Obviously, the existing prediction methods are not reliable and must not be trusted. Perhaps searching for a single consensus motif is futile, since two enzymes are responsible for Tyr sulfation, and they may have different preferences. Or perhaps, the tertiary structure of the protein modified is also a determinant factor. We do not know, and without acquiring more, reliable information one cannot tell.
Unfortunately, Tyr sulfation presently does not belong to those PTMs that can be studied in a high-throughput manner. First, a selective and sensitive enrichment method has to be developed. Then, the resulting mixtures could be used to develop/fine-tune mass spectrometric workflows for efficient and reliable sulfopeptide detection and characterization, including unambiguous site assignments. We think, presently, HCD/ETD analysis or chemical derivatiza- tion and HCD analysis seem to be the most sensitive solutions, compatible with nano-HPLC fractionation. But we predict that the future most certainly will offer better alternatives, and then our knowledge on protein sulfation and its biological function will further expand.
KFM was supported by the NIH grant NIGMS 8P41GM103481, the Howard Hughes Medical Institute, and the Miriam and Sheldon Adelson Medical Research Foundation (to the Bioorganic Biomedical Mass Spectrometry Resource at UCSF, director A.L. Burlingame). All authors were also supported by the Hungarian Scientific Research Fund Grant OTKA #105611. The Janos Bolyai Fellowship to EK is gratefully acknowledged.