Secondary and quaternary structural transition of the halophilic archaeon nucleoside diphosphate kinase under high- and low-salt conditions

Most halophilic enzymes from extremely halophilic archaea are denatured immediately after transfer from high-salt to low-salt medium. However, nucleoside diphosphate kinase (HsNDK) from the extremely halophilic archaeon Halobacterium salinarum seems to be exceptional, since the enzyme exhibited catalytic activity even under the low-salt condition. Here we show the mechanism how HsNDK is active under both high- and low-salt conditions that the HsNDK hexamer in high-salt medium dissociates into a dimer in the low-salt medium without denaturation. The observed change of the subunit structure was accompanied by a large decrease of alpha-helical content and lowered thermal sensitivity, yet keeping the conformations. This novel hexamer to,dimer conversion under high- and low-salt conditions, respectively, seems to be the mechanism by which HsNDK is avoided from the irreversible denaturation. (C) 2002 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.