Identification of tyrosine sulfation in extracellular leucine-rich repeat proteins using mass spectrometry

Multiple and variable tyrosine sulfation in extracellular class II leucine-rich repeat proteins/proteoglycans were characterized by mass spectrometry. The sulfogroup on tyrosine is labile and is released from peptides under normal mass spectrometric conditions. Thus, special approaches must be considered in order to identify this modification. By using a combination of mass spectrometry studies operating in negative and positive ion mode, tyrosine sulfation could be identified. In positive mode, the peptides normally appeared non-sulfated, whereas in negative mode a mixture of sulfated and non-sulfated species was observed. A combination of peptides released by different proteinases was used to obtain details on the locations of sulfate groups. Multiple tyrosine sulfates were observed in the N-terminal region of fibromodulin (up to 9 sites), osteoadherin (up to 6 sites), and lumican (2 sites). Osteoadherin contains two additional sulfated tyrosine residues close to its C terminus. We also identified an error in the published sequence of bovine fibromodulin, resulting in the replacement of Thr37 by Tyr37-Gly38, thus increasing its homology with its human counterpart.