Protein prenyltransferase (PTase) domain, beta subunit (alpha 6 - alpha 6 barrel fold). The protein prenyltransferase family of lipid-modifying enzymes includes protein farnesyltransferase (FTase) and geranylgeranyltransferase types I and II (GGTase-I and GGTase-II). They catalyze the carboxyl-terminal lipidation of Ras, Rab, and several other cellular signal transduction proteins, facilitating membrane associations and specific protein-protein interactions. Prenyltransferases employ a Zn2+ ion to alkylate a thiol group catalyzing the formation of thioether linkages between the C1 atom of farnesyl (15-carbon by FTase) or geranylgeranyl (20-carbon by GGTase-I, II) isoprenoid lipids and cysteine residues at or near the C-terminus of protein acceptors. FTase and GGTase-I prenylate the cysteine in the terminal sequence, "CAAX"; and GGTase-II prenylates both cysteines in the "CC" (or "CXC") terminal sequence. These enzymes are heterodimeric with both alpha and beta subunits required for catalytic activity. In contrast to other prenyltransferases, GGTase-II does not recognize its protein acceptor directly but requires Rab to complex with REP (Rab escort protein) before prenylation can occur. These enzymes are found exclusively in eukaryotes.
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