Structural characterization of monomeric folding intermediates of recombinant human macrophage-colony stimulating factor beta (rhM-CSFbeta) by chemical trapping, chromatographic separation and mass spectrometric peptide mapping

"De-novo" amino acid sequence elucidation of protein G'e by combined "top-down" and "bottom-up" mass spectrometry

Mass spectrometric de-novo sequencing was applied to review the amino acid sequence of a commercially available recombinant protein G´ with great scientific and economic importance. Substantial deviations to the published amino acid sequence (Uniprot Q54181) were found by the presence of 46 additional amino acids at the N-terminus, including a so-called "His-tag" as well as an N-terminal partial α-N-gluconoylation and α-N-phosphogluconoylation, respectively. The unexpected amino acid sequence of the commercial protein G' comprised 241 amino acids and resulted in a molecular mass of 25,998.9 ± 0.2 Da for the unmodified protein. Due to the higher mass that is caused by its extended amino acid sequence compared with the original protein G' (185 amino acids), we named this protein "protein G'e." By means of mass spectrometric peptide mapping, the suggested amino acid sequence, as well as the N-terminal partial α-N-gluconoylations, was confirmed with 100% sequence coverage. After the protein G'e sequence was determined, we were able to determine the expression vector pET-28b from Novagen with the Xho I restriction enzyme cleavage site as the best option that was used for cloning and expressing the recombinant protein G'e in E. coli. A dissociation constant (K(d)) value of 9.4 nM for protein G'e was determined thermophoretically, showing that the N-terminal flanking sequence extension did not cause significant changes in the binding affinity to immunoglobulins.

High detection sensitivity achieved with cryogenic detectors in combination with matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry

Cryogenic detectors directly measure the impact energy of any impinging particle independent of its velocity. Thus a very high, mass-independent, detection efficiency is expected from their application in TOF-MS. The cryogenic detector applied here is based on a superconducting phase-transition thermometer and was implemented in a dual reflector time-of-flight mass spectrometer (N-geometry). A dilution series using standard sample preparation procedures shows that the detection limit for insulin (Mr: 5,734) can be decreased by several orders of magnitude, down to 0.5 amol on the MALDI target. Detection limits for rhM-CSF beta (Mr: 49,032) and for polyclonal IgG (Mr: ca 150,000) in the high femtomole and low picomole range, respectively, were established.