Computational identification of interplay between phosphorylation and O-ß-glycosylation of human occludin as potential mechanism to impair hepatitis C virus entry.

Hepatitis C virus (HCV) is one of the leading causes of liver diseases. Several host factors that facilitate the attachment and entry of HCV have been discovered, of which human occludin seems to be the most promising. Studies have shown that activity of occludin is dependent upon its phosphorylation status, and that during HCV infection deregulation of phosphorylated occludin collectively leads to a reduction in tight junction (TJ) integrity of hepatocytes and favors HCV entry. However, detailed information of the posttranslational modifications (PTMs) of occludin still remains largely unknown. In addition to phosphorylation, serine/threonine residues of several proteins are also regulated by a unique type of modification known as O-ß-glycosylation and this crosstalk serves as a functional switch. To identify the O-ß-glycosylation potential and how interplay between phosphorylation and O-ß-glycosylation can be exploited for the inhibition of HCV entry, here we report a computational analysis of PTMs of human occludin. Several conserved phosphorylation residues and kinases that can alter the ability of occludin to regulate the integrity of TJs were identified. In addition to previously reported Tyr residues, two additional Tyr residues (Tyr29 and Tyr287) were identified as target sites of Src kinase. To our knowledge, this is the first study to report the O-ß-GlcNAc potential of occludin and target sites of ERK (Ser8, Ser310, and Thr345), GSK-3 (Ser8, Ser341) and Cdk5 (Thr376). Furthermore, based on findings from this study, a potential novel interplay between phosphorylation and O-ß-glycosylation at the two Yin Yang sites (Ser408 and Ser490) is also proposed.