Regulation of phosphatidylinositol 3'-kinase by tyrosyl phosphoproteins. Full activation requires occupancy of both SH2 domains in the 85-kDa regulatory subunit.
Phosphatidylinositol 3'-kinase (PI 3'-kinase) is activated in insulin-stimulated cells by the binding of the SH2 domains in its 85-kDa regulatory subunit to insulin receptor substrate-1 (IRS-1). We have previously shown that both tyrosyl-phosphorylated IRS-1 and mono-phosphopeptides containing a single YXXM motif activate PI 3'-kinase in vitro. However, activation by the monophosphopeptides was significantly less potent than activation by the multiply phosphorylated IRS-1. We now show that the increased potency of PI 3'-kinase activation by IRS-1 relative to phosphopeptide is not due to tertiary structural features IRS-1, as PI 3'-kinase is activated normally by denatured, reduced, and carboxymethylated IRS-1. Furthermore, activation of PI 3'-kinase by bis-phosphorylated peptides containing two YXXM motifs is 100-fold more potent than the corresponding mono-phosphopeptides and similar to activation by IRS-1. These data suggest that tyrosyl-phosphorylated IRS-1 or bis-phosphorylated peptides bind simultaneously to both SH2 domains of p85. However, these data cannot differentiate between an activation mechanism that requires two-site occupancy for maximal activity as opposed to one in which bivalent binding enhances the occupancy of a single activating site. To distinguish between these possibilities, we produced recombinant PI 3'-kinase containing either wild-type p85 or p85 mutated in its N-terminal, C-terminal, or both SH2 domains. We find that mutation of either SH2 domains significantly reduced phosphopeptide binding and decreased PI 3'-kinase activation by 50%, whereas mutation of both SH2 domains completely blocked binding and activation. These data provide the first direct evidence that full activation of PI 3'-kinase by tyrosylphosphorylated proteins requires occupancy of both SH2 domains in p85.
Rordorf-Nikolic
T
T
Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York 10461.
Van Horn
D J
DJ
Chen
D
D
White
M F
MF
Backer
J M
JM
D000595:Amino Acid Sequence; D000818:Animals; D002478:Cells, Cultured; D003001:Cloning, Molecular; D004789:Enzyme Activation; D055504:Insulin Receptor Substrate Proteins; D008969:Molecular Sequence Data; D009154:Mutation; D019869:Phosphatidylinositol 3-Kinases; D010748:Phosphopeptides / Q000378:metabolism*; D010750:Phosphoproteins / Q000378:metabolism; D017853:Phosphotransferases (Alcohol Group Acceptor) / Q000235:genetics / Q000378:metabolism*; D051381:Rats; D018411:Spodoptera; D013379:Substrate Specificity; D014443:Tyrosine / Q000378:metabolism*
D055504:Insulin Receptor Substrate Proteins; C526296:Irs1 protein, rat; D010748:Phosphopeptides; D010750:Phosphoproteins; D014443:Tyrosine; D019869:Phosphatidylinositol 3-Kinases; D017853:Phosphotransferases (Alcohol Group Acceptor)