microfilaments allow exocytosis by disorganizing their barrier system and then, by encircling the discharged secretory granule membranes, provide forces for the extrusion of secretory products through the action of the acto-myosin contractile system.     

Secretion of saliva in X-irradiated rat submandibular glands

The mechanism of radiation-induced dysfunction in rat submandibular glands was investigated at the cellular level. After X irradiation (single dose, 15 Gy), a vacuolation in the acinar cells or an enlargement of the acinar lumen was observed as a typical morphological change for 2 weeks. As observed using a video-enhanced contrast differential interference contrast (VEC-DIC) microscope, exocytosis and shrinkage of the acinar cells induced by application of pilocarpine (100 microM) were markedly suppressed for 5 days and then recovered to 80% of the control levels. Using an immunohistochemical method, no significant change was observed in amylase distribution, but a marked loss of aquaporin 5 was found in the acinar cells after the irradiation. The extent and time course of pilocarpine-induced mobilization of intracellular Ca(2+) did not change after the irradiation. We conclude that radiation-induced dysfunction in the salivary glands is due to an impairment of exocytosis and a reduction of water secretion. The loss of aquaporin 5 and possibly other membrane-fusion proteins in acinar cells may be the major mechanism underlying such a dysfunction.     

Syntaxin isoform specificity in the regulation of renal H+-ATPase exocytosis

Intercalated and inner medullary collecting duct (IMCD) cells of the kidney mediate the transport of H+ by a plasma membrane H+-ATPase. The rate of H+ transport in these cells is regulated by exocytic insertion of H+-ATPase-laden vesicles into the apical membrane. We have shown that the exocytic insertion of proton pumps (H+-ATPase) into the apical membrane of rat IMCD cells, in culture, involves SNARE proteins (syntaxin (synt), SNAP-23, and VAMP). The membrane fusion complex observed in IMCD cells with the induction of proton pump exocytosis not only included these SNAREs but also the H+-ATPase. Based on these observations, we suggested that the targeting of these vesicles to the apical membrane is mediated by an interaction between the H+-ATPase and a specific t-SNARE. To evaluate this hypothesis, we utilized a "pull-down" assay in which we identified, by Western analysis, the proteins in a rat kidney medullary homogenate that complexed with glutathione S-transferase (GST) fusion syntaxin isoforms attached to Sepharose 4B-glutathione beads. The syntaxin isoforms employed were 1A, 1B, 2, 4, 5, and also 1A that was truncated to exclude the H3 SNARE binding domain (synt-1ADeltaH3). All full-length syntaxin isoforms formed complexes with SNAP-23 and VAMP. Neither GST nor synt-1ADeltaH3 formed complexes with these SNAREs. H+-ATPase (subunits E, a, and c) bound to syntaxin-1A and to a lesser extent to synt-1B but not to synt-1ADeltaH3 or synt-2, -4, and -5. In cultured IMCD cells transfected to express syntaxin truncated for the membrane binding domain (synt-DeltaC), expression of synt-1ADeltaC, but not synt-4DeltaC, inhibited H+-ATPase exocytosis. In conclusion, because all full-length syntaxins examined bound VAMP-2 and SNAP-23, but only non-H3-truncated syntaxin-1 bound H+-ATPase, and synt-1ADeltaC expression by intact IMCD cells inhibited H+-ATPase exocytosis, it is likely that the H+-ATPase binds directly to the H3 domain of syntaxin-1 and not through VAMP-2 or SNAP-23. Interaction between the syntaxin-1A and H+-ATPase is important in the targeted exocytosis of the proton pump to the apical membrane of intercalated cells.     

Novel Interactions of CAPS (Ca2+-dependent Activator Protein for Secretion) with the Three Neuronal SNARE Proteins Required for Vesicle Fusion

CAPS (aka CADPS) is required for optimal vesicle exocytosis in neurons and endocrine cells where it functions to prime the exocytic machinery for Ca²⁺-triggered fusion. Fusion is mediated by trans complexes of the SNARE proteins VAMP-2, syntaxin-1, and SNAP-25 that bridge vesicle and plasma membrane. CAPS promotes SNARE complex formation on liposomes, but the SNARE binding properties of CAPS are unknown. The current work revealed that CAPS exhibits high affinity binding to syntaxin-1 and SNAP-25 and moderate affinity binding to VAMP-2. CAPS binding is specific for a subset of exocytic SNARE protein isoforms and requires membrane integration of the SNARE proteins. SNARE protein binding by CAPS is novel and mediated by interactions with the SNARE motifs in the three proteins. The C-terminal site for CAPS binding on syntaxin-1 does not overlap the Munc18-1 binding site and both proteins can co-reside on membrane-integrated syntaxin-1. As expected for a C-terminal binding site on syntaxin-1, CAPS stimulates SNARE-dependent liposome fusion with N-terminal truncated syntaxin-1 but exhibits impaired activity with C-terminal syntaxin-1 mutants. Overall the results suggest that SNARE complex formation promoted by CAPS may be mediated by direct interactions of CAPS with each of the three SNARE proteins required for vesicle exocytosis.     

Calcium transport in human salivary glands: a proposed model of calcium secretion into saliva

Salivary calcium plays a vital role in bio-mineralization of dental enamel and exposed dentin. In order to elucidate the yet unknown cellular and molecular mechanisms of calcium secretion in human salivary glands the presence of various relevant plasma membrane transport systems for calcium were investigated. Using an RT-PCR approach, expression of the epithelial calcium channel (CaT-Like), the calcium binding protein (calbindin-2), the endoplasmic reticulum pumps (SERCA-2 and -3), and the plasma membrane calcium ATPases (PMCA-1, -2, and -4), were found in parotid and submandibular glands. Immunohistochemistry revealed that CaT-Like is located in the basolateral plasma membrane of acinar cells; while calbindin-2, SERCA-2 and SERCA-3 were found inside the acinar cells; and PMCA-2 was found in the apical membrane and in the secretory canaliculi between the cells. Based on these findings, we propose the following model of calcium secretion in human salivary glands: (1) calcium enters the acinar cell at the basolateral side via calcium channel CaT-Like (calcium influx); (2) intracellular calcium is taken up into the endoplasmic reticulum by SERCA-2 and possibly SERCA3 or bound to calbindin-2 (intracellular calcium pool); and (3) calcium is secreted by PMCAs at the apical plasma membrane (calcium efflux).Evamaria Kinne-Saffran deceased on 6 December 2002     

Reinternalization of secretory proteins during membrane recycling in rat pancreatic acinar cells

Membrane recycling in pancreatic acinar cells involves endocytic vesicle formation at the apical cell surface and rapid membrane traffic to the Golgi complex. During this process a small amount of extracellular content is taken up from the acinar lumen. In order to determine whether secretory proteins already released into the pancreatic acinar lumen are reinternalized during membrane retrieval, 3H-labeled amylase or 125I-labeled secretory proteins were reinfused through the pancreatic duct until the lumina were reached. Tissue samples from various time points were prepared for light and electron microscope autoradiography. The observations showed that [3H]amylase and, to a lesser extent, the 125I-labeled secretory proteins were internalized at the apical cell surface and rapidly (within 2-5 min) transferred to the Golgi cisternae and the condensing vacuoles; only a minor proportion of silver grains was observed over lysosomes. In addition, at later time points, mature secretion granules close to the Golgi complex became labeled. The results indicate that exocytosis in the rat exocrine pancreas does not operate at 100% efficiency; part of the exported amylase and part of the total secretion product are reinternalized concomitantly with the endocytic removal of plasma membrane and are copackaged together with newly synthesized secretory proteins.     

Role of actin in regulated exocytosis and compensatory membrane retrieval: insights from an old acquaintance

This review summarizes new insights into the role of the actin cytoskeleton in exocytosis and compensatory membrane retrieval from mammalian regulated secretory cells. Data from our lab and others now indicate that the actin cytoskeleton is involved in exocytosis both as a negative regulator of membrane fusion under resting conditions and as a facilitator of movement of secretory granules to their site of fusion with the apical plasmalemma. Coating of docked secretory granules with actin filaments correlates with the dissociation of secretory-granule-associated rab3D, pointing out a novel role for rab proteins in modulating the actin cytoskeleton during regulated exocytosis. Compensatory membrane retrieval following regulated exocytosis is also critically dependent on the actin cytoskeleton both in initiating the formation of clathrin-coated retrieval vesicles and subsequent trafficking back into the cell. We propose that insertion of secretory granule membrane into the plasmalemma initiates a trigger for membrane retrieval, possibly by exposing sites where proteins involved in compensatory membrane retrieval are assembled. The results summarized in this review were derived primarily from investigations on the pancreatic acinar cell, an old friend who is providing modern wisdom not attainable in other simpler systems.     

Histochemical and biochemical determination of calcium in salivary glands of cat

Although feline salivary glands have been used in investigations on secretion and microlithiasis and both processes involve calcium, nothing is known about its distribution in these glands. Therefore we have demonstrated the presence of calcium by a histochemical technique using glyoxal bis(2-hydroxyanil) and a biochemical technique using dry ashing. The histochemical technique stained serous acinar cells weakly and rarely found mucous acinar cells strongly in the parotid gland, mucous acinar cells moderately to strongly and serous acinar cells weakly in the sublingual gland, and central and demilunar acinar cells moderately to strongly in the submandibular gland. The biochemical technique revealed less calcium in the parotid than in the submandibular and sublingual glands. Both techniques revealed a decrease of calcium in submandibular and sublingual glands following parasympathetic stimulation. The histochemical distribution of calcium, which corresponds to that of acinar secretory glycoprotein, and the loss of calcium following parasympathetic stimulation, which causes release of secretory granules, indicate the presence of calcium in secretory granules. The concentration of calcium in the different types of acinar cell corresponds to the acidity of the secretory glycoprotein and suggests that calcium is present as a cationic shield to allow the condensation of polyionic glycoprotein in secretory granules.     

The exocyst complex in exocytosis and cell migration

Exocytosis is a fundamental membrane trafficking event in eukaryotic cells in which membrane proteins or lipids are incorporated into the plasma membrane and vesicle contents are secreted to the exterior of the cell. The exocyst, an evolutionarily conserved octameric protein complex, plays a crucial role in the targeting of secretory vesicles to the plasma membrane during exocytosis. The exocyst has been shown to be involved in diverse cellular processes requiring polarized exocytosis such as yeast budding, epithelial polarity establishment, and neurite outgrowth. Recently, the exocyst has also been implicated in cell migration through mechanisms independent of its role in exocytosis. In this review, we will first summarize our knowledge on the exocyst complex at a molecular and structural level. Then, we will discuss the specific functions of the exocyst in exocytosis in various cell types. Finally, we will review the emerging roles of the exocyst during cell migration and tumor cell invasion.     

Large scale protein identification in intracellular aquaporin-2 vesicles from renal inner medullary collecting duct

Vasopressin acts on renal collecting duct cells to stimulate translocation of aquaporin-2 (AQP2)-containing membrane vesicles from throughout the cytoplasm to the apical region. The vesicles fuse with the plasma membrane to increase water permeability. To identify the intracellular membrane compartments that contain AQP2, we carried out LC-MS/MS-based proteomic analysis of immunoisolated AQP2-containing intracellular vesicles from rat inner medullary collecting duct. Immunogold electron microscopy and immunoblotting confirmed heavy AQP2 labeling of immunoisolated vesicles. Vesicle proteins were separated by SDS-PAGE followed by in-gel trypsin digestion in consecutive gel slices and identification by LC-MS/MS. Identification of Rab GTPases 4, 5, 18, and 21 (associated with early endosomes); Rab7 (late endosomes); and Rab11 and Rab25 (recycling endosomes) indicate that a substantial fraction of intracellular AQP2 is present in endosomal compartments. In addition, several endosome-associated SNARE proteins were identified including syntaxin-7, syntaxin-12, syntaxin-13, Vti1a, vesicle-associated membrane protein 2, and vesicle-associated membrane protein 3. Rab3 was not found, however, either by mass spectrometry or immunoblotting, suggesting a relative lack of AQP2 in secretory vesicles. Additionally, we identified markers of the trans-Golgi network, components of the exocyst complex, and several motor proteins including myosin 1C, non-muscle myosins IIA and IIB, myosin VI, and myosin IXB. Beyond this, identification of multiple endoplasmic reticulum-resident proteins and ribosomal proteins indicated that a substantial fraction of intracellular AQP2 is present in rough endoplasmic reticulum. These results show that AQP2-containing vesicles are heterogeneous and that intracellular AQP2 resides chiefly in endosomes, trans-Golgi network, and rough endoplasmic reticulum.