MAPPING PEPTIDE BINDING SITES OF SP & SK RECEPTORS

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Electrospray (ESI) and matrix-assisted laser desorption/ionization (NLALDI) mass spectrometry, in combination with proteolyic digestion and BPLC separation, have been used to verify the cDNA-predicted amino acid sequence of chicken type II collagen. The type II collagen molecule is composed of three identical polypeptide chains [alpha, (11) chains] intertwined in a triple helix. The single triple helical domain comprises about 96% of the amino acid sequence and consists of repeating tripeptides of the form glycine-X-Y (G-X-Y). The helical domain is flanked on both the N-terminal and C-terminal ends by short non-helical domains known as telopeptides. Themolecular weight for the unmodified chicken type 11 collagen triple helix predicted from the cDNA sequence is 294 kDa. Posttranslational modifications occur at numerous sites; the best studied are hydroxylation of prolines and lysines in the Y position of the G-X-Y sequence, glycosylation of hydroxylated lysines, and interhelical cross-linkages between lysines and hydroxylysines. Mass spectrometric analysis of this protein is very challenging because several factors produce heterogeneity. Incomplete modification at the large number of potential sites for post-translational modifications produces a variety of forms. During the solubilization of type 11 collagen from chicken sterna by pepsin digestion, the C-terminal telopeptide may be cleaved at several different points, producing additicrnal heterogeneity. The digestion may also generate "tags" from the crosslinked peptides that are attached to the main chain. Chicken type 11 collagen hasbeen heat denatured and digested with endoproteinase Lys-C. The Lys-C peptides were separated by reversed-phase FIPLC, collected and examined by MALDI-TOF MS and ESI MS. A total of 131 different peptides with molecular weights ranging from 711 Da to 14,027 Da have been identified to date. MS/MS analyses and secondary digestions have been performed to determine peptide sequence and to locate modification sites within a peptide. Our mass spectrometric analysis has identified numerous sites of hydroxylation and glycosylation and has identified several C-termini. This analysis has enabled us to confirm most of the residues predicted from the cDNA sequence and to correct a few misassignments The results show a high degree of heterogeneity in hydroxylation and glycosylation with attendant shifts in chromatographic behavior.