High metal concentrations are required for self-association of synaptotagmin II.
Several members of the synaptotagmin (syt) family of vesicle proteins have been proposed to act as Ca2+ sensors on synaptic vesicles. The mechanism by which calcium activates this class of proteins has been the subject of controversy, yet relatively few detailed biophysical studies have been reported on how isoforms other than syt I respond to divalent metal ions. Here, we report a series of studies on the response of syt II to a wide range of metal ions. Analytical ultracentrifugation studies demonstrate that Ca2+ induces protein dimerization upon exposure to 5 mM Ca2+. Whereas Ba2+, Mg2+, or Sr2+ do not potentiate self-association as strongly as Ca2+, Pb2+ triggers self-association of syt II at concentrations as low as 10 microM. Partial proteolysis studies suggest that the various divalent metals cause different changes in the conformation of the protein. The high calcium concentrations required for self-association of syt II suggest that the oligomerized state of this protein is not a critical intermediate in vesicle fusion; however, low-affinity calcium sites on syt II may play a critical role in buffering calcium at the presynaptic active zone. In addition, the high propensity of lead to oligomerize syt II offers a possible molecular explanation for how lead interferes with calcium-evoked neurotransmitter release.
García
Ricardo A
RA
Department of Biochemistry, Molecular Biology, and Cell Biology, Northwestern University, Evanston, Illinois 60208, USA.
Godwin
Hilary Arnold
HA
D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D002413:Cations, Divalent / Q000494:pharmacology*; D019281:Dimerization; D051379:Mice; D018625:Microscopy, Atomic Force; D008958:Models, Molecular; D009419:Nerve Tissue Proteins / Q000378:metabolism*; D011487:Protein Conformation / Q000187:drug effects; D011994:Recombinant Proteins / Q000378:metabolism*; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization; D013572:Synaptic Vesicles / Q000378:metabolism; D050861:Synaptotagmin II; D014461:Ultracentrifugation
D002413:Cations, Divalent; D009419:Nerve Tissue Proteins; D011994:Recombinant Proteins; D050861:Synaptotagmin II; C494263:Syt2 protein, mouse; D002118:Calcium