Isopeptide linkage between N-alpha-monomethylalanine and lysine in ribosomal protein S11 from Escherichia coli.

Protein S11 from the Escherichia coli ribosome has a unique NH2-terminal structure not previously observed among ribosomal proteins. Owing to the formation of an isopeptide bond between a secondary amino acid (N-alpha-monomethylalanine) and the epsilon-amino group of the NH2-terminal lysine residue, a "branching point" is formed. Therefore, two amino acids are seen when the NH2 terminus of the protein is determined.