Insights into the structure of mature streptavidin C1 from Streptomyces cinnamonensis reveal the self-binding of the extension C-terminal peptide to biotin-binaing sites

Abstract

The members of the avidin protein family are well known for their high affinity towards D-biotin and their structural stability. These properties make avidins a valuable tool for various biotechnological applications. In the present study, two avidin-like biotin-binding proteins (named streptavidin C1 and C2) from Streptomyces cinnamonensis were newly identified while exploring antifungal proteins against Fusarium oxysporum f. sp. cucumerinum. Streptavidin C1 reveals a low correlation (a sequence identity of approximately 64%) with all known streptavidins, whereas streptavidin C2 shares a sequence identity of approximately 94% with other streptavidins. Here, the crystal structures of streptavidin C1 in the mature form and in complex with biotin at 2.1 and 2.5 angstrom resolution, respectively, were assessed. The overall structures present similar tetrameric features with D-2 symmetry to other (strept)avidin structures. Interestingly, the long C-terminal region comprises a short alpha-helix (C-Lid; residues 169-179) and an extension C-terminal peptide (ECP; residues 180-191) which stretches into the biotin-binding sites of the same monomer. This ECP sequence (-(180)VTSANPPAS(188)-) is a newly defined biotin-binding site, which reduces the ability to bind to (strept)avidin family proteins. The novel streptavidin C1 could help in the development of an engineered tetrameric streptavidin with reduced biotin-binding capacity as well as other biomaterial tools.