Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 102, Issue 14, Pages 5210-5214Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.0500941102
Keywords
Ca2+ sensor; kinetics; liposome; disassembly; isoform
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Funding
- NIGMS NIH HHS [GM 56827, R01 GM056827] Funding Source: Medline
- NIMH NIH HHS [MH61876, R01 MH061876] Funding Source: Medline
- PHS HHS [13742] Funding Source: Medline
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Synaptotagmins (syts) are a family of membrane proteins present on a variety of intracellular organelles. In vertebrates, 16 isoforms of syt have been identified. The most abundant isoform, syt 1, appears to function as a Ca2+ sensor that triggers the rapid exocytosis of synaptic vesicles from neurons. The functions of the remaining syt isoforms are less well understood. The cytoplasmic domain of syt I binds membranes in response to Ca2+, and this interaction has been proposed to play a key role in secretion. Here, we tested the Ca2+-triggered membrane-binding activity of the cytoplasmic domains of syts I-XII; eight isoforms tightly bound to liposomes that contained phosphatidylserine as. a function of the concentration of Ca2+. We then compared the disassembly kinetics of Ca2+-syt-membrane complexes upon rapid mixing with excess Ca2+ chelator and found that syts can be classified into three distinct kinetic groups. syts I, II, and III constitute the fast group; syts V, VI, IX, and X make up the medium group; and syt VII exhibits the slowest kinetics of disassembly. Thus, isoforms of syt, which have much slower disassembly kinetics than does syt 1, might function as Ca2+ sensors for asynchronous release, which occurs after Ca2+ domains have collapsed. We also compared the temperature dependence of Ca2+-syt-membrane assembly and disassembly reactions by using squid and rat syt I. These results indicate that syts have diverged to release Ca2+ and membranes with distinct kinetics.
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