Dimer–tetramer assembly of nucleoside diphosphate kinase from moderately halophilic bacterium Chromohalobacter salexigens DSM3043: Both residues 134 and 136 are critical for the tetramer assembly

The subunit structure of Halomonas nucleoside diphosphate kinase (HaNDK) is a dimer, different from NDKs of other species. We have shown before that it is due to Glu134 in HaNDK, which results in steric hindrance and charge repulsion between two dimeric units and prevents further assembly into the tetramer and that changing the Glu134 to neutral Ala results in formation of a stable tetramer. To our surprise, both wild-type NDK from moderately halophilc Chromohalobacter salexigens (CsNDK/GNE: GNE represents Gly134-Asn135-Glu136) and mutant CsNDK/ANE, both of which have a neutral amino acid at residue 134, were found to form a dimer. These constructs contain Glu136, which may also cause steric barrier and charge repulsion. A double mutant, CsNDK/ANT, having Thr at 136 resulted in stable tetrameric assembly, supporting the above notion. A mutant CsNDK/GNT reverted, however, to a dimer again, indicating that the introduced Ala residue at 134th in the double mutant generated a hydrophobic cluster consisting of the Ala residues and thereby stabilized dimer–dimer association of CsNDK assembly, while Gly destabilized it due to the loss of this cluster. Based on these observations, it is evident that both residues 134 and 136 contribute to the subunit assembly of CsNDK.