Figure 5

Structure of EcFrmR.

(a) Cartoon representations of uncross-linked (left) and cross-linked (right) EcFrmR monomers colored blue (N-terminal) to red (C-terminal). Secondary structure elements (α-helices, α1 to α3; loops, L1 and L2) are labeled and the amino acid residues (single letter code, P2, C35 and C70) involved in cross-linking and disulfide bond formation are shown as sticks. The disordered N-terminal region in the uncross-linked subunit is represented by the blue dashed line. (b) A comparison of the overall size and shape of the uncross-linked (left) and cross-linked (right) faces of the EcFrmR tetramer. The upper images show the arrangement of the helices on each face of the tetramer, the positions of the methylene bridges (P2′-C35) and the Cys70-Cys70′ disulfide bonds (S-S). The homodimer (A/B) on the uncross-linked face is drawn in shades of green and the cross-linked face (A′/B′) in shades of orange. The middle images show the expansion of the surface envelope upon cross-linking (black double headed arrow drawn between Arg14 Cα atoms, highlighted in pink). The lower images show the surface-charge on either side of the tetramer (red represents negative charge, blue positive charge and white neutral). (c) Section of the 2Fo-Fc map between chains A and A′ obtained when the coordinates for Pro2 and the methylene bridge were omitted from the refinement (black mesh, contoured at 1σ). Residues are indicated by their single letter codes.