SUMO modification of the HSF2 loop is regulated by residues neighboring the consensus site. A. Schematic presentation of the nonconserved regions between the HSF1 and HSF2 loops. Mutations I, II, and III were introduced at the shaded sites, in which the amino acids of HSF2 were replaced with the corresponding amino acids of HSF1. B. Mutation of the GPV motif (mutation III) reduces HSF2 loop sumoylation. Extracts from cells transfected with wild-type HSF2 (WT) or HSF2 mutants containing HSF1-specific residues (I, II, and III), together with His-HA-SUMO-2 (+) or empty plasmid (−), were analyzed by Western blotting (WB) using an αHSF2 antibody. Equal protein input was determined by Hsc70 levels. C. Both the deletion of G87 and a P88D mutation reduce the sumoylation of the HSF2 loop. The experiment was done as for panel B. D. Close-up depiction of the interface between the ΨKXE motif from HSF1 (silver) or HSF2 (gold) and Ubc9 (blue). In comparison with HSF2, the presence of proline in HSF1 would lead to a much stronger hydrogen bond with Y87 of Ubc9 (the distance is smaller, 2.6 versus 3.3 Å, and the angle of bonding is ideal, nearly 180°). Such tight binding might prevent Y87 from altering its position as part of a catalytic mechanism and slow the release of sumoylated product. The effect of proline on its own would not be expected to completely prevent catalysis in the case of HSF1, since proline would not affect the relative positioning of the lysine and glutamate of the SUMO consensus motif. E. The region C terminal to the motif in HSF1 relative to HSF2 would limit the ability of the loop to place the sumoylation motif in an appropriate binding position. Here, the HSF model structures were superimposed over the elements of regular secondary structure of the DBDs but not over the loop regions, in order to reveal any relative displacements of the sumoylation motif. As a consequence of the unique structural properties of G87 and P88, in HSF2 two separate strong hydrogen bonds would be formed between main-chain atoms of V89 and V38, connecting the loop region to the final β-strand of the DBD sheet, stabilizing the structure, and facilitating better coordination of the VKQE tetrapeptide within the active site of Ubc9. In comparison, these strong hydrogen bonds would be replaced in HSF1 by a single, long and weak interaction between the main-chain nitrogen of E98 and oxygen atom of V46. Thus, in HSF1, the C-terminal end of the loop would be more detached from the β-sheet and more flexible than in HSF2, hampering Ubc9 from recognizing and docking VKPE of the loop to the active site of Ubc9. Due to the sequence differences C terminal to the sumoylation motif, in the modeled structures the nonsumoylated lysine of HSF1 is displaced by >4 Å with respect to the sumoylated lysine of HSF2.