The structure of a trimeric archaeal adenylate kinase

The adenylate kinase from the hyperthermophilic archaean species Sulfolobus acidocaldarius has been cloned, expressed in Escherichia coli, purified and crystallized. The crystal structure was elucidated by multiple isomorphous replacement and non-crystallographic density averaging. The structure was refined at 2.6 A (1 A=0.1 nm) resolution. The enzyme is trimeric, in contrast to previous solution measurements that suggested a dimeric structure, and in contrast to the vast majority of adenylate kinases, which are monomeric. In large parts of each subunit the chain fold resembles the known enzyme structure from eubacteria and eukaryotes although the sequence homology is negligible. Since the asymmetric unit contains two trimers with and without bound AMP at the AMP sites and with an ADP at one of the six ATP sites, the analysis shows the enzyme in several states. The conformational differences between these states resemble those of other adenylate kinases. Because of sequence homology, the structure presented provides a good model for the methanococcal adenylate kinases.