Cab45b, a Munc18b-interacting partner, regulates exocytosis in pancreatic beta-cells.
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.
Zhang
Yi
Y
Departments of Medicine and Physiology, University of Toronto, Toronto, Ontario M5S 1A8, Canada.
Kang
You-Hou
YH
Chang
Nathan
N
Lam
Patrick P L
PP
Liu
Yunfeng
Y
Olkkonen
Vesa M
VM
Gaisano
Herbert Y
HY
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
D000906:Antibodies; D002135:Calcium-Binding Proteins; D051938:Munc18 Proteins; D050505:Mutant Proteins; D050765:Qa-SNARE Proteins; D034741:RNA, Small Interfering