MALDI in-source decay of high mass protein isoforms: application to alpha- and beta-tubulin variants.

Tubulin is one of the major targets in cancer chemotherapy and the target of more than twenty percent of the cancer chemotherapic agents. The modulation of isoform content has been hypothesized as being a cause of resistance to treatment. Isoform differences lie mostly in the C-terminus part of the protein. Extensive characterization of this polypeptide region is therefore of critical importance. MALDI-TOF fragmentation of tubulin C-terminal domains was tested using synthetic peptides. Then, isotypes from HeLa cells were successfully characterized for the first time by in-source decay (ISD) fragmentation of their C-terminus coupled to a pseudo MS(3) technique named T(3)-sequencing. The fragmentation occurred in-source, preferentially generating y(n)-series ions. This approach required guanidination for the characterization of the beta(III)-tubulin C-terminus peptide. This study is, to our knowledge, the first example of reflectron in-source decay (reISD) of the C-terminus of a 50 kDa protein. This potentially occurs via a CID-like mechanism occurring in the MALDI plume. There are now new avenues for top-down characterization of important clinical biomarkers such as beta(III)-tubulin isotypes, a potential marker of drug resistance and tumor progression. This paper raises the challenge of protein isotypes characterization for early cancer detection and treatment monitoring.