Divergent regulation of phosphatidylinositol 3-kinase p85α and p85β isoforms upon T cell activation

Phosphatidylinositol (PtdIns) 3-kinase is composed of a catalytic p110 subunit and a regulatory p85 subunit. A synthetic phosphopeptide corresponding to the kinase insert of the human PDGF β subunit receptor and monoclonal antibodies raised against the two described p85 isoforms, p85α and p85β were used to isolate PtdIns 3-kinase from human T lymphocytes. We demonstrate that T cells express both p85α and p85β proteins. Both isoforms tightly associate with a p110 protein and with PtdIns 3-kinase activity in T cells. Upon triggering of the T cell antigen receptor (TCR)/CD3 complex or activation of protein kinase C (PKC) the p110 protein complexed to p85α becomes rapidly phosphorylated exclusively on serine residues. p85α does not appear to undergo a change in its basal serine phosphorylation during T cell activation. In contrast, stimulation of the TCR/CD3 complex or PKC, results in a marked and rapid increase in phosphorylation of p85β on threonine residues. These data show that PtdIns 3-kinase can be a substrate for serine/threonine kinases in T cells. The differential phosphorylation of p85α and p85β reveals the potential for divergent regulation and function of these two PtdIns 3-kinase isoforms during T cell activation.