STRUCTURE OF TRANSLATION INITIATION-FACTOR 5A FROM PYROBACULUM-AEROPHILUM AT 1.75 ANGSTROM RESOLUTION

Background: Translation initiation factor 5A (IF-5A) is reported to be involved in the first step of peptide bond formation in translation, to be involved in cell-cycle regulation and to be a cofactor for the R ev and Rex transactivator proteins of human immunodeficiency virus-1 a nd T-cell leukemia virus I, respectively. IF-5A contains an unusual am ino acid, hypusine (N-epsilon-(4-aminobutyl-2-hydroxy)lysine), that is required for its function. The first step in the post-translational m odification of lysine to hypusine is catalyzed by the enzyme deoxyhypu sine synthase, the structure of which has been published recently. Res ults: IF-5A from the archebacterium Pyrobaculum aerophilum has been he terologously expressed in Escherichia coli with selenomethionine subst itution. The crystal structure of IF-5A has been determined by multiwa velength anomalous diffraction and refined to 1.75 Angstrom. Unmodifie d P. aerophilum IF-5A is found to be a beta structure with two domains and three separate hydrophobic cores. Conclusions: The lysine (Lys42) that is post-translationally modified by deoxyhypusine synthase is fo und at one end of the IF-5A molecule in a turn between beta strands be ta 4 and beta 5; this lysine residue is freely solvent accessible, The C-terminal domain is found to be homologous to the cold-shock protein CspA of E. coli, which has a well characterized RNA-binding fold, sug gesting that IF-5A is involved in RNA binding.