IDENTIFICATION OF 2 SH3-BINDING MOTIFS IN THE REGULATORY SUBUNIT OF PHOSPHATIDYLINOSITOL 3-KINASE

Src homology 3 (SH3) domains have been recently shown to bind to proli ne-rich sequences contained in 3BP1, 3BP2, and SOS. In a recent study we demonstrated that phosphatidylinositol 3-kinase (PI 3-kinase) assoc iates with the Fyn SH3 domain. Here we show that p85, the regulatory s ubunit of PI 3-kinase, binds directly to the SH3 domains of Abl, Lck, Fyn, and p85 itself. An examination of p85 amino acid sequence reveale d two proline-rich sequences in its N-terminal region similar to those present in 3BP1, 3BP2, and SOS. To test whether these sequences media te the association of p85 with SH3 domains two peptides with amino aci d composition corresponding to the p85alpha proline-rich sequences wer e synthesized and used in competition assays. Both peptides worked equ ally well in inhibiting the binding of PI 3-kinase activity and p85alp ha to Fyn SH3 domain, whereas a control peptide had no effect. These r esults indicate that, as in 3BP1 and SOS, the proline-rich sequences i n p85 mediate its interaction with SH3 domains. These results also sug gest that the SH3 domain of p85 may ''self-associate'' with the prolin e-rich motifs of the same subunit as part of the PI 3-kinase regulator y mechanism.