Analysis of the C-terminal proteolysis of prenylated proteins
A variety of proteins, including many GTP-binding proteins, are modified by an isoprenoid lipid which is attached at a C-terminal motif. This motif consists of the tetrapeptide CaaX, where 6C5 is the cysteine on which prenylation occurs, 6a6 are commonly aliphatic residues, and 6X5 can be one of several amino acids. Following prenylation of the CaaX motif, the prenylated protein undergoes two additional processing steps. First, a specific protease cleaves the -aaX tripeptide from the C-terminus of the protein. After this proteolytic cleavage, the now exposed cagboxyl group of the prenylated cysteine residue is methylated by a specific carboxylmethyltransferase. Using purified, recombinant prenylated proteins and a recombinant yeast carboxytmethyltransferase, we have developed an assay by which we can observe the proteolysis of prenylated proteins in vitro. Using this assay, we have characterized the specific prenyl protein protease activity using a variety of prenytated protein substrates, and have eliminated non-specific proteolysis events that occur in prenyl peptide-based assays.
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