Figure 1. Constraining the relative orientation between the C2A and C2B domains of syt using poly-proline linkers.

(a) A seven-residue poly-proline (7Pro) helical rod is shown in the left panel; the pitch is three residues per turn of the helix. Right panel: Magnified view in which the dihedral angels, which underlie the periodicity of three, are shown. Nitrogen atoms are represented with blue balls, and the oxygen atoms are represented in Red. (b–d) The relative orientations of C2A and C2B are predicted to be constrained, and thus altered, by varying the number of residues in the poly-proline linker; these findings are based on the analysis of the 1a-2b conformer (see definitions and discussion in Supplementary Note 1, Supplementary Figs 7–9 and Supplementary Table 8). Left panels show representative snapshots for the tandem C2-domains connected by poly-proline linkers consisting 9 (b), 10 (c) or 11 (d) residues. Right panels: Results from MD simulations, revealing the predicted relative C2-domain orientations as a function of time. The green and blue traces indicate the relative orientation between the proline linker and either C2A or C2B, respectively. Black traces indicate the relative orientation between C2A and C2B; histograms and numerical averages of relative orientation angles are provided on the right side of each panel.