Membrane-bound orientation and position of the synaptotagmin C2B domain determined by site-directed spin labeling

Elisabeth Rufener, April A. Frazier, Catherine M. Wieser, Anne Hinderliter, David S. Cafiso

Research output: Contribution to journalArticle

58 Scopus citations

Abstract

Site-directed spin labeling is used to determine the orientation and depth of insertion of the second C2 domain from synaptotagmin I (C2B) into membrane vesicles composed of phosphatidylcholine (PC) and phosphatidylserine (PS). EPR line shapes of spin-labeled mutants located with the Ca2+-binding loops of C2B broaden in the presence of Ca2+ and PC/PS vesicles, indicating that these loops undergo a Ca2+-dependent insertion into the membrane interface. Power saturation of the EPR spectra provides a position for each spin-labeled site along the bilayer normal, and these EPR-derived distance constraints, along with a high-resolution structure of the C2B domain, are used to generate a model for the domain orientation and position at the membrane interface. Our data show that the isolated C2B domain from synaptotagmin I penetrates PC/PS membranes, and that the backbone of Ca 2+-binding loops 1 and 3 is inserted below the level of a plane defined by the lipid phosphates. The side chains of several loop residues are within the bilayer interior, and both Ca2+-binding sites are positioned near a plane defined by the lipid phosphates. A Tb3+-based fluorescence assay is used to compare the membrane affinity of the C2B domain to that of the first synaptotagmin C2 domain (C2A). Both C2A and C2B bind PC/PS (75:25) membrane vesicles with a micromolar lipid affinity in the presence of metal ion. These results indicate that C2A and C2B have a similar membrane affinity and position when bound to PC/PS (75:25) membrane vesicles. EPR spectroscopy indicates that the C2B domain has different interactions with PC/PS membranes containing 1 mol % phosphatidylinositol 4,5-bisphosphate.

Original languageEnglish (US)
Pages (from-to)18-28
Number of pages11
JournalBiochemistry
Volume44
Issue number1
DOIs
StatePublished - Jan 11 2005

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