Three extremely thermostable proteins from Sulfolobus and a reappraisal of the 'traffic rules'.

Three cytosolic enzymes from the extremely thermoacidophilic archaeon Sulfolobus acidocaldarius (DSM 639) have been investigated: adenylate kinase, pyrophosphatase and superoxide dismutase. The latter was isolated from S. acidocaldarius cells, the others were heterologically overproduced in Escherichia coli. Long-term thermostability, flexibility, catalytic activity, and thermal denaturation were investigated by biochemical and physical methods. Superoxide dismutase is hyperthermostable over several days. The other enzymes have Tm values between 87 degrees C - 98 degrees C depending on conditions and reveal long-term stability in the range of hours. On the basis of sequence alignments, core structures were defined and compared to mesophilic homologues selected by growth temperature of organisms from 25 degrees C to 88 degrees C. The data set confirms none of the simple sequence based 'traffic rules' previously proposed by others. Some aspects of thermostability based on molecular modeling studies are discussed which remain to be proved by the 3D structures. All three enzymes could be crystallized.