Membrane domain formation by calcium-dependent, lipid-binding proteins: insights from the C2 motif

We propose a novel role in cellular function for some membrane-binding prot eins and, specifically, the C2 motif. The C2 motif binds phospholipid in a manner that is modulated by Ca2+ and is thought to confer membrane-binding ability on a wide variety of proteins, primarily proteins involved in signa l transduction and membrane trafficking events. We hypothesize that in the absence of Ca2+ the C2 motif couples the free energy of binding to a bilaye r membrane comprised of zwitterionic and negatively charged lipids to the f ormation of a domain enriched in the negative lipids. This in turn leads to the dynamic clustering of bound homologous or heterologous proteins incorp orating the C2 motif, or other acidic lipid-binding motifs. In the presence of Ca2+, the protein clusters may be further stabilized. In support of thi s hypothesis we present evidence for membrane domain formation by the first C2 domain of synaptotagmin in the absence of Ca2+. Fluid state phospholipi d mixtures incorporating a pyrene-labeled phospholipid probe exhibited a ch ange in pyrene excimer/monomer fluorescence ratio consistent with domain fo rmation upon binding the C2 domain. In addition, we present the results of simulations of the interaction of the C2 domain with the membrane that indi cate that protein clusters and lipid domains form in concert. (C) 1998 Else vier Science B.V. All rights reserved.