Synaptotagmins form a hierarchy of exocytotic Ca(2+) sensors with distinct Ca(2+) affinities.
Synaptotagmins constitute a large family of membrane proteins implicated in Ca(2+)-triggered exocytosis. Structurally similar synaptotagmins are differentially localized either to secretory vesicles or to plasma membranes, suggesting distinct functions. Using measurements of the Ca(2+) affinities of synaptotagmin C2-domains in a complex with phospholipids, we now show that different synaptotagmins exhibit distinct Ca(2+) affinities, with plasma membrane synaptotagmins binding Ca(2+) with a 5- to 10-fold higher affinity than vesicular synaptotagmins. To test whether these differences in Ca(2+) affinities are functionally important, we examined the effects of synaptotagmin C2-domains on Ca(2+)-triggered exocytosis in permeabilized PC12 cells. A precise correlation was observed between the apparent Ca(2+) affinities of synaptotagmins in the presence of phospholipids and their action in PC12 cell exocytosis. This was extended to PC12 cell exocytosis triggered by Sr(2+), which was also selectively affected by high-affinity C2-domains of synaptotagmins. Together, our results suggest that Ca(2+) triggering of exocytosis involves tandem Ca(2+) sensors provided by distinct plasma membrane and vesicular synaptotagmins. According to this hypothesis, plasma membrane synaptotagmins represent high-affinity Ca(2+) sensors involved in slow Ca(2+)-dependent exocytosis, whereas vesicular synaptotagmins function as low-affinity Ca(2+) sensors specialized for fast Ca(2+)-dependent exocytosis.
Sugita
Shuzo
S
The Center for Basic Neuroscience, Department of Molecular Genetics, and Howard Hughes Medical Institute, UT Southwestern Medical Center, Dallas, TX 75390, USA.
Shin
Ok-Ho
OH
Han
Weiping
W
Lao
Ye
Y
Südhof
Thomas C
TC
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002135:Calcium-Binding Proteins / Q000378:metabolism*; D005089:Exocytosis; D008562:Membrane Glycoproteins / Q000378:metabolism*; D009419:Nerve Tissue Proteins / Q000378:metabolism*; D016716:PC12 Cells; D010743:Phospholipids / Q000378:metabolism; D011485:Protein Binding; D051381:Rats; D050857:Synaptotagmins
D002135:Calcium-Binding Proteins; D008562:Membrane Glycoproteins; D009419:Nerve Tissue Proteins; D010743:Phospholipids; D050857:Synaptotagmins; D002118:Calcium