Band 3 protein is a major erythrocyte transmembrane glycoprotein. We compared the degradation of band 3 protein by calpain I (a cytoplasmic, micromolar-Ca2(+)-requiring thiol proteinase) in the cells from old individuals (greater than 70 years old) to that in the cells from young ones (20-30 years old). In the young, little degradation of band 3 protein occurred in calpain-treated erythrocyte ghosts. In the old, significant band 3 protein degradation was found in erythrocyte ghosts treated similarly. The difference between young and old in the susceptibility of band 3 protein to calpain was retained in membrane vesicles (membranes stripped of peripheral proteins by NaOH) and in chymotrypsin-generated 60 kDa fragment (CH-60). The isolated N-terminal cytoplasmic 43 kDa fragment was degraded by calpain to a similar extent in old and in young. The separated 17 kDa membrane domain of the CH-60 and the trypsin-generated C-terminal 55 kDa membrane-spanning fragment were not degraded by calpain I in the young, nor in the old. Thus the N-terminal cytoplasmic domain is the domain degraded by calpain I. Enhanced sensitivity in the old is observed in intact band 3 protein and in CH-60, the isolated cytoplasmic domain being equally susceptible in young and old. The observed age-related enhanced sensitivity to calpain is consistent with the presence of modifications in band 3 protein and alterations in the association with the calpain-calpastatin system. Band 3 protein has several important functions, with modifications in the protein having implications for altered cell behaviour in the old individual.
