The small GTP-binding protein G25K and the protein K-Ras 4B contain prenyl groups (geranylgeranyl and farnesyl, respectively) that are thioether linked to a C-terminal cysteine which is methylated on its alpha-carboxyl group. These proteins, like many other prenyl proteins, also have a string of basic residues near their C-termini. A series of prenylated peptides based on the C-terminal sequences of human brain G25K and human K-Ras 4B were synthesized and analyzed for their membrane binding affinities. G25K peptides containing an N-terminal N-acetyltryptophan group were studied because their binding to membranes containing a trace of dansylated phospholipid could be detected by fluorescence resonance energy transfer. The G25K peptide lacking a prenyl group and a C-terminal methyl ester did not detectably bind to vesicles, and binding was enhanced by more than 500-fold if the peptide was geranylgeranylated. For the farnesylated peptide, methylation of the C-terminus increased membrane affinity by at least 60-fold if the vesicles contained phosphatidylserine and by 3-fold if they lacked this acidic lipid. The geranylgeranylated and methylated G25K peptide remains irreversibly attached to vesicles over several minutes only if the vesicles contain phosphatidylserine, whereas the corresponding nonmethylated or farnesylated and methylated peptides dissociate rapidly (less than a few seconds) from neutral or anionic vesicles. Farnesylation of the nonmethylated K-Ras 4B peptide enhances its affinity to vesicles containing acidic phospholipids (phosphatidylglycerol or phosphatidylserine) by 70-fold, and methylation leads to an additional dramatic (150-fold) increase in membrane affinity. Replacement of the farnesyl group by the longer geranylgeranyl group enhances membrane binding by 8-fold. Methylation of the geranylgeranylated K-Ras peptide enhances its binding to vesicles with acidic lipids by 100-fold; the corresponding enhancement with pure phosphatidylcholine vesicles (no net charge) is only 3-fold. The presence of 20% acidic phospholipid in phosphatidylcholine vesicles enhances the binding of the farnesylated and geranylgeranylated K-Ras 4B methylated peptides by 300- and 700-fold, respectively, compared to pure phosphatidylcholine vesicles. A simple model is presented to rationalize the synergistic effects of a C-terminal prenylated cysteine methyl ester combined with a polybasic peptide sequence on the binding of prenylated peptides to vesicles containing acidic phospholipids.
