CHARACTERIZATION OF A MUTANT RECOMBINANT S100 PROTEIN USING ELECTROSPRAY-IONIZATION MASS-SPECTROMETRY

Two recombinant proteins derived by thrombin cleavage of a fusion prot ein between glutathione-S-transferase and CP10 (Chemotactic protein 10 kDa) were separated by C4 reversed-phase high-performance liquid chro matography (RP-HPLC). Both proteins were recognised by a polyclonal an tibody to native CP10 following sodium dodecyl sulphate/polyacryamide gel electrophoresis (SDS/PAGE) and Western blotting. The major form (s imilar to 90%) had a mass of 10 308 Da, by electrospray mass spectrome try (ESI-MS), which compared well with the theoretical mass of rCP10 ( 10 307.6 Da) whereas the minor component (similar to 10%) had a mass o f 11 333 Da, 1025 mass units greater than expected. One sequence was o btained by N-terminal sequencing, suggesting that the N-terminus was n ot modified. The mass of peptides isolated after Asp-N digestion and C 18 RP-HPLC were determined by ESI-MS and each assigned a probable sequ ence based on the expected peptide map of rCP10. The mutant protein pr oduced one additional peak at 10.0 min with mass 1639 Da and the seque nce DSHKEQQRGIPGNSS by Edman degradation. The first 5 amino acids corr esponded to the last 5 C-terminal amino acids of rCP10. Analysis of th e cDNA sequence of the expression vector used to produce rCP10 indicat ed that the 10 additional C-terminal amino acids were translated after the insertion of glutamine at the normal TAG stop codon. Another stop codon (TGA) located 27 base pairs downstream halts translation. The c alculated mass of the mutant protein is 11 332.7 Da, in good agreement with the experimental mass. Readthrough occurs in strains of E.coli ( eg JPA101) with the amber mutation supE, and this allowed substitution of glutamine at TAG codons in similar to 5-10% of transcripts. (C) 19 97 by John Wiley & Sons, Ltd.