ACTIVATION OF PROTEIN-KINASE-B-BETA AND PROTEIN-KINASE-B-GAMMA ISOFORMS BY INSULIN IN-VIVO AND BY 3-PHOSPHOINOSITIDE-DEPENDENT PROTEIN KINASE-1 IN-VITRO - COMPARISON WITH PROTEIN-KINASE-B-ALPHA

The regulatory and catalytic properties of the three mammalian isoform s of protein kinase B (PKB) have been compared. All three isoforms (PK B alpha, PKB beta and PKB gamma) were phosphorylated at similar rates and activated to similar extents by 3-phosphoinositide-dependent prote in kinase-1 (PDK1). Phosphorylation and activation of each enzyme requ ired the presence of PtdIns(3,4,5)P-3 or PtdIns(3,4)P-2, as well as PD K1. The activation of PKB beta and PKB gamma by PDK1 was accompanied b y the phosphorylation of the residues equivalent to Thr(308) in PKB al pha, namely Thr(309) (PKB beta) and Thr(305) (PKB gamma). PKB gamma wh ich had been activated by PDK1 possessed a substrate specificity ident ical with that of PKB alpha and PKB beta towards a range of peptides, The activation of PKB gamma and its phosphorylation at Thr(305) was tr iggered by insulin-like growth factor-1 in 293 cells. Stimulation of r at adipocytes or rat hepatocytes with insulin induced the activation o f PKB alpha and PKB beta with similar kinetics. After stimulation of a dipocytes, the activity of PKB beta was twice that of PKB alpha, but i n hepatocytes PKB alpha activity was four-fold higher than PKB beta. I nsulin induced the activation of PKB alpha in rat skeletal muscle in v ivo, with little activation of PKB beta. Insulin did not induce PKB ga mma activity in adipocytes, hepatocytes or skeletal muscle, but PKB ga mma was the major isoform activated by insulin in rat L6 myotubes (a s keletal-muscle cell line).