A GTP-binding protein modulates a Ca2+ pump present in reticulocyte endocytic vesicles.

Rat reticulocytes were used to prepare endocytic vesicles to study calcium fluxes across the endosomal membrane. We used 45Ca2+, and found that endocytic vesicles from reticulocytes present a Ca(2+)-ATPase that pumps Ca2+ into the lumen of vesicles. This activity was sensitive to vanadate and the calmodulin antagonists: trifluoperazine and calmidazolium. Western blot analysis using a monoclonal antibody evidenced that Ca(2+)-ATPase present in reticulocyte endocytic vesicles is probably the same as the erythrocyte Ca2+ pump. Ca2+ pump activity was shown to be partially inhibited by GTP gamma S. Moreover, mastoparan and benzalkonium chloride, both activators of heterotrimeric G proteins, were found to decrease 45Ca2+ uptake by endocytic vesicles. These results suggest the involvement of a trimeric G protein in the modulation of Ca(2+)-ATPase.