Role of alternative phosphorylation and O-glycosylation of erythropoietin receptor in modulating its function: an in silico study

Detailed knowledge of the three-dimensional (3D) structure of a protein is essential for the proper understanding of its function(s) that could be modified through posttranslational modifications (PTMs). Among these PTMs, alterations of serine/threonine residues of a protein through phosphorylation and O-glycosylation are extremely dynamic and could modulate the functions of a protein by affecting their 3D structure. Potential of a protein for certain PTMs could be evaluated through computer-based methods. Erythropoietin (EPO) is a multifunctional protein that primarily regulates red blood cell production and is also involved in other nonhematopoietic functions; for example, EPO also has cardioprotective and neuroprotective effects. In this study, multifunctional EPO behavior has been revealed based on transient modifications of its receptor. In this study, PTMs of erythropoietin receptor (EPO-R) were predicted using neural network tools, and the possible effects of these modifications are suggested. Phosphorylation and O-glycosylation at serine 380 and 444 of the cytoplasmic domain of EPO-R seem to have an antagonistic role in controlling signaling events induced by EPO. O-glycosylation at threonine 423 might hinder beta-TrCP (a ubiquitin ligase) binding, which ubiquitinates at K 428, and ultimately results in the recycling of EPO-R, thus increasing EPO sensitivity. In contrast, the phosphorylated form of the same residue inhibits the recycling of EPO-R and thereby decreases the EPO sensitivity. Additionally, the interplay of O-glycosylation modification at serine 478 and phosphorylation at tyrosine 479 might help in controlling the duration of EPO-induced signaling.