THE INTERACTION OF SMALL DOMAINS BETWEEN THE SUBUNITS OF PHOSPHATIDYLINOSITOL 3-KINASE DETERMINES ENZYME-ACTIVITY

Previous studies have suggested that the two subunits of phosphatidyli nositol (PI) 3-kinase, p85 and p110, function as localizing and cataly tic subunits, respectively. Using recombinant p85 and p110 molecules, we have reconstituted the specific interaction between the two subunit s of mouse PI 3-kinase in cells and in vitro. We have previously shown that the region between the two Src homology 2 (SH2) domains of p85 i s able to form a functional complex with the 110-kDa subunit in vivo. In this report, we identify the corresponding domain in p110 which dir ects the binding to p85. We demonstrate that the interactive domains i n p85 and p110 are less than 103 and 124 amino acids, respectively, in size. We also show that the association of p85 and p110 mediated by t hese domains is critical for PI 3-kinase activity. Surprisingly, a com plex between a 102-amino-acid segment of p85 and the full-length p110 molecule is catalytically active, whereas p110 alone has no activity. In addition to the catalytic domain in the carboxy-terminal region, 12 3 amino acids at the amino terminus of p110 were required for catalyti c activity and were sufficient for the interaction with p85. These res ults indicate that the 85-kDa subunit, previously thought to have only a linking role in localizing the p110 catalytic subunit, is an import ant component of the catalytic complex.