title: Cab45b, a Munc18b-interacting partner, regulates exocytosis in pancreatic beta-cells.
abstract: Cab45b is a cytosolic Ca(2+)-binding protein reported to regulate zymogen secretion in pancreatic acini. We now show that Cab45b is also expressed in pancreatic islet beta-cells and interacts there with the Sec1-Munc18 protein Munc18b. We employed patch clamp cell capacitance measurements to show that antibodies against Cab45b inhibited depolarization-evoked membrane capacitance increments, suggesting an impact on beta-cell granule exocytosis, both the readily releasable granule pool and refilling of this pool. Site-specific mutants in the Cab45b EF-hands were used to dissect the molecular interactions involved in Cab45b function. Mutants in EF-hands 2 and 3 had no detectable effects on interaction of Cab45b with Munc18b and did not affect the depolarization-evoked calcium currents, but remarkably, they facilitated the complex formation of Munc18b with syntaxin-2 and -3. As a result, these two EF-hand mutants inhibited beta-cell membrane capacitance increments. This inhibition is mediated via Munc18b because Munc18b silencing with small interfering RNA abolished the effects of these two mutants. The results suggest a mechanism for Cab45b action that involves regulating the dynamic association of Munc18b with SNAREs to impact beta-cell granule exocytosis.
authors.affiliation: Departments of Medicine and Physiology, University of Toronto, Toronto, Ontario M5S 1A8, Canada.
authors.forename: Yi
authors.initials: Y
authors.lastname: Zhang
authors.forename: You-Hou
authors.initials: YH
authors.lastname: Kang
authors.forename: Nathan
authors.initials: N
authors.lastname: Chang
authors.forename: Patrick P L
authors.initials: PP
authors.lastname: Lam
authors.forename: Yunfeng
authors.initials: Y
authors.lastname: Liu
authors.forename: Vesa M
authors.initials: VM
authors.lastname: Olkkonen
authors.forename: Herbert Y
authors.initials: HY
authors.lastname: Gaisano
mesh_terms: D000818:Animals; D000906:Antibodies / Q000378:metabolism; D020013:Calcium Signaling; D002135:Calcium-Binding Proteins / Q000737:chemistry / Q000378:metabolism*; D003594:Cytoplasmic Granules / Q000378:metabolism; D005089:Exocytosis; D050416:Glucagon-Secreting Cells / Q000166:cytology / Q000378:metabolism; D050417:Insulin-Secreting Cells / Q000166:cytology* / Q000378:metabolism*; D008564:Membrane Potentials; D051938:Munc18 Proteins / Q000378:metabolism*; D050505:Mutant Proteins / Q000378:metabolism; D055099:Neuroendocrine Cells / Q000166:cytology / Q000378:metabolism; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D050765:Qa-SNARE Proteins / Q000378:metabolism; D034741:RNA, Small Interfering / Q000378:metabolism; D051381:Rats
chemical_list: D000906:Antibodies; D002135:Calcium-Binding Proteins; D051938:Munc18 Proteins; D050505:Mutant Proteins; D050765:Qa-SNARE Proteins; D034741:RNA, Small Interfering