YMXM motifs of IRS-1 define substrate specificity of the insulin receptor kinase.

Of 34 tyrosine residues in insulin receptor substrate 1 (IRS-1), 14 are adjacent to acidic residues, suggesting that they might be phosphorylation sites. Synthetic peptides corresponding to sequences surrounding these tyrosines were used as substrates of the insulin receptor kinase. Surprisingly six of these, each within YMXM motifs, were phosphorylated with greatest efficiency (Km, 24-92 microM; kcat/Km, 0.6-2.1 x 10(4) M-1.sec-1). Substituted YMXM peptides revealed a strong preference of the insulin receptor kinase for methionine at Y + 1 and Y + 3 positions. When phosphorylated, related YMXM sequences are recognition motifs for binding to proteins with src-homology (SH2) domains. The combined hydrophobic and flexible nature of methionine side chains adjacent to the targeted tyrosines provides a versatile contact for recognition by diverse proteins involved in signal transduction.