Phosphatidylinositol 4,5-Bisphosphate Alters Synaptotagmin 1 Membrane Docking and Drives Opposing Bilayers Closer Together

• Published in: Biochemistry (Easton) Vol. 50; no. 13; pp. 2633 - 2641
• Main Authors: Kuo, Weiwei, Herrick, Dawn Z, Cafiso, David S
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 05.04.2011
• Subjects: Animals; Computer Simulation; Electron Spin Resonance Spectroscopy; Kinetics; Lipid Bilayers - chemistry; Lipid Bilayers - metabolism; Liposomes - chemistry; Liposomes - metabolism; Membrane Fusion; Models, Biological; Mutagenesis, Site-Directed; Mutant Proteins - chemistry; Mutant Proteins - metabolism; Peptide Fragments - chemistry; Peptide Fragments - genetics; Peptide Fragments - metabolism; Phosphatidylinositol 4,5-Diphosphate - metabolism; Protein Conformation; Protein Interaction Domains and Motifs; Rats; Spin Labels; Surface Properties; Synaptotagmin I - chemistry; Synaptotagmin I - genetics; Synaptotagmin I - metabolism
• ISSN: 0006-2960; 1520-4995
• DOI: 10.1021/bi200049c

Synaptotagmin 1 (syt1) is a synaptic vesicle-anchored membrane protein that acts as the calcium sensor for the synchronous component of neuronal exocytosis. Using site-directed spin labeling, the position and membrane interactions of a fragment of syt1 containing its two C2 domains (syt1C2AB) were assessed in bilayers containing phosphatidylcholine (PC), phosphatidylserine (PS), and phosphatidylinositol 4,5-bisphosphate (PIP2). Addition of 1 mol % PIP2 to a lipid mixture of PC and PS results in a deeper membrane penetration of the C2A domain and alters the orientation of the C2B domain so that the polybasic face of C2B comes into the proximity of the bilayer interface. The C2B domain is found to contact the membrane interface in two regions, the Ca2+-binding loops and a region opposite the Ca2+-binding loops. This suggests that syt1C2AB is configured to bridge two bilayers and is consistent with a model generated previously for syt1C2AB bound to membranes of PC and PS. Point-to-plane depth restraints, obtained by progressive power saturation, and interdomain distance restraints, obtained by double electron−electron resonance, were obtained in the presence of PIP2 and used in a simulated annealing routine to dock syt1C2AB to two membrane interfaces. The results yield an average structure different from what is found in the absence of PIP2 and indicate that bilayer−bilayer spacing is decreased in the presence of PIP2. The results indicate that PIP2, which is necessary for bilayer fusion, alters C2 domain orientation, enhances syt1−membrane electrostatic interactions, and acts to drive vesicle and cytoplasmic membrane surfaces closer together.