Fig. 4.

MS/MS analysis of the signal at m/z 658.3597 identifying a conjugate comprising residues 21–23 and 1037–1042 of the regulatory β subunit of PhK. A, fragmentation pattern of the cross-linked peptide. Small letters denote ions arising from amide cleavages of the peptide backbone and are color-coded with the respective peptide in the conjugate pair (green and black for residues 21–23 and 1037–1042, respectively). B, structure of the GMBS cross-link between Lys-22 and Arg-1040. The capital letters indicate ions resulting from amide cleavage of the reagent and ɛ-amine of Lys-22. C, fragmentation pattern of ions corresponding to cleavage (∼) of the intervening cross-link. D, composition of ions identifying the cross-linked peptide. Intact covalent links formed between peptides are indicated by a colon (:), and covalent links between fragments of the cross-link and peptide are indicated by a centered dot (·). For singly charged ions, it should be noted that one of the two peptide ions covalently attached to either functional group of GMBS is a neutral product of the indicated backbone amide cleavage. * and ^o, loss of ammonia and water.