Influence of metal ions on substrate binding and catalytic activity of mammalian protein geranylgeranyltransferase type-I

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Abstract
Protein geranylgeranyltransferase type-I (GGTase-I) transfers a geranylgeranyl group from the prenyl donor geranylgeranyl diphosphate (GGPP) to the cysteine residue of substrate proteins containing a C-terminal CaaX-motif (a sequence motif of proteins consisting of an invariant Cys residue fourth from the C-terminus). The GGTase-I heterodimer contains one atom of zinc, and this metal is required for enzyme activity. In this regard, GGTase-I is similar to the related enzyme protein farnesyltransferase (FTase); the latter enzyme also requires Mg2+ for activity. The current studies were undertaken in an attempt to explore further the role of bivalent metal ions in the activity of GGTase-I. Surprisingly, we found that GGTase-I and FTase have different metal requirements. Specifically, in marked contrast to FTase, GGTase-I does not require Mg2+ for activity. Direct binding assays, including a novel fluorescence-based technique, were employed to obtain quantitative information on the interaction of substrates with GGTase-I. Using these assays, we demonstrate that the Zn2+ in GGTase-I is required for peptide, but not for isoprenoid, substrate binding. Moreover, binding of GGPP protects GGTase-I from inactivation by zinc-chelating reagents; this protective effect is not seen with binding of peptide substrates. Metal substitution studies show that the Zn2+ in GGTase-I can be replaced by Cd2+, and that the Cd form of GGTase-I has altered specificity with regard to utilization of both peptide and isoprenoid substrates. The significance of these findings in relation to proposed mechanisms for the GGTase-I reaction is discussed.