Heterotrimeric G proteins couple cell surface receptors to intracellular effectors at the cytoplasmic face of membranes. The heterotrimer consists of a subunits which bind the guanine nucleotide and bg subunits which are tightly bound together. The membrane localization of G proteins is critical for their ability to relay signal between membrane-bound receptors and effectors. Palmitoylation, the reversible addition of 16 carbon palmitic acid to cysteine residues, occurs on most a subunits and participates in membrane localization. Receptor activation of as causes turnover of the palmitate group suggesting that palmitoylation regulates G protein signaling. The function of a subunit palmitoylation on bg and receptor interactions was investigated. In vitro, palmitoylated as bound bg with a 5 fold higher affinity than nonpalmitoylated as indicating that bg binding and palmitoylation can reciprocally potentiate membrane attachment of as. To evaluate receptor-a subunit coupling, membranes were treated with and without hydroxylamine to remove the palmitate and ligand binding affinity was determined. Preliminary results showed that hydroxylamine treatment caused a shift from a high affinity binding receptor state to a low affinity state suggesting that palmitoylation increased the affinity of the a subunit for the receptor. We have also investigated whether palmitoylation directs proteins to specific intracellular membranes by transfecting cells with the wild-type and palmitoylation-defective mutants of XLas. This protein is a splice variant of as with an additional 367 amino acids on the amino-terminus of as. Wild -type XLas was predominantly localized to the Golgi. A series of cysteine-to-serine mutations which gradually diminished palmitoylation of the protein did not effect distribution except to decrease membrane attachment. In contrast, deletion of a 24 amino acid proline-rich in the extra long portion reduced Golgi localization without effecting expression level or membrane attachment. A proline-rich region is also a putative signal for Golgi localization of a Gi2a splice variant suggesting that this domain could be a general signal for Golgi localization of a subunits and possibly other proteins as well.
