Photoaffinity labeling using photoreactive isoprenoid analogs

Protein prenylation is a post-translational modification where a farnesy] (C15) or a geranylgeranyl (C20) group is covalently attached to a cysteine residue of a protein by a thioether linkage. Prenylation has been shown to be important for membrane association, and there is some evidence that it is involved in protein-protein interactions; however, identifying proteins which specifically recognize the isoprenoids has proven difficult. Because farnesylation of oncogenic Ras has been shown to be necessary for membrane association and cellular transformation, there is considerable interest in developing farnesyl protein transferase inhibitors as possible anticancer agents. We have synthesized farnesyl and geranylgeranyl pyrophosphate analogs which incorporate a 2-diazo-3,3,3-trifluoropropionyl photoreactive group linked by either an ester or a more stable amide bond. Using these analogs, preliminary photoalfinity labeling studies have been performed with yeast farnesyl protein transferase which show that the analogs are substrates for the enzyme, photoinactivate the enzyme in a time-dependent manner, and speciiically label the α subunit. Because the analogs are substrates, we have also been able to enzymatically incorporate the photoreactive farnesyl moiety onto a Ras protein, generating a molecule which should be useful for identifying proteins involved in isoprenoid recognition. Currently, peptides modified with the photoreactive isoprenoid are being used in preliminary studies for this purpose.