Large-scale analysis of phosphorylation site occupancy in eukaryotic proteins

Many recent high throughput technologies have enabled large-scale discoveries of new phosphorylation sites and phosphoproteins. Although they have provided a number of insights into protein phosphorylation and the related processes, an inclusive analysis on the nature of phosphorylated sites in proteins is currently lacking. We have therefore analyzed the occurrence and occupancy of phosphorylated sites (~100,281) in a large set of eukaryotic proteins (~22,995). Phosphorylation probability was found to be much higher in both the termini of protein sequences and this is much pronounced in transmembrane proteins. A large proportion (51.3%) of occupied sites had a nearby phosphorylation within a distance of 10 amino acids; however, this proportion is very high compared to the expected one (16.9%). The distribution of phosphorylated sites in proteins showed a strong deviation from the expected maximum randomness. An analysis of phosphorylation motifs indicated that just 40 motifs and a much lower number of associated kinases might account for nearly 50% of the known phosphorylations in eukaryotic proteins. Our results provide a broad picture of the phosphorylation sites in eukaryotic proteins.