Synaptotagmins form a hierarchy of exocytotic Ca2+sensors with distinct Ca2+affinities

Open Access
Publisher Website
Google Scholar
Abstract
Synaptotagmins constitute a large family of membrane proteins implicated in Ca2+‐triggered exocytosis. Structurally similar synaptotagmins are differentially localized either to secretory vesicles or to plasma membranes, suggesting distinct functions. Using measurements of the Ca2+ affinities of synaptotagmin C2‐domains in a complex with phospholipids, we now show that different synaptotagmins exhibit distinct Ca2+ affinities, with plasma membrane synaptotagmins binding Ca2+ with a 5‐ to 10‐fold higher affinity than vesicular synaptotagmins. To test whether these differences in Ca2+ affinities are functionally important, we examined the effects of synaptotagmin C2‐domains on Ca2+‐triggered exocytosis in permeabilized PC12 cells. A precise correlation was observed between the apparent Ca2+ affinities of synaptotagmins in the presence of phospholipids and their action in PC12 cell exocytosis. This was extended to PC12 cell exocytosis triggered by Sr2+, which was also selectively affected by high‐affinity C2‐domains of synaptotagmins. Together, our results suggest that Ca2+ triggering of exocytosis involves tandem Ca2+ sensors provided by distinct plasma membrane and vesicular synaptotagmins. According to this hypothesis, plasma membrane synaptotagmins represent high‐affinity Ca2+ sensors involved in slow Ca2+‐dependent exocytosis, whereas vesicular synaptotagmins function as low‐affinity Ca2+ sensors specialized for fast Ca2+‐dependent exocytosis.