Recombinant Mouse Complement Component C3d Protein, CF

Component C3 plays key role in activation of complement system. Proteolytic cleavage of full-length mouse C3 generates C3 beta (aa 25-666) and C3 alpha (aa 671-1663). C3 alpha is activated by proteolytic enzyme ‘C3 convertase', generating the small C3a peptide (aa 671-748), also called anaphylatoxin, and the large C3b alpha' chain (aa 749-1663). After activation, C3a functions as a mediator of local inflammatory process, while an internal thioester on C3b is exposed to mediate covalent attachment of C3b to foreign pathogens (1). C3b is further cleaved by several proteases to smaller fragments including C3g (aa 955-1001), C3d (aa 1002-1303), C3f (aa 1304-1320) and two C3c fragments (aa 749-954 and aa 1321-1663). The C3d fragment is a complement receptor 2 (CR2)-binding protein.The native C3d has an alpha-alpha barrel structure (2) which contains an isoglutamyl cysteine thioester cross link between Cys1010 and Gln1013 and a disulfide bond between Cys1101 and Cys1158, which allows C3d to covalently attach to cell surface molecules (2). In this recombinant mouse C3d protein, the Cys1010Ser mutation is introduced to ensure correct disulfide bonding between Cys1101 and Cys1158, since Cys1010 contributes the thiol moiety of the thioester, which would only be expected to form in native C3 (1). The biological function of this mutant C3d and its binding to CR2 is comparable to serum derived C3 fragment (2). Mouse C3d fragment shares 94% aa identity with rat c3d and 84% aa identity with human c3d. The interaction of C3d with CR2 enhances antibody responses by reducing the threshold for B cell activation (3). It also facilitates retention and internalization of antigens (4). C3d regulates immune checkpoint blockade and enhances antitumor immunity (5).