Characterizing the Evolutionary Dynamics of Protein Phosphorylation Sites for Functional Phospho-proteomics

Protein phosphorylation is a prevalent reversible post-translational modification that influences protein functions. The advent of phospho-proteomic technologies now enables proteome-wide quantitative detection of residues phosphorylated under different physiological conditions. The functional consequences of the majority of these phosphorylation events are unknown. This calls for endeavors to characterize their molecular functions and cellular effects. This can be facilitated by systematic approaches to categorize phosphorylation events, interpret their importance and infer their functions. I carried out comparative, evolutionary and integrative analyses on in vivo phosphorylation events to address these challenges. First, I performed cross-species comparative phospho-proteomic analysis to identify evolutionarily conserved phosphorylation events in human. A sequence alignment approach was used to identify phosphorylation events conserved at similar sequence positions across orthologous proteins and a network alignment approach was applied to identify potential evolutionarily conserved kinase-substrate interactions. Conserved human phosphoproteins identified are found enriched for proteins encoded by known cancer- and disease-associated genes. Next, I developed a new approach to analyze the sequence conservation of known phosphorylated residues on human, mouse and yeast proteins that factored in the background mutational rates of protein and phosphorylatable residue. Furthermore, sites were analyzed according to (i) characterized functions, (ii) prevalence, (iii) stoichiometry, their occurrence in (iv) structurally disordered/ordered protein regions, in (v) proteins of various abundance and in (vi) proteins with different protein interaction propensity to identify the factors influencing sequence conservation of phosphorylated residues. Importantly, my analysis suggests that false positives and randomly phosphorylated residues are present in existing phosphorylation datasets and they are more common on high abundance proteins. Lastly, I characterized the theoretical maximum phosphorylation capacity in terms of phosphorylatable residues and discovered that genomic tyrosine frequency correlates negatively and significantly with tyrosine kinase frequency and cell type in metazoan. This observation suggests that fidelity of phosphotyrosine signaling occurred partially through global tyrosine depletion.Ph