The reversible phosphorylation of proteins is one of the most important post-translational modifications, which is involved in varieties of cellular processes and plays a critical role in activation, inactivation and regulation of proteins. Two types of enzymes, known as protein kinase and protein phosphatase, are responsible for this reversible reaction and almost constitute about 2~4% of the genes in a typical eukaryotic genome (Moorhead et al., 2009). Protein kinase is a type of well understood enzyme which modifies other proteins by chemically adding phosphate groups to them (phosphorylation). Protein kinases have been found to be involved in varieties of cellular processes, including metabolism (Viollet and Andreelli 2011), transcription, cell cycle progression (Moniz et al., 2011), cytoskeleton rearrangement and cell movement (Huang et al., 2009), cell apoptosis (Wang, 2000), and differentiation (Taylor et al., 2011). Contrary to phosphorylation, dephosphorylation is catalyzed by protein phosphatase through a way of removing a phosphate group from its substrate. Protein phosphatases contain two groups, PSP and PTP, which are responsible for the ser/thr dephosphorylation and tyr dephosphorylation respectively. Like protein kinases, protein phosphatases also play an important role in a lot of cellular processes, including proliferation (Zhang et al., 2012), differentiation, cell adhesion (Bessette et al., 2008), motility and cell death (Gallego et al., 2005). As the critical roles in biological processes, abnormal level of phosphorylation or dephosphorylation is implicated in diseases, such as myotonic muscular dystrophy, hirschsprungis disease, leprechaunism, diabetes and so on (Cohen, 2001).

    Although the studies on phosphorylation and dephosphorylation last for a long time, and many related resources have been developed, however, a comprehensive classification of all protein kinases and protein phosphatases and how many potential two types of enzymes exist in usual and rare species still remains not to be available. In this work, we have collected 347 protein phosphatase data from literatures, integrated and curated five protein kinase complements of H. sapiens, M. musculus, D. melanogaster, C. elegans and S. cerevisiae from KinBase with a total number of 1,855. The data are further classified into 10 groups and 33 families for protein phosphatase, 10 groups and 149 families for protein kinase, respectively. The protein kinase and protein phosphatase identification in 84 eukaryotic species is based on the Hidden Markov Model (HMM) of their catalytic domains (Eddy, 1998). Currently, the Eukaryotic Kinase and Phosphatase Database -- EKPD is the most comprehensive resource to get protein kinases and protein phosphatases information in eukaryotic organisms.

    The EKPD 1.1 was lastly updated on Sep 10th, 2013, containing 61,729 unique protein entries including 50,433 protein kinases and 11,296 protein phosphatases. The online service of EKPD was implemented in PHP + MySQL + JavaScript. The database will be updated routinely as new protein kinases or protein phosphatases are reported.