Conserved arginine and aspartate residues are critical for function of MjNhaP1, a Na+/H+ antiporter of M. jannaschii.

Recently MjNhaP1 was identified as a pH-regulated Na(+)/H(+) antiporter of Methanococcus jannaschii [Hellmer, J. et al. (2002) FEBS Lett. 527, 245-249]. The antiporter is active at pH 6.0 and displays continuously decreasing activity towards alkaline pH. We have performed a site-directed mutagenesis study on all histidines as well as on conserved Asp, Glu and Arg residues of MjNhaP1, and analyzed the mutated proteins for activity. The mutants fall into three classes, i.e. normally active mutants, mutants with intermediate activity and mutants which are completely inactive. None of the histidine residues appears to be essential unlike in the bacterial proteins. The results point at an important role of a number of aspartate and arginine residues.