Spidroin N-terminal domain: A pH sensor in the spider silk assembly process

Spider silks are protein-based fibers with remarkable mechanical qualities. Perhaps even more impressive is the spinning process in which the spider silk proteins (spidroins) are assembled from a highly soluble storage state into a well-ordered and insoluble fiber. Indeed, the ordered arrangement of spidroins, which is endowed by the spinning process, is the basis of fiber strength. However, the forces driving fiber assembly and the mechanisms by which spidroins respond those forces are only poorly understood. Spidroins have a tripartite domain architecture consisting of a large and repetitive central domain flanked by small, non-repetitive N- and C-terminal domains. Both terminal domains are well conserved among different types of spidroins, and they are believed to act as coordinators of spidroin assembly.;This dissertation represents an important advancement of knowledge on the sequence, structure, and biochemical behavior of the highly conserved but greatly under studied N-terminal domain of major ampullate silks (MaSp-NTD). In my analysis of MaSp-NTD sequence, I demonstrated for the first time that one of the two types of MaSp genes (MaSp1) had been duplicated in the golden orb-weaving spider (Nephila clavipes) generating MaSp1A and MaSp1B genes. My publication of this work was contemporaneous with publications by research groups finding similar MaSp gene duplications in other spider species. Characterization of the RNA transcripts from MaSp1A and MaSp1B genes indicated that they are simultaneously co-expressed in adult female spiders. It also allowed experimental verification the transcription start sites in the MaSp genes.;My biochemical analysis of three recombinant MaSp-NTDs indicate that they are highly pH responsive. Upon experiencing a mild acidification, as occurs in the spider silk gland, they undergo a dramatic change in tertiary conformation. Concurrent with the pH-dependent refolding, the interaction between MaSp-NTD molecules is strengthened resulting in considerably more stable MaSp-NTD homo-dimers. Through a collaboration with another research group, we are working to determine the structure of MaSp-NTD in both conformational states by NMR. By transducing the duct pH change into specific protein-protein interactions, this conserved spidroin domain likely contributes significantly to the silk spinning process. Based on these results, we propose a model of spider silk assembly dynamics as mediated through the MaSp-NTD.