Comprehensive analysis of CXXX sequence space reveals that Saccharomyces cerevisiae GGTase-I mainly relies on a2X substrate determinants

Many proteins undergo a post-translational lipid attachment, which increases their hydrophobicity, thus strengthening their membrane association properties or aiding in protein interactions. Geranylgeranyltransferase-I (GGTase-I) is an enzyme involved in a 3-step post-translational modification (PTM) pathway that attaches a 20-carbon lipid group called geranylgeranyl at the carboxy-terminal cysteine of proteins ending in a canonical CaaL motif (C-cysteine, a-aliphatic, L-often leucine, but can be phenylalanine, isoleucine, methionine, or valine). Genetic approaches involving 2 distinct reporters were employed in this study to assess Saccharomyces cerevisiae GGTase-I specificity, for which limited data exist, toward all 8,000 CXXX combinations. Orthogonal biochemical analyses and structure-based alignments were also performed to better understand the features required for optimal target interaction. These approaches indicate that yeast GGTase-I best modifies the Cxa[L/F/I/M/V] sequence that resembles but is not an exact match for the canonical CaaL motif. We also observed that minor modification of noncanonical sequences is possible. A consistent feature associated with well-modified sequences was the presence of a nonpolar a2 residue and a hydrophobic terminal residue, which are features recognized by mammalian GGTase-I. These results thus support that mammalian and yeast GGTase-I exhibit considerable shared specificity.