Selective loss of substrate recognition induced by the tumour-associated D294G point mutation in protein kinase Calpha.

The tumour-associated D294G mutant of protein kinase Calpha (PKCalpha) was recently shown not to be translocated to the plasma membrane on stimulation with PMA, in contrast with the wild-type enzyme. Using recombinant wild-type and mutant PKCalpha, we establish here that, although the PKCalpha intrinsic lipid-dependent catalytic activity remains unaltered by the D294G mutation, the mutant enzyme exhibits a selective loss of substrate recognition. Indeed, whereas the mutant enzyme is still able to phosphorylate histone IIIS with comparable efficiency to that of the wild-type enzyme, it exhibits a lack of kinase activity towards the previously cloned 35F and 35H substrates for PKC. Overlay experiments demonstrate that this selective loss of kinase activity is correlated with a decrease in binding of D294G PKCalpha to the 35F and 35H proteins compared with that of the wild-type enzyme. Because the 35H and 35F proteins are predicted to be PKCalpha-anchoring proteins, these findings suggest a selective loss of PKCalpha-protein interactions that might fail to stabilize the location of the PKCalpha mutant at the plasma membrane.