Role of the polybasic sequence in the Doc2α C2B domain in dense‐core vesicle exocytosis in PC12 cells

J. Neurochem . (2010) 114 , 171–181. Abstract The double C2 (Doc2) family is characterized by an N‐terminal Munc13‐1‐interacting domain and C‐terminal tandem C2 domains, and it comprises three isoforms, Doc2α, Doc2β, and Doc2γ, in humans and mice. Doc2α, the best‐characterized, brain‐specific isoform, exhibits Ca 2+ ‐dependent phospholipid‐binding activity through its C2A domain, and the Ca 2+ ‐binding activity is thought to be important for the regulation of Ca 2+ ‐dependent exocytosis. In contrast to the C2A domain, however, nothing is known about the physiological functions of the C2B domain in regulated exocytosis. In this study, we demonstrated by a mutation analysis that the polybasic sequence in the C2B domain of Doc2α (306 KKSKHKTCVKKK 317) is required for binding of syntaxin‐1a/synaptosome‐associated protein of 25 kDa (SNAP‐25) heterodimer. We also investigated the effect of Lys‐to‐Gln (named KQ) mutations in the polybasic sequence of the C2B domain on vesicle dynamics by total internal reflection fluorescence microscopy in PC12 cells. A Doc2α(KQ) mutant, which lacks binding activity toward syntaxin‐1a/SNAP‐25 heterodimer, significantly decreased the number of plasma membrane‐docked vesicles before stimulation and strongly inhibited high‐KCl‐induced exocytosis from the plasma membrane‐docked vesicles. These results indicate that the polybasic sequence in the C2B domain functions as a binding site for syntaxin‐1a/SNAP‐25 heterodimer and controls the number of ‘readily releasable’ vesicles in neuroendocrine cells.