Characterization of the in vivo forms of lacrimal-specific proline-rich proteins in human tear fluid

The tear film is complex and is rich in both peptides and proteins. Physiological factors have been shown to alter the balance of the protein components in the tear film, however, little is known of the precise stimuli that initiate these changes, or their nature and extent. Attention has been directed at the role of tear proteins in the protection of the external ocular surface, and their potential role in the pathogenesis of inflammatory and autoimmune diseases, but few lacrimal-specific proteins have been identified and demonstrated to offer a protective function at the ocular surface. The biological importance of proline-rich proteins is uncertain, although there is some evidence to indicate a potential antimicrobial function for these proteins in saliva. Despite the detection of mRNA for proline-rich proteins in lacrimal gland, the translated protein product has not been detected in tear fluid. In this study we investigate the presence of proline-rich proteins in the tear film. Human reflex tear fluid was examined by matrix-assisted laser desorption/ionization-time of flight mass spectrometry directly, and following size exclusion high performance liquid chromatography. This revealed significant levels of a truncated form of lacrimal proline-rich protein, and a series of peptides derived the C-terminus of this protein. None of these had previously been identified in tear. Our study highlights the dangers inherent in proteomic strategies that assign an identity to a protein based on limited coverage of tryptic peptides.