The development changes in GTP-binding proteins and the regulation of their appearance by calcium ions were investigated during early sexual development in Dictyostelium discoideum. GTP gamma S strongly inhibited gamete cell fusion, while GDP beta S slightly augmented it, suggesting that G-proteins have a critical role in cell fusion. A 52-kDa protein recognized by an anti-GTP-binding site-specific immune serum, was abundant during calcium-dependent early sexual development but decreased in amount concomitant with cell fusion. This protein remained at high levels in Ca(2+)-deficient cultures, suggesting that its down-regulation is linked to the events of sexual development. Analysis of substrates for cholera and pertussis toxin-mediated [32P]ADP-ribosylation in D. discoideum extracts determined that the 52-kDa protein is a G-alpha subunit similar to mammalian Gs. The 52-kDa protein was also detected in vegetative, asexual amoebae, but diminished rapidly within the first 2 h of starvation. Together these data indicate that the 52-kDa protein functions during the growth phase and is lost upon entry into either the sexual or asexual developmental programs. The amounts of several lower molecular weight GTP-binding proteins, ranging from 21- to 28 kDa, increased during the stage of zygote differentiation and their increases were calcium dependent. These data provide the first analysis of G-proteins during sexual development of D. discoideum and lay the foundation for continued analysis of the signal transduction events mediating cell fusion and zygote differentiation.