Polylysine specifically activates the insulin-dependent insulin receptor protein kinase

We have extended these observations to examine the role of polylysine on the divalent metal ion requirement for ligand-stimulated protein kinase activity and the transmembrane signaling mechanism of both the human placenta insulin and insulin-like growth factor 1 (IGF-1) receptors. Polylysine (0.2-1 microM) was found to activate maximally the alpha 2 beta 2 heterotetrameric insulin receptor autophosphorylation and exogenous substrate protein kinase activity 25-50-fold in the presence of insulin without significantly affecting the basal protein kinase activity in the absence of insulin. The polylysine-dependent insulin stimulation of protein kinase activity required the presence of both magnesium and manganese but at relatively low divalent metal ion concentrations (0.1 mM) compared to the typical 2-10 mM Mg/Mn used in the standard in vitro kinase assays. The stimulation of the insulin receptor kinase by insulin in the presence of polylysine occurred primarily due to an increase in Vmax with no significant effect on the Km for ATP. In addition, autophosphorylated insulin receptors which are protein kinase-active and insulin-independent at high metal ion concentrations still displayed the polylysine-dependent insulin stimulation of protein kinase activity to the same extent as nonphosphorylated insulin receptors at low Mg/Mn (0.1 mM) concentrations. Surprisingly, polylysine was completely unable to stimulate the IGF-1-dependent protein kinase activity of the homologous human placenta IGF-1 receptor. These data suggest that the insulin receptor tyrosine-specific protein kinase activity may be regulated by unique endogenous basic proteins that are distinct from those which modify the IGF-1 receptor.