Crystal Structure of Catalytic Domain of the Initiation Factor 2B Epsilon Subunit from Saccharomyces cerevisiae by Thomas Boesen1, Graham Pavitt2, and Gregers Rom Andersen1* 1Department of Molecular Biology, University of Aarhus, Gustav Wieds Vej 10C, DK-8000 Århus C, Denmark 2Department of Biomolecular Sciences, UMIST, PO Box 88, Manchester, M60 1QD UK *To whom correspondence should be addressed: firstname.lastname@example.org, Tel: (+45) 8942 5024. Fax: (+45) 8612 3178 Abstract The crystal structure of the C-terminal catalytic domain of the initiation factor 2B epsilon subunit, residues 544-704, from Saccharomyces cerevisiae has been determined to 2.3 Å resolution by the multiple wavelength anomalous dispersion technique using selenomethionine substituted protein. The structure consists of two four helix bundles. The N-terminal four helix bundle contain the catalytic part of the domain, whereas the C-terminal domain harbors the two aa-box motifs involved in binding to the N-terminal part of the initiation factor 2 β subunit. Aliphatic residues in the aa-box motifs are involved in specific contacts in the hydrophobic core of the C-terminal bundle important for maintaining the overall structure, whereas, acidic residues in the motifs form a surface exposed acidic patch which might interact with the lysine boxes of initiation factor 2 β. Interestingly, tryptophan 699 was found to be solvent exposed and involved in crystal packing. This residue could possibly be important for the specific interaction with initiation factor 2 β. Furthermore, the structure shows the location of residues important for catalytic function and disease.