PMID: 25187573

 

    Legend: Sugar

Title : The O-glycomap of lubricin , a novel mucin responsible for joint lubrication, identified by site-specific glycopeptide analysis

Abstract :
  1. The lubricative, heavily glycosylated mucin-like synovial glycoprotein lubricin has previously been observed to contain glycosylation changes related to rheumatoid and osteoarthritis
  2. Thus, a site-specific investigation of the glycosylation of lubricin was undertaken, in order to further understand the pathological mechanisms involved in these diseases
  3. Lubricin contains an serine/threonine/proline ( STP )-rich domain composed of imperfect tandem repeats (EPAPTTPK), the target for O-glycosylation
  4. In this study, using a liquid chromatography-tandem mass spectrometry approach, employing both collision-induced and electron-transfer dissociation fragmentation methods, we identified 185 O-glycopeptides within the STP-rich domain of human synovial lubricin
  5. This showed that adjacent threonine residues within the central STP-rich region could be simultaneously and/or individually glycosylated
  6. In addition to core 1 structures responsible for biolubrication, core 2 O-glycopeptides were also identified, indicating that lubricin glycosylation may have other roles
  7. Investigation of the expression of polypeptide N-acetylgalactosaminyltransferase genes was carried out using cultured primary fibroblast-like synoviocytes, a cell type that expresses lubricin in vivo
  8. This analysis showed high mRNA expression levels of the less understood polypeptide N-acetylgalactosaminyltransferase 15 and 5 in addition to the ubiquitously expressed polypeptide N-acetylgalactosaminyltransferase 1 and 2 genes
  9. This suggests that there is a unique combination of transferase genes important for the O-glycosylation of lubricin
  10. The site-specific glycopeptide analysis covered 82% of the protein sequence and showed that lubricin glycosylation displays both micro- and macroheterogeneity
  11. The density of glycosylation was shown to be high: 168 sites of O-glycosylation, predominately sialylated, were identified
  12. These glycosylation sites were focused in the central STP-rich region , giving the domain a negative charge
  13. The more positively charged lysine and arginine residues in the N and C termini suggest that synovial lubricin exists as an amphoteric molecule
  14. The identification of these unique properties of lubricin may provide insight into the important low-friction lubricating functions of lubricin during natural joint movement