Abstract
Synaptotagmin (Syt) is a calcium sensor to regulate fusion of secretory vesicle with the plasma membrane (PM) in exocytsis. Syt proteins contain a single transmembrane domain in N-terminus and two tandem calcium-binding domains (C2A and C2B) in C-terminus. Here, we show that the localization of an Arabidopsis synaptotagmin homolog, SYT1, to the PM is modulated by tandem C2A-C2B domains. A biochemical analysis suggested that SYT1 is integrated to the plasma membrane, and the two tandem C2A-C2B domains are oriented to cytosol. We transiently expressed a series of truncated proteins in protoplasts, and determined that only those proteins containing tandem C2A-C2B domains were localized to the PM. The PM localization of SYT1 was greatly reduced following mutation of the calcium-binding motifs of the C2B domain, based on sequence comparisons with other homologs (SYT2, SYT4, SYT5). The localization of SYT1 to the PM may have been required for the functional divergence that occurred in the molecular evolution of plant synaptotagmins.
