REGIONS OUTSIDE OF THE CAAX MOTIF INFLUENCE THE SPECIFICITY OF PRENYLATION OF G-PROTEIN GAMMA-SUBUNITS

A family of GTP-binding regulatory proteins (G proteins) transduces si gnals across the plasma membrane from a large number of receptors to a smaller number of effecters. Recent studies indicate that a series of post-translational modifications are required for their association w ith the plasma membrane and for their function. In the case of the G p rotein gamma subunits, the post-translational modifications include th e prenylation of a cysteine residue within a carboxyl-terminal CAAX mo tif. Although prenylation has been shown to involve the addition of ei ther a C-15 farnesyl or a C-20 geranylgeranyl group to proteins, the s tructural requirements and functional consequences of adding different types of prenyl groups to various members of the gamma subunit family have not been examined. In the present study, we have employed the ba culovirus expression system to study the structural requirements for a ttaching different types of prenyl groups to various members of the ga mma subunit family. We show that the gamma(2) subunit is modified by a C-20 geranylgeranyl group, consistent with the presence of a geranylg eranylation target sequence in this protein. However, we found that th e gamma(1) and mutant gamma(2(ser-71)) subunits are modified by both C -15 farnesyl and C-20 geranylgeranyl groups, despite the presence of a n accepted farnesylation target sequence in both of these proteins. Us ing chimeras of the gamma(1) and gamma(2) subunits, we provide evidenc e indicating that structural elements upstream of the carboxyl-termina l CAAX motif play a role in the recognition of members of the gamma su bunit family by the appropriate insect and mammalian prenyltransferase s.