ACTIVE LYN PROTEIN-TYROSINE KINASE IS SELECTIVELY ENRICHED WITHIN MEMBRANE MICRODOMAINS OF RESTING PLATELETS

Circulating platelets are primed to respond very rapidly to thrombogen ic stimuli, but most platelets complete their lifespan without ever be coming activated. Platelet activation is accompanied by waves of seque ntial tyrosine phosphorylation thought to involve members of the Src f amily of protein tyrosine kinases (PTKs). We show here that resting pl atelets contain highly active pp53/56(Lyn) PTK within membrane microdo mains (rafts) isolated biochemically with or without the use of deterg ent. This fraction is also greatly enriched in the transmembrane glyco protein CD36, known to associate with Lyn PTK, but in transfection stu dies we could find no evidence to suggest that CD36 affects the distri bution or function of Lyn. Upon platelet activation Lyn activity remai ns constant or diminishes and pp60(c-src) PTK within this fraction bec omes highly activated, indicating the dynamic nature of the membrane m icrodomains. It is suggested that the function of active Lyn PTK in th e resting platelet is to allow prolonged survival of this anucleate ce ll.