WNK1, a novel mammalian serine/threonine protein kinase lacking the catalytic lysine in subdomain II

We have cloned and characterized a novel mammalian serine/threonine protein kinase WNK1 ((w) under bar ith (n) under bar o lysine (K) under bar)) from a rat brain cDNA library. WNK1 has 2126 amino acids and can be detected as a protein of similar to 230 kDa in various cell lines and rat tissues. WNK 1 contains a small N-terminal domain followed by the kinase domain and a lo ng C-terminal tail. The WNK1 kinase domain has the greatest similarity to t he MEKK protein kinase family. However, overexpression of WNK1 in HEK293 ce lls exerts no detectable effect on the activity of known, co transfected mi togen-activated protein kinases, suggesting that it belongs to a distinct p athway. WNK1 phosphorylates the exogenous substrate myelin basic protein as well as itself mostly on serine residues, confirming that it is a serine/t hreonine protein kinase. The demonstration of activity was striking because WNK1, and its homologs in other organisms lack the invariant catalytic lys ine in subdomain II of protein kinases that is crucial for binding to ATP. A model of WNK1 using the structure of cAMP-dependent protein kinase sugges ts that lysine 233 in kinase subdomain I may provide this function. Mutatio n of this lysine residue to methionine eliminates WNK1 activity, consistent with the conclusion that it is required for catalysis. This distinct organ ization of catalytic residues indicates that WNK1 belongs to a novel family of serine/threonine protein kinases.