Abstract
During translation initiation, the heterotrimeric archaeal translation initiation factor 2 (aIF2) recruits the initiator tRNA(i) to the small ribosomal subunit. In the stationary growth phase and/or during nutrient stress, Sulfolobus solfataricus aIF2 has a second function: It protects leaderless mRNAs against degradation by binding to their 5'-ends. The S. solfataricus protein Sso2509 is a translation recovery factor (Trf) that interacts with aIF2 and is responsible for the release of aIF2 from bound mRNAs, thereby enabling translation re-initiation. It is a member of the domain of unknown function 35 (DUF35) protein family and is conserved in Sulfolobales as well as in other archaea. Here, we present the X-ray structure of S. solfataricus Trf solved to a resolution of 1.65 Å. Trf is composed of an N-terminal rubredoxin-like domain containing a bound zinc ion and a C-terminal oligosaccharide/oligonucleotide binding fold domain. The Trf structure reveals putative mRNA binding sites in both domains. Surprisingly, the Trf protein is structurally but not sequentially very similar to proteins linked to acyl-CoA utilization-for example, the Sso2064 protein from S. solfataricus-as well as to scaffold proteins found in the acetoacetyl-CoA thiolase/high-mobility group-CoA synthase complex of the archaeon Methanothermococcus thermolithotrophicus and in a steroid side-chain-cleaving aldolase complex from the bacterium Thermomonospora curvata. This suggests that members of the DUF35 protein family are able to act as scaffolding and binding proteins in a wide variety of biological processes.