p85 alpha gene generates three isoforms of regulatory subunit for phosphatidylinositol 3-kinase (PI 3-kinase), p50 alpha, p55 alpha, and p85 alpha, with different PI 3-kinase activity elevating responses to insulin

Phosphatidylinositol 3-kinase (PI 3-kinase) is stimulated by association with a variety of tyrosine kinase receptors and intracellular tyrosine-phosphorylated substrates. We isolated a cDNA that encodes a 50-kDa regulatory subunit of PI 3-kinase with an expression cloning method using P-32-labeled insulin receptor substrate-1 (IRS-1), This 50-kDa protein contains two SH2 domains and an inter-SH2 domain of p85 alpha, but the SH3 and bcr homology domains of p85 alpha were replaced by a unique 6-amino acid sequence, Thus, this protein appears to be generated by alternative splicing of the p85 alpha gene product, We suggest that this protein be called p50 alpha. Northern blotting using a specific DNA probe corresponding to p50 alpha revealed 6.0- and 2.8-kb bands in hepatic, brain, and renal tissues, The expression of p50 alpha protein and its associated PI 3-kinase were detected in lysates prepared from the liver, brain, and muscle using a specific antibody against p50 alpha. Taken together, these observations indicate that the p85 alpha gene actually generates three protein products of 85, 55, and 50 kDa. The distributions of the three proteins (p85 alpha, p55 alpha, and p50 alpha), in various rat tissues and also in various brain compartments, were found to be different, Interestingly, p50 alpha forms a heterodimer with p110 that can as well as cannot be labeled with wortmannin, whereas p85 alpha and p55 alpha associate only with p110 that can be wortmannin-labeled. Furthermore, p50 alpha exhibits a markedly higher capacity for activation of associated PI 3-kinase via insulin stimulation and has a higher affinity for tyrosine-phosphorylated IRS-1 than the other isoforms. Considering the high level of p50 alpha expression in the liver and its marked responsiveness to insulin, p50 alpha appears to play an important role in the activation of hepatic PI 3-kinase, Each of the three alpha isoforms has a different function and may have specific roles in various tissues.