Purified preparations of human erythrocyte G-proteins contain a 43 kDa pertussis toxin substrate which appears to be the α-subunit of a heterotrimeric GTP-binding protein. The 43 kDa protein is recognized by antisera that are sequence-specific for peptides encoding a sequence common to all 39–53 kDa G-protein α-subunits. Gαo-specific antiserum did not recognize 43 or 40–41 kDa α-subunits. AS/6, which recognizes the αi proteins, recognized 43 kDa as well as 40–41 kDa proteins. Of the three antisera specific for individual members of the αi family, only the Gi3-specific antiserum recognized the 43 kDa erythrocyte G-protein. However, 40–41 kDa forms of all three αis are present. These observations indicate that human erythrocytes contain a novel 43 kDa form of Gi3.
