Role of N1-Domain, Linker, N2-Domain, and Latch in the Binding Activity and Stability of the Collagen-Binding Domain for the Collagen-Binding Protein Cbm from Streptococcus mutans

Abstract

A special type of Streptococcus mutans expressing collagen-binding proteins (CBPs), Cnm, and Cbm, on the cell surface has been shown to be highly pathogenic. It is believed that S. mutans with CBPs that has entered the blood vessel attaches to collagen molecules exposed from the damaged blood vessel, inhibiting aggregation by platelets and increasing bleeding. Therefore, it is crucial to understand the molecular characteristic features of CBPs to protect against and cure S. mutans-related diseases. In this work, we highlighted the Cbm/collagen-binding domain (CBD) and examined its binding ability and thermal stability using its domain/region exchange variants in more detail. The CBD comprises the N1-domain, a linker, N2-domain, and a latch (N1–N2~), where the latch interacts with the N1-domain to form a β-sheet. The collagen-binding activity of the Cbm/CBD domain/region exchange variants was investigated using ELISA. Binding assays demonstrated that the N-domain_linker_N-domain composition is necessary for collagen binding as previously reported, newly that the latch is involved in binding through the β-sheet with the N1-domain when the N1-domain is present at the N-terminal position, and that the N2-domain is particularly important for collagen binding at both the N- and C-terminal positions. Thermal denaturation experiments newly revealed that the linker and latch bound to the N-domain contribute to N-domain stabilization but have no effect on the N-domain_linker_N-domain molecule, which contains two N-domains. It has also been shown that the N-terminal N2-domain destabilizes the N-domain_linker_N-domain structure. The results of this study will contribute to the rapid detection of CBP, development of CBP-targeted therapies, and application of CBPs to protein engineering using their collagen-binding ability.