Regions Outside of the CAAX Motif Influence the Specificity of Prenylation of G Protein γ Subunits

A family of GTP-binding regulatory proteins (G proteins) transduces signals across the plasma membrane from a large number of receptors to a smaller number of effectors. Recent studies indicate that a series of post-translational modifications are required for their association with the plasma membrane and for their function. In the case of the G protein γ subunits, the post-translational modifications include the prenylation of a cysteine residue within a carboxyl-terminal CAAX motif. Although prenylation has been shown to involve the addition of either a C15 farnesyl or a C20 geranylgeranyl group to proteins, the structural requirements and functional consequences of adding different types of prenyl groups to various members of the γ subunit family have not been examined. In the present study, we have employed the baculovirus expression system to study the structural requirements for attaching different types of prenyl groups to various members of the γ subunit family. We show that the γ2 subunit is modified by a C20 geranylgeranyl group, consistent with the presence of a geranylgeranylation target sequence in this protein. However, we found that the γ1 and mutant γ2(Ser-71) subunits are modified by both C15 farnesyl and C20 geranylgeranyl groups, despite the presence of an accepted farnesylation target sequence in both of these proteins. Using chimeras of the γ1 and γ2 subunits, we provide evidence indicating that structural elements upstream of the carboxyl-terminal CAAX motif play a role in the recognition of members of the γ subunit family by the appropriate insect and mammalian prenyltransferases.