Phosphatidylinositol phosphates as co-activators of Ca2+ binding to C2 domains of synaptotagmin 1.
Ca2+-dependent phospholipid binding to the C2A and C2B domains of synaptotagmin 1 is thought to trigger fast neurotransmitter release, but only Ca2+ binding to the C2B domain is essential for release. To investigate the underlying mechanism, we have compared the role of basic residues in Ca2+/phospholipid binding and in release. Mutations in a polybasic sequence on the side of the C2B domain beta-sandwich or in a basic residue in a top Ca2+-binding loop of the C2A domain (R233) cause comparable decreases in the apparent Ca2+ affinity of synaptotagmin 1 and the Ca2+ sensitivity of release, whereas mutation of the residue homologous to Arg233 in the C2B domain (Lys366) has no effect. Phosphatidylinositol polyphosphates co-activate Ca2+-dependent and -independent phospholipid binding to synaptotagmin 1, but the effects of these mutations on release only correlate with their effects on the Ca2+-dependent component. These results reveal clear distinctions in the Ca2+-dependent phospholipid binding modes of the synaptotagmin 1 C2 domains that may underlie their functional asymmetry and suggest that phosphatidylinositol polyphosphates may serve as physiological modulators of Ca2+ affinity of synaptotagmin 1 in vivo.
Li
LiYi
L
Department of Neuroscience, Baylor College of Medicine, Houston, Texas 77030, USA.
Shin
Ok-Ho
OH
Rhee
Jeong-Seop
JS
Araç
Demet
D
Rah
Jong-Cheol
JC
Rizo
Josep
J
Südhof
Thomas
T
Rosenmund
Christian
C
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D006224:Cricetinae; D051379:Mice; D016297:Mutagenesis, Site-Directed; D018377:Neurotransmitter Agents / Q000378:metabolism; D019906:Nuclear Magnetic Resonance, Biomolecular; D018129:Phosphatidylinositol Phosphates / Q000378:metabolism*; D011485:Protein Binding; D051381:Rats; D050863:Synaptotagmin I / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*
D018377:Neurotransmitter Agents; D018129:Phosphatidylinositol Phosphates; D050863:Synaptotagmin I; D002118:Calcium