PEST Sequences Do Not Influence Substrate Susceptibility to Calpain Proteolysis

Mutations lowering the PEST score of domains surrounding the calmodulin (CaM)-binding region of the plasma membrane Ca(2+)-ATPase failed to influence the susceptibility of the enzyme to mu-calpain (mu-CANP). Synthetic peptides corresponding to the high PEST score C-terminal sequences A18 and B28 had no effect on the rate of pump proteolysis by mu-CANP, i.e. the peptides did not compete for a putative high PEST score recognition site for mu-CANP in the pump molecule. An accessible CaM-binding region appears to be critical for substrate (i.e. the Ca2+ pump) proteolysis and probably also for its recognition by mu-CANP; phosphorylation of the CaM-binding domain of the pump or its occupation by CaM significantly decreased the rate of proteolysis.