Binding to erythrocyte membrane is the physiological mechanism for activation of Ca2+-dependent neutral proteinase.

In the presence of micromolar concentrations of Ca2+ the catalytic 80 kDa subunit of human erythrocyte procalpain binds to the cytosolic surface of the erythrocyte membrane. Binding is rapid, highly specific and is reversed by the removal of Ca2+. In the bound form the 80 kDa catalytic subunit undergoes a rapid conversion to calpain, the active 75 kDa Ca2+-requiring proteinase. The activated proteinase produces extensive degradation of membrane components, particularly of band 4.1 and 2.1 proteins. Binding to membranes may represent an obligatory physiological mechanism for the conversion of procalpain to calpain.