Mutagenesis of the pseudosubstrate site of protein kinase C leads to activation

Protein kinase C is maintained in an inactive state by the action of an inhibitory region within the effector binding domain of the kinase. It has been suggested that a short stretch of amino acids (pseudosubstrate site) mediates this inhibition by binding to the active site and preventing substrate interaction [House, C. and Kemp, B. E. (1987) Science 238, 1726-1728]. A mutated version of protein kinase C-alpha containing a glutamic acid for alanine substitution in this region has been analysed for biochemical properties and biological function. Consistent with the importance of this pseudosubstrate site in regulating kinase activity, this altered protein has a significantly increased effector-independent kinase activity relative to wild-type protein kinase C-alpha and shows increased sensitivity to activation by proteolysis. The increased activity of this protein in the intact cell was confirmed by its ability to stimulate expression from a phorbol-ester-inducible reporter construct in a transient transfection system. Expression of a mutant kinase with the pseudosubstrate sequence deleted causes greater induction in this transient expression system, consistent with this kinase being independent of effectors and thus constitutively active.