Regions Outside of the CAAX Motif Influence the Specificity of Prenylation of G Protein 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 subunits, the post-translational modifications include the prenylation of a cysteine residue within a carboxyl-terminal C AAX motif. Although prenylation has been shown to involve the addition of either a C farnesyl or a C geranylgeranyl group to proteins, the structural requirements and functional consequences of adding different types of prenyl groups to various members of the 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 subunit family. We show that the 2 subunit is modified by a C geranylgeranyl group, consistent with the presence of a geranylgeranylation target sequence in this protein. However, we found that the 1 and mutant subunits are modified by both C farnesyl and C geranylgeranyl groups, despite the presence of an accepted farnesylation target sequence in both of these proteins. Using chimeras of the 1 and 2 subunits, we provide evidence indicating that structural elements upstream of the carboxyl-terminal C AAX motif play a role in the recognition of members of the subunit family by the appropriate insect and mammalian prenyltransferases.