Isolation and characterization of sea urchin egg lipid rafts and their possible function during fertilization

Specialized membrane microdomains called rafts are thought to play a role i n many types of cell-cell interactions and signaling. We have investigated the possibility that sea urchin eggs contain these specialized membrane mic rodomains and if they play a role in signal transduction at fertilization. A low density, TX-100 insoluble membrane fraction, typical of lipid rafts, was isolated by equilibrium gradient centrifugation. This raft fraction con tained proteins distinct from cytoskeletal complexes. The fraction was enri ched in tyrosine phosphorylated proteins and contained two proteins known t o be involved in signaling during egg activation tan egg Src-type kinase an d PLC gamma). This fraction was further characterized as a prototypical raf t fraction by the release of proteins in response to in vitro treatment of the rafts with the cholesterol binding drug, methyl-beta -cyclodextrin (M b eta CD). Furthermore, treatment of eggs with M beta CD inhibited fertilizat ion, suggesting that egg lipid rafts play a physiological role in fertiliza tion. (C) 2001 Wiley-Liss, Inc.