Global Phosphoproteomic Analysis of Daphnia pulex Reveals Evolutionary Conservation of Ser/Thr/Tyr Phosphorylation

Reversible protein phosphorylations of serine, threonine, and tyrosine are critical processes in organisms ranging from prokaryotes to eukaryotes. Water fleas (Daphnids) have been used widely in ecologic and ecotoxicological studies, with more than 80% of ecotoxicological publications over the last 10 years involving planktonic genera, including Daphnia. However, the substrate proteins and the functions of phosphorylation in Daphnia remain largely unknown. Here, we report the first global screening of phosphoproteins and their sites of phosphorylation in D. pulex. We identified 103 phosphorylation sites in 91 Daphnia proteins by phosphopeptide enrichment using titanium dioxide isolation technology and an online two-dimensional liquid chromatography (2D-LC) system supported by high accuracy mass spectrometry. The identified Serine/threonine/tyrosine phosphorylation sites showed enrichment in the unstructured regions. Using Gene Ontology analysis, phosphorylated proteins were identified mainly as membrane proteins with essential biological roles such as protein binding, catalytic activity and nucleotide binding. BLASTP searching identified 21 phosphorylated sites in 20 D. pulex proteins that were evolutionally conserved between D. pulex and human. Here, we report the phosphorylation in Daphnia proteins and the predicted biological and functional roles of these phosphorylations. D. pulex might provide a promising model for examining the role of phosphorylation in biological functions.