Post-translation modification of proteins in tears.

This is the first 2-DE study using sequential dyes to analyse phospho-, glyco- and total tear protein profiles (Pro-Q Diamond for phosphoprotein, Pro-Q Emerald for glycoprotein and Sypro Ruby for total protein). This method minimised the gel-gel variations, allowing better comparisons among the three profiles and generated a whole map of PTM profiles of tear protein. A novel tear protein, dermcidin, was identified for the first time in this study. The identification of this antimicrobial protein suggests a new model of defence in tears. In addition, we are able to present the first experimental evidence of the presence of glycosylated lipocalin 1 and cystatin S. Nucleobindin 2 was only detected using phospho staining, suggesting it is only phosphorylated in tears. This study provides the groundwork for understanding the PTM of tear proteins and consequently these methods could be useful in the search for biomarkers in tears.