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1 Medicine, University of Cambridge, Cambridge, United Kingdom
* To whom correspondence should be addressed. E-mail: kmo22{at}medschl.cam.ac.uk.
The novel Serine/Threonine kinases (With No Lysine kinases or WNKs), WNK1 and WNK4, are encoded by the disease genes for Gordon syndrome (PRKWNK1 and PRKWNK4), a rare monogenic syndrome of hypertension and hyperkalemia. These proteins alter the expression of the thiazide-sensitive Na/Cl co-transporter (NCCT) in Xenopus oocytes, although the details are controversial. We describe here our own experience and confirm that kinase-dead WNK4 (318D>A) is unable to affect Na+ fluxes through NCCT or its membrane expression as an ECFP-NCCT fusion protein. However, the kinase domain is not sufficient for a functional WNK4 since deletion of the acidic motif (a motif unique to WNK family members) completely abolishes functional activity. Indeed, the N-terminal of WNK4 (1-620) containing the kinase domain and acidic motif retains full activity, but does not interact directly with NCCT in pull-down assays. Co-expression of WNK1 antagonises the action of WNK4, and kinase-dead WNK1 (368D>A) or WNK1 carrying a WNK4 disease mutation (565Q>E) behaves in the same way as wild-type WNK1. This suggests kinase activity and charge-conservation within the acidic motif are not essential for the WNK1-WNK4 interaction. We also report that WNK4 probably reduces surface expression largely through an effect on forward trafficking. Hence, the effect of WNK4 on NCCT expression is mimicked by dynamin, but the dominant-negative K44A dynamin mutant does not block the action of WNK4 itself. These results further highlight important differences in the mechanism by which WNK kinases affect expression of NCCT versus other membrane proteins such as ROMK.
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