A NOVEL 55-KDA REGULATORY SUBUNIT FOR PHOSPHATIDYLINOSITOL 3-KINASE STRUCTURALLY SIMILAR TO P55PIK IS GENERATED BY ALTERNATIVE SPLICING OF THE P85-ALPHA GENE

Phosphatidylinositol 3-kinase, which is composed of a 110-kDa catalyti c subunit and a regulatory subunit, plays important roles in various c ellular signaling mechanisms. We screened a rat brain cDNA expression library with P-32-labeled human IRS-1 protein and cloned cDNAs that we re very likely to be generated by alternative splicing of p85 alpha ge ne products. These cDNAs were demonstrated to encode a 55-kDa protein (p55 alpha) containing two SH2 domains and an inter-SH2 domain of p85 alpha but neither a bcr domain nor a SH3 homology domain. Interestingl y, p55 alpha contains a unique 34-amino acid sequence at its NH2 termi nus, which is not included in the p85 alpha amino acid sequence. This 34-amino acid portion was revealed to be comparable with p55PIK (p55 g amma) in length, with a high homology between the two, suggesting that these NH2-terminal domains of p55 alpha and p55 gamma may have a spec ific role that p85 does not. The expression of p55 alpha mRNA is most abundant in the brain, but expression is ubiquitous in most rat tissue s. Furthermore, it should be noted that the expression of p85 alpha mR NA in muscle is almost undetectably low by Northern blotting with a cD NA probe coding for the p85 alpha SH3 domain, while the expression of p55 alpha can be readily detected. These results suggest that p55 alph a may play an unique regulatory role for phosphatidylinositol 3-kinase in brain and muscle.