Bendahmane, Mounir; Bohannon, Kevin P; Bradberry, Mazdak M M; Rao, Tejeshwar C; Schmidtke, Michael W; Abbineni, Prabhodh S; Chon, Nara L; Tran, Sherleen; Lin, Hai; Chapman, Edwin R; Knight, Jefferson D; Anantharam, Arun
description
In chromaffin cells, the kinetics of fusion pore expansion vary depending on which synaptotagmin isoform (Syt-1 or Syt-7) drives release. Our recent studies have shown that fusion pores of granules harboring Syt-1 expand more rapidly than those harboring Syt-7. Here we sought to define the structural specificity of synaptotagmin action at the fusion pore by manipulating the Ca-binding C2B module. We generated a chimeric Syt-1 in which its C2B Ca-binding loops had been exchanged for those of Syt-7. Fusion pores of granules harboring a Syt-1 C2B chimera with all three Ca-binding loops of Syt-7 (Syt-1:7C2B) exhibited slower rates of fusion pore expansion and neuropeptide cargo release relative to WT Syt-1. After fusion, this chimera also dispersed more slowly from fusion sites than WT protein. We speculate that the Syt-1:7 C2B and WT Syt-1 are likely to differ in their interactions with Ca and membranes. Subsequent in vitro and in silico data demonstrated that the chimera exhibits a higher affinity for phospholipids than WT Syt-1. We conclude that the affinity of synaptotagmin for the plasma membrane, and the rate at which it releases the membrane, contribute in important ways to the rate of fusion pore expansion.
