Evolutionary Conservation of Human Phosphorylation Sites

The primary objective of this work was to identify those human phosphorylation sites (phosphosites) that are highly conserved in other species as this may reveal functionally important phosphosites. We wondered whether human phosphosites (e.g. tyrosine, serine or threonine) that are known to be activatory upon phosphorylation, are commonly replaced by a glutamic or aspartic acid residues in other species. This type of alteration might mimic constitutive phosphorylation of cognate proteins in other species, which would indicate that phosphorylation of these sites in humans may confer functionality. To investigate this, we developed an algorithm to identify ortholog proteins in different species for each given human phospho-protein and also predict phosphosites in every extracted ortholog cognate protein. The results demonstrate that relatively few human phosphosites are highly conserved; for instance from about 90,000 human phosphosites, about 75% of these were conserved in mammals, but less than 16% were detected in most model organisms. These extremely well conserved phosphosites did not display any increased preponderance for acidic amino acid substitutions. However, we observed that the most conserved functional phosphosites occurred on threonine phosphosites that were found in protein kinases and these were 8-times more likely to be stimulatory than inhibitory.