Phosphatidylserine binding sites in red cell spectrin.

Spectrin has been shown to interact with phosphatidylserine (PS), however, the precise binding sites for PS in spectrin have not been defined. In the present study, we have identified specific PS binding sites in spectrin using recombinant spectrin fragments encompassing the entire sequences of both spectrin chains. We show that sites of high affinity are located within eight of the 38 triple-helical structural repeats which make up the bulk of both chains: these are: alpha8 and alpha9-10, and beta2, beta3, beta4, beta12, beta13 and beta14, and PS affinity was also found in the non-homologous N-terminal domain of the beta-chain. It is noteworthy that the PS-binding sites in beta-spectrin are clustered in close proximity to the sites of attachment both of ankyrin and of 4.1R, the proteins engaged in attachment of spectrin to the membrane. We conjecture that direct interaction of spectrin with PS in the membrane complements modulates its interactions with the proteins, and that (considering also the known affinity of 4.1R for PS) the formation of PS-rich lipid domains, which have been observed in the red cell membrane, may be a result.