CALMODULIN MODULATES PROTEIN-4.1 BINDING TO HUMAN ERYTHROCYTE-MEMBRANES

Calmodulin, an abundant protein in the red cell cytosol, exerts its ef fects on erythrocyte membrane properties via interactions with numerou s proteins. To evaluate whether calmodulin might regulate association of protein 4.1 with one of its integral membrane protein anchors, prot ein 4.1 binding to inside-out erythrocyte membrane vesicles (IOVs) in the presence and absence of calmodulin and Ca2+ was examined. Ca2+ plu s calmodulin was found to competitively inhibit protein 4.1 associatio n with IOVs with a K-i of 1.4 mu M and a maximal inhibition of 83%. In the absence of Ca2+, calmodulin still reduced protein 4.1 binding by 43%, consistent with the known Ca2+ independent association of calmodu lin with protein 4.1. Ca2+ alone had no effect on protein 4.1-membrane interactions. Digestion studies revealed that both band 3 and glycoph orin sites were similarly affected by calmodulin competition, suggesti ng all major protein 4.1 anchors are potentially regulated. In light o f other data showing regulation of the same interactions by phosphoino sitides, protein kinases, and the concentration of free cytosolic 2,3- diphosphoglycerate, it can be argued that association of protein 4.1 w ith integral protein anchors constitutes one of the more sensitively r egulated interactions of the membrane.