Identification of methylated amino acids during sequence analysis. Application to the Escherichia coli ribosomal protein L11.

Three methylated amino acid residues, one residue of N-trimethylalanine and two of N epsilon,N epsilon,N epsilon-trimethyllysine residues, are present in protein L11. The methods used for the identification and location of these unusual amino acids in the sequence of protein L11 are described. Temperature and pH modifications to the eluting buffers enabled the detection of the methylated derivatives of lysine and arginine with a Durrum analyser using routine 90 min amino acid analyses. The presence of N epsilon,N epsilon-dimethyllysine in the hydrolysate of proteins, was revealed by ascending chromatography on thin-layer cellulose plates. The blocked N-terminal amino acid of protein L11, N,N,N-trimethylalanine, although non-volatile, was identified by field desorption mass spectrometry. The identification was confirmed by comparing the N-terminal dipeptide of protein L11 with the synthesised dipeptide Me3Ala-Lys. The behaviour of these methylated amino acids during sequence analysis is described.