A synthetic peptidic substrate of minimal size and semioptimal sequence for the protein tyrosine kinase pp60c-src.

We used a novel approach to determine the minimal size and semioptimal sequence of a peptide to serve as an inhibitor and/or substrate for the protein tyrosine kinase pp60c-src. The preferred amino acids surrounding tyrosine were determined by a systematic study in which we increased the length of a series of linear peptides starting from the tripeptide EYG. Using an iterative cycle, the size of the peptide was increased one residue at a time, first at the amino terminus and then at the carboxy terminus. A series of six analogs were synthesized at each position and assayed as inhibitors and substrates. The amino acids G, A, L, F, E, and K were used to semioptimize each position. The tripeptide EYG was not a substrate nor an efficient inhibitor. With increasing size of the peptide, the Ki decreased from 10.0 to 0.10 mM. The smallest peptide to serve as a substrate was a hexapeptide. The best overall peptide obtained from this method, EFEYAFF, had a Ki value of 0.13 mM with Km and Vmax values of 0.21 mM and 680 nmol/min/mg, respectively. Our best peptide was found to have higher substrate specificity than all other commerically available peptidic substrates for pp60c-src.