C-Terminal Domains of Spider Silk Proteins Having Divergent Structures but Conserved Functional Roles.

Spider silk is self-assembled from silk proteins or spidroins. C-terminal domains (CTDs) of various types of spidroins are relatively conserved in amino acid sequences and are suggested to adopt similar structures and perform similar functional roles in spidroin storage and silk formation. Here, we solved the structure of the CTD from a capture-spiral silk protein (CTD Fl ) and characterized its stability and fibril formation in the presence and absence of a reducing agent at different pH values. CTD Fl adopts a dimeric structure with 8 helices, but the CTDs of other types of spidroins exist in a domain-swapped dimeric structure with 10 helices. Despite the structural differences, CTD Fl is pH-responsive in stability and fibril formation, similar to the CTDs from minor and major ampullate spidroins. Thus, the functional role of CTDs in silk fiber formation seems conserved. Comparing wild-type CTD Fl and its mutants, we found that the pH-responsive behavior results from the protonation of H76, which is conserved from different spider species. In addition, the fibril formation rate of CTD Fl correlates with its instability, suggesting that structural changes are involved in fibril formation.