An effector candidate for G protein action, GRIN1, was identified by screening a cDNA expression library with phosphorylated GTPgammaS-G(z)alpha as a probe. GRIN1 is a novel protein without substantial homology to known protein domains. It is expressed largely in brain and binds specifically to activated G(z)alpha, G(o)alpha, and G(i)alpha through its carboxyl-terminal region. The protein KIAA0514 (GRIN2) is homologous to GRIN1 at its carboxyl terminus and also binds to activated G(o)alpha. Both GRIN1 and G(o)alpha are membrane-bound proteins that are enriched in the growth cones of neurites. Coexpression of GRIN1 or GRIN2 with activated G(o)alpha causes formation of a network of fine processes in Neuro2a cells, suggesting that these pathways may function downstream of G(o)alpha to control growth of neurites.