Dimerization of the Conserved N-Terminal Domain of a Spider Silk Protein Controls the Self-Assembly of the Repetitive Core Domain.

Spider silk proteins comprise a repetitive core domain with polyalanine and glycine/proline-rich stretches flanked by highly conserved nonrepetitive N- and C-terminal domains. The termini are responsive to assembly triggers, sensing changes in the ionic (H+, phosphate) and mechanical (shear stress) environment along the spinning duct. The N-terminal domain dimerizes in a pH-dependent manner induced by protonation of conserved acidic residues. To date, dimerization of N-terminal spider silk domains has been individually investigated in the absence of large core domains. In this work, the impact of an engineered 50 kDa (AQ) core domain was studied on N-terminal dimerization by circular dichroism, fluorescence and absorbance spectroscopy, multiangle light scattering, as well as scanning electron and transmission electron microscopy. Although the core domain showed no apparent influence on the dimerization behavior of the N-terminal domain, the N-terminal domain in contrast influenced the behavior of the core domain: the monomeric state enhanced (AQ)'s solubility, and dimer formation triggered self-assembly. The monomer-dimer equilibrium was influenced by using several previously established mutants, confirming these results. This work thereby provides molecular insights into how key residues of the N-terminal domain control the dimerization-mediated transformation of soluble spidroins into fibrillary assemblies.