PI 3-kinase activation in liver and muscle of Pik3r1 mutant mice. Lysates from the liver of animals were immunoprecipitated with the indicated antibodies and subjected to a PI 3-kinase assay as described in Methods. PI 3-kinase activities associated with total regulatory subunit (αp85pan) (a), associated with p85α regulatory subunit (αp85α) (b), associated with the p110 catalytic subunit (αp110) (c), associated with tyrosine-phosphorylated proteins (αPY) (d), and associated with p85β regulatory subunit (αp85β) (e) were assessed. The upper panels shows representative PI 3-kinase assays, while each bar in the lower panels represents the mean ± SEM of the relative PI 3-kinase activity (% of unstimulated wild-type) calculated from at least three independent experiments. *P < 0.05, wild-type vs. p85^+/–. (f) Activation of PI 3-kinase in muscle. Lysates were immunoprecipitated with the p85pan antibody (αp85pan, top panel), tyrosine-phosphorylated proteins (αPY, middle panel), or the p85β regulatory subunit (αp85β, bottom panel), and subjected to PI 3-kinase assay as above.