Supplementary Figure 5: SLC26Dg-Nb5776 interactions.
From: Structure of a prokaryotic fumarate transporter reveals the architecture of the SLC26 family
(a) Ribbon representation of the nanobody Nb5776 (blue) and the STAS domain of SLC26Dg (red). Both proteins interact via a parallel β-sheet formed by β5 of Nb5776 (containing the variable region CDR2) and β4 of the STAS domain and via side-chain interactions with a long loop of the nanobody connecting β-strands 8 and 9 (part of the variable region CDR3, *) that has changed its conformation upon binding to the transport protein. (b) Side-chain interactions between the Nb5776 and the STAS domain of SLC26Dg. (c) Ribbon representation of the Nb5776 structure. (d) Superposition of Cα-traces of the isolated Nb5776 (beige) and of its structure observed in the complex (blue). The difference in the conformation of the CDR3 region is apparent. (e) Stereo view of 2Fo-Fc density (calculated at 2.4 Å and contoured at 1σ) of a Nb5776 crystal superimposed on the structure. An asterisk indicates the region of CDR3 that has changed its conformation upon binding to SLC26Dg. (f) Interactions of the nanobody and the STAS domain between symmetry-related molecules in the crystals of the SLC26Dg-Nb5776 complex. Transmembrane and STAS domains of SLC26Dg are colored in green and red respectively, the nanobody is colored in blue. (g) SLC26Dg structure placed in a model of a lipid bilayer (obtained from http://www.lobos.nih.gov/mbs/coords.shtml). The molecular surface of the protein is shown with polar residues colored in green, acidic residues in red and basic residues in blue. The observed conformation of SLC26Dg would place the hydrophilic STAS domain (indicated by brackets) within the hydrophobic core of the lipid bilayer.
