Figure 6. Scheme of pH-induced dimerization and structural conversion of wtNRN1_L.h.

(a) Physiological solvent conditions (pH 7.2, NaCl) during spidroin storage cause deprotonated D39, E76, E81 (red spheres) and E114 (orange sphere) to stabilize monomeric NRN1_L.h. in conformation I. A decreasing NaCl concentration along the spinning duct enables electrostatic interactions between the acidic and basic pole of both subunits, prearranging the dimer in an antiparallel manner. Decreasing ionic strength as well as pH effects residue E114 (orange → white sphere) which induces a slight altered conformation I*, and, thereby, triggers stable dimer formation. (b) The structural change into the tight conformation II is initiated simultaneously in the spinning duct but is independently controlled by successive protonation of clustered D39, E76 and E81 (red → white spheres). Thereby, the only W of the domain is shifted towards the surface (yellow → green sphere).