The first crystal structure of an archaeal helical repeat protein

The crystal structure of ST1625p, a protein encoded by a hypothetical open reading frame ST1625 in the genome of the hyperthermophilic archaeon Sulfolobus tokodaii , was determined at 2.2 Å resolution. The only sequence similarity exhibited by the amino‐acid sequence of ST1625p was a 33% identity with the sequence of SSO0983p from S. solfataricus . The 19 kDa monomeric protein was observed to consist of a right‐handed superhelix assembled from a tandem repeat of ten α‐helices. A structural homology search using the DALI and MATRAS algorithms indicates that this protein can be classified as a helical repeat protein.