TI - Activation of protein kinase B beta and gamma isoforms by insulin in vivo and by 3-phosphoinositide -dependent protein kinase -1 in vitro : comparison with protein kinase B alpha . AB - The regulatory and catalytic properties of the three mammalian isoforms of protein kinase B ( PKB ) have been compared . All three isoforms ( PKBalpha , PKBbeta and PKBgamma ) were phosphorylated at similar rates and activated to similar extents by 3-phosphoinositide-dependent protein kinase-1 ( PDK1 ) . Phosphorylation and activation of each enzyme required the presence of PtdIns ( 3,4,5 ) P3 or PtdIns ( 3,4 ) P2 , as well as PDK1 . The activation of PKBbeta and PKBgamma by PDK1 was accompanied by the phosphorylation of the residues equivalent to Thr308 in PKBalpha , namely Thr309 ( PKBbeta ) and Thr305 ( PKBgamma ) . PKBgamma which had been activated by PDK1 possessed a substrate specificity identical with that of PKBalpha and PKBbeta towards a range of peptides . The activation of PKBgamma and its phosphorylation at Thr305 was triggered by insulin-like growth factor-1 in 293 cells . Stimulation of rat adipocytes or rat hepatocytes with insulin induced the activation of PKBalpha and PKBbeta with similar kinetics . After stimulation of adipocytes , the activity of PKBbeta was twice that of PKBalpha , but in hepatocytes PKBalpha activity was four-fold higher than PKBbeta . Insulin induced the activation of PKBalpha in rat skeletal muscle in vivo , with little activation of PKBbeta . Insulin did not induce PKBgamma activity in adipocytes , hepatocytes or skeletal muscle , but PKBgamma was the major isoform activated by insulin in rat L6 myotubes ( a skeletal-muscle cell line ) .