Fig. 5: p18m Dimer-1 interface mutants assembly and structure.

a, b SEC-MALLS analysis of p18m-A273V and p18m-I269F, sample loading concentrations; 400 µM (cyan), 200 µM (magenta), and 100 µM (wheat). dRI is plotted against retention time. The molar mass, determined at 1 s intervals throughout peak elution, is plotted as points. Monomer and dimer molar masses are indicated with the dashed lines. c C(S) distributions were derived from sedimentation velocity data recorded from p18m-A273V at 40 µM (wheat), 86 µM (magenta), and 181 µM (cyan). Curves are the distribution of the sedimentation coefficients that best fit the sedimentation data (RMSD 0.009-0.025), see also Supplementary Table 3. (d) Thermal denaturation of p18m, p18m-A273V, and p18m-I269F monitored by CD at 222 nm upon heating from 10–80 °C. Curves are the best fit spline function to the data points, dashed lines indicate the T_m of transitions determined from the 1st derivative of the fitted curves. Error bars are standard deviations from three independent measurements. Data shown are from one representative experiment, the lower panel shows the residuals to each fit. Source data for c and d is provided in the Source Data file (e) Asymmetric unit of the p18m-A273V crystal structure. The backbone of the three p18m protomers are shown in the cartoon, coloured lime, slate and pink. The Fo-Fc map contoured at 3σ (orange mesh), produced after molecular replacement, contains residual positive density for the valine γ-methyl groups at residue 273. f 3D superimposition of the 3 chains shown in (e). Structures were aligned using backbone Cα atoms (rmsd = 0.16 ± 0.04 Å over 78 ± 3.6 Cα). g Comparison of p18m (grey) and p18m-A273V (green) dimer interfaces. Structures are shown in cartoons with A273 or V273 sidechains as sticks. The PISA-calculated average solvation energy contribution to each dimer from either A273 (p18m) or V273 (p18m-A273V) are shown below. h Close-up view of the dimer interface, boxed in g. Local shifts in the backbone conformation at the C-terminus of α1 prevent steric clashes and allow the packing of the additional γ-methyl groups of V273 preserving a favourable 3.6 Å Van der Waals distance.