Polylysine activates and alters the divalent cation requirements of the insulin receptor protein tyrosine kinase

Abstract

Protamine and poly(Lys) activate the protein tyrosine kinase of both the human placental insulin receptor and its purified recombinant cytoplasmic domain. Spermidine, poly(Arg) (average molecular mass 15 kDa), poly(Glu), Arg or Lys are not effective. Activation is stable, reversible, and optimal when the enzyme is preincubated with activator, divalent cation and ATP prior to the addition of exogenous protein substrates. The most striking feature of the activation is that it results in 20–30-fold stimulation of the kinase in the presence of 0.2–0.4 mM Mn²⁺ and induces equivalent activity in the presence of Mg²⁺ alone (0.4–4.0 mM). The activated protein tyrosine kinase has a specific activity (0.25–0.5 μmol/mg protein) that approaches that of well characterized protein serine kinases.