Three distinct kinetic groupings of the synaptotagmin family: candidate sensors for rapid and delayed exocytosis.
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 I, 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 Ca(2+)-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 I, 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.
Hui
Enfu
E
Department of Physiology, University of Wisconsin, Madison, WI 53706, USA.
Bai
Jihong
J
Wang
Ping
P
Sugimori
Mutsuyuki
M
Llinas
Rodolfo R
RR
Chapman
Edwin R
ER
D000818:Animals; D002118:Calcium / Q000378:metabolism; D002135:Calcium-Binding Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D005089:Exocytosis / Q000502:physiology*; D066298:In Vitro Techniques; D007700:Kinetics; D008081:Liposomes; D008562:Membrane Glycoproteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D008954:Models, Biological; D008958:Models, Molecular; D009419:Nerve Tissue Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D009474:Neurons / Q000378:metabolism; D020033:Protein Isoforms / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017434:Protein Structure, Tertiary; D051381:Rats; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D013572:Synaptic Vesicles / Q000378:metabolism; D050857:Synaptotagmins
D002135:Calcium-Binding Proteins; D008081:Liposomes; D008562:Membrane Glycoproteins; D009419:Nerve Tissue Proteins; D020033:Protein Isoforms; D011994:Recombinant Proteins; D050857:Synaptotagmins; D002118:Calcium