Purification and properties of Rabphilin-3A

A low-affinity Ca2+-dependent association of calmodulin with the Rab3A effector domain inversely correlates with insulin exocytosis

The stimulus-response coupling pathway for glucose-regulated insulin secretion has implicated a rise in cytosolic [Ca2+]i as a key factor to induce insulin exocytosis. However, it is unclear how elevated [Ca2+]i communicates with the pancreatic beta-cell's exocytotic apparatus. As Rab3A is a model protein involved in regulated exocytosis, we have focused on its role in regulating insulin exocytosis. By using a photoactivatable cross-linking synthetic peptide that mimics the effector domain of Rab3A and microsequence analysis, we found calmodulin to be a major Rab3A target effector protein in pancreatic beta-cells. Coimmunoprecipitation analysis from pancreatic islets confirmed a Rab3A-calmodulin interaction in vivo, and that it inversely correlated with insulin exocytosis. Calmodulin affected neither GTPase nor guanine nucleotide exchange activity of Rab3A. The calmodulin-Rab3A interaction was pH- and Ca2+-dependent, and it was preferential for GTP-bound Rab3A. However, Rab3A affinity for calmodulin was relatively low (Kd = 18-22 micromol/l at 10(-5) mol/l [Ca2+]) and competed by other calmodulin-binding proteins that had higher affinity (e.g., Ca2+/calmodulin-dependent protein kinase-2 [CaMK-2] [Kd = 300-400 nmol/l at 10(-5) mol/l [Ca2+]]). Moreover, the Ca2+ dependence of the calmodulin-Rab3A interaction (K0.5 = 15-18 micromol/l [Ca2+], maximal at 100 micromol/l [Ca2+]) was significantly lower compared with that of the calmodulin-CaMK-2 association (K0.5 = 40 micromol/l [Ca2+], maximal at 1 mmol/l [Ca2+]). The data suggested that a transient Rab3A-calmodulin interaction might represent a means of directing calmodulin to the cytoplasmic face of a beta-granule, where it can be subsequently transferred for activation of other beta-granule-associated calmodulin-binding proteins as local [Ca2+]i rises to promote insulin exocytosis.

GRAB: a physiologic guanine nucleotide exchange factor for Rab3A, which interacts with inositol hexakisphosphate kinase

Diphosphoinositol-pentakisphosphate (InsP7) and bis-diphosphoinositol tetrakisphosphate (InsP8) possess pyrophosphate bonds. InsP7 is formed from inositol hexakisphosphate (InsP6) by recently identified InsP6 kinases designated InsP6K1 and InsP6K2. We now report the identification, cloning, and characterization of a novel protein, GRAB (guanine nucleotide exchange factor for Rab3A), which interacts with both InsP6K1 and Rab3A, a Ras-like GTPase that regulates synaptic vesicle exocytosis. GRAB is a physiologic GEF (guanine nucleotide exchange factor) for Rab3A. Consistent with a role of Rab3A in synaptic vesicle exocytosis, GRAB regulates depolarization-induced release of dopamine from PC12 cells and nicotinic agonist-induced hGH release from bovine adrenal chromaffin cells. The association of InsP6K1 with GRAB fits with a role for InsP7 in vesicle exocytosis.

Structural basis of activation and GTP hydrolysis in Rab proteins

BACKGROUND

Rab proteins comprise a large family of GTPases that regulate vesicle trafficking. Despite conservation of critical residues involved in nucleotide binding and hydrolysis, Rab proteins exhibit low sequence identity with other GTPases, and the structural basis for Rab function remains poorly characterized.

RESULTS

The 2. 0 A crystal structure of GppNHp-bound Rab3A reveals the structural determinants that stabilize the active conformation and regulate GTPase activity. The active conformation is stabilized by extensive hydrophobic contacts between the switch I and switch II regions. Serine residues in the phosphate-binding loop (P loop) and switch I region mediate unexpected interactions with the gamma phosphate of GTP that have not been observed in previous GTPase structures. Residues implicated in the interaction with effectors and regulatory factors map to a common face of the protein. The electrostatic potential at the surface of Rab3A indicates a non-uniform distribution of charged and nonpolar residues.

CONCLUSIONS

The major structural determinants of the active conformation involve residues that are conserved throughout the Rab family, indicating a common mode of activation. Novel interactions with the gamma phosphate impose stereochemical constraints on the mechanism of GTP hydrolysis and provide a structural explanation for the large variation of GTPase activity within the Rab family. An asymmetric distribution of charged and nonpolar residues suggests a plausible orientation with respect to vesicle membranes, positioning predominantly hydrophobic surfaces for interaction with membrane-associated effectors and regulatory factors. Thus, the structure of Rab3A establishes a framework for understanding the molecular mechanisms underlying the function of Rab GTPases.

Rabphilin-3A: a multifunctional regulator of synaptic vesicle traffic

We have investigated the function of the synaptic vesicle protein Rabphilin-3A in neurotransmitter release at the squid giant synapse. Presynaptic microinjection of recombinant Rabphilin-3A reversibly inhibited the exocytotic release of neurotransmitter. Injection of fragments of Rabphilin-3A indicate that at least two distinct regions of the protein inhibit neurotransmitter release: the NH2-terminal region that binds Rab3A and is phosphorylated by protein kinases and the two C2 domains that interact with calcium, phospholipid, and beta-adducin. Each of the inhibitory fragments and the full-length protein had separate effects on presynaptic morphology, suggesting that individual domains were inhibiting a subset of the reactions in which the full-length protein participates. In addition to inhibiting exocytosis, constructs containing the NH2 terminus of Rabphilin-3A also perturbed the endocytotic pathway, as indicated by changes in the membrane areas of endosomes, coated vesicles, and the plasma membrane. These results indicate that Rabphilin-3A regulates synaptic vesicle traffic and appears to do so at distinct stages of both the exocytotic and endocytotic pathways.

Involvement of Rabphilin3 in endocytosis through interaction with Rabaptin5

Rabphilin3 and rabaptin5 are downstream target molecules of the Rab3 and -5 subfamily small G proteins that are implicated in exocytosis and endocytosis, respectively. We examined here the physical and functional relationship between the Rab3-rabphilin3 and Rab5-rabaptin5 systems. Rabphilin3 interacted with rabaptin5 at the N-terminal region (amino acids 1-280), which GTP-Rab3A interacted with. The interaction of rabphilin3 with rabaptin5 was inhibited by guanosine 5'-(3-O-thio)triphosphate-Rab3A. Overexpression of the N-terminal fragment of rabphilin3 (amino acids 1-280) inhibited the receptor-mediated endocytosis of transferrin, and this inhibition was overcome by co-transfection with a dominant active mutant of Rab3A or rabaptin5 in PC12 and HeLa cells. These results suggest that rabphilin3, free of GTP-Rab3A, regulates endocytosis through interaction with rabaptin5 after rabphilin3 complexed with GTP-Rab3A regulates exocytosis.

Physical and functional interaction of rabphilin-3A with alpha-actinin

Rabphilin-3A is a downstream target molecule of Rab3A small GTP-binding protein and implicated in Ca2+-dependent neurotransmitter release. Here we have isolated a rabphilin-3A-interacting molecule from a human brain cDNA library by the yeast two-hybrid method and identified it to be alpha-actinin, known to cross-link actin filaments into a bundle. alpha-Actinin interacts with the N-terminal region of rabphilin-3A, with which GTP-Rab3A interacts, and this interaction stimulates the activity of alpha-actinin to cross-link actin filaments into a bundle. The interaction of rabphilin-3A with alpha-actinin is inhibited by guanosine 5'-(3-O-thio)triphosphate-Rab3A. These results suggest that the Rab3A-rabphilin-3A system regulates the alpha-actinin-regulated reorganization of actin filaments. It has been shown that reorganization of actin filaments is also involved in Ca2+-dependent exocytosis. Therefore, rabphilin-3A may serve as a linker for Rab3A and cytoskeleton.

Involvement of Rabphilin-3A in cortical granule exocytosis in mouse eggs

Rabphilin-3A is a putative target protein for Rab3A, a member of the small GTP-binding protein superfamily that has been suggested to play a role in regulated exocytosis in presynapses. In this study we determined the expression and the function of Rabphilin-3A in mouse eggs at fertilization. Rabphilin-3A mRNA and protein were detected by reverse transcriptase-PCR and immunoblot analysis, respectively, in metaphase II mouse eggs. Immunofluorescence analysis showed that Rabphilin-3A protein was distributed in the cortical region in eggs. Sperm induces cortical granule (CG) exocytosis via an increase in cytosolic Ca2+ at fertilization. We microinjected the NH2- or COOH-terminal fragment of recombinant Rabphilin-3A into metaphase II eggs. Neither treatments altered the sperm-induced cytosolic Ca2+ increase, but both inhibited CG exocytosis in a dose-dependent manner. The NH2-terminal fragment was more effective than the COOH-terminal fragment. Full-length Rabphilin-3A did not affect CG exocytosis, but it attenuated the inhibition of CG exocytosis by the NH2-terminal fragment. These results show that Rabphilin-3A is involved in Ca(2+)-dependent CG exocytosis at fertilization in mouse eggs.