Human Molecular Genetics Advance Access originally published online on July 2, 2007
Human Molecular Genetics 2007 16(20):2394-2410; doi:10.1093/hmg/ddm167
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A novel family of transmembrane proteins interacting with ß subunits of the Na,K-ATPase
1 Laboratory of Molecular Biology, Howard Hughes Medical Institute, The Rockefeller University, New York, NY 10021, USA and 2 Department of Pharmacology and Toxicology, University of Lausanne, Lausanne 1005, Switzerland
* To whom correspondence should be addressed. Tel: +1 2123277956; Fax: +1 2123277878; Email: heintz{at}rockefeller.edu
Received May 31, 2007; Revised May 31, 2007; Accepted June 26, 2007
We characterized a family consisting of four mammalian proteins of unknown function (NKAIN1, 2, 3 and 4) and a single Drosophila ortholog dNKAIN. Aside from highly conserved transmembrane domains, NKAIN proteins contain no characterized functional domains. Striking amino acid conservation in the first two transmembrane domains suggests that these proteins are likely to function within the membrane bilayer. NKAIN family members are neuronally expressed in multiple regions of the mouse brain, although their expression is not ubiquitous. We demonstrate that mouse NKAIN1 interacts with the ß1 subunit of the Na,K-ATPase, whereas Drosophila ortholog dNKAIN interacts with Nrv2.2, a Drosophila homolog of the Na,K-ATPase ß subunits. We also show that NKAIN1 can form a complex with another ß subunit-binding protein, MONaKA, when binding to the ß1 subunit of the Na,K-ATPase. Our results suggest that a complex between mammalian NKAIN1 and MONaKA is required for NKAIN function, which is carried out by a single protein, dNKAIN, in Drosophila. This hypothesis is supported by the fact that dNKAIN, but not NKAIN1, induces voltage-independent amiloride-insensitive Na+-specific conductance that can be blocked by lanthanum. Drosophila mutants with decreased dNKAIN expression due to a P-element insertion in the dNKAIN gene exhibit temperature-sensitive paralysis, a phenotype also caused by mutations in the Na,K-ATPase 
subunit and several ion channels. The neuronal expression of NKAIN proteins, their membrane localization and the temperature-sensitive paralysis of NKAIN Drosophila mutants strongly suggest that this novel protein family may be critical for neuronal function.