  Synaptotagmin (syt) 7 is one of three syt isoforms found in all metazoans; it
is ubiquitously expressed, yet its function in neurons remains obscure. Here,
we resolved Ca2+-dependent and Ca2+-independent synaptic vesicle (SV)
replenishment pathways, and found that syt 7 plays a selective and critical
role in the Ca2+-dependent pathway. Mutations that disrupt Ca2+-binding to syt
7 abolish this function, suggesting that syt 7 functions as a Ca2+-sensor for
replenishment. The Ca2+-binding protein calmodulin (CaM) has also been
implicated in SV replenishment, and we found that loss of syt 7 was phenocopied
by a CaM antagonist. Moreover, we discovered that syt 7 binds to CaM in a
highly specific and Ca2+-dependent manner; this interaction requires intact
Ca2+-binding sites within syt 7. Together, these data indicate that a complex
of two conserved Ca2+-binding proteins, syt 7 and CaM, serve as a key regulator
of SV replenishment in presynaptic nerve terminals.
