PHOSPHOINOSITIDE 3-KINASE IS ACTIVATED BY PHOSPHOPEPTIDES THAT BIND TO THE SH2 DOMAINS OF THE 85-KDA SUBUNIT

Tyrosine-phosphorylated peptides based on the regions of polyoma virus middle t antigen and the platelet-derived growth factor receptor that bind phosphoinositide 3-kinase are shown to activate this enzyme 2-3- fold in vitro. The concentrations of the peptides required to activate the enzyme are at least 10-1000-fold higher than the dissociation con stants of these peptides for the individual SH2 domains of the 85-kDa subunit (K(D) < 100 nM). Doubly phosphorylated peptides are more effec tive than singly phosphorylated peptides. The results suggest that a f raction of the cellular phosphoinositide 3-kinase has SH2 domains with relatively low affinity for phosphopeptides and that binding of phosp hopeptides to these enzymes causes activation. Thus, SH2 domains may b e involved not only in recruiting the enzyme but also in regulating ac tivity.