Synexin (Annexin VII) Hypothesis for Ca2+/GTP-Regulated Exocytosis

Abstract

The current fusion machine hypothesis envisions a core complex formed between plasma membrane syntaxin and SNAP-25 and the synaptic vesicle protein synaptobrevinNAMP (S), with vesicular synaptotagmin identified as a low-affinity calcium sensor that interacts with regulatory syntaxin 1. The site of GTP action in exocytosis has been thought to be closely associated with the site of calcium action at some common site. The affinity of this site for Ca2+ has been shown to be in the 50- to 200-μM range, as estimated from electrophysiological studies with caged calcium in chromaffin cells, neurons, and digitonin-permeabilized chromaffin cells. Therefore, a good experimental goal would seem to be to search for a embrane fusion protein that is activated by both calcium and GTP. Chromaffin and many other cell types contain a membrane fusion protein, synexin (annexin VII), associated with secretory vesicles and plasma membranes, which have an appropriate intrinsic Kd for Ca2+ of approximately 200 μM. In addition to binding of GTP and Ca2+, a further identifying feature of the hypothetical “fusion scaffold” protein has been proposed to be self-association. Indeed, it has been shown by light scattering and electron microscopy studies that the Ca2+-activated form of synexin has a polymeric structure. Consistently, the protein concentration dependence for many synexindependent reactions has consistently been sigmoidal, with Hill coefficients (nH). It is true that synexin alone is able to mediate efficiently both membrane contact and fusion reactions in vitro. However, the exocytosis process is likely to be much more complicated.