FIG 4.

An inactive form of C3b is associated with CHIKV. (A) Schematic diagram depicting C3b and the first order of inactivation to iC3b. C3b comprises a 102-kDa α′ subunit and a 75-kDa β subunit linked by an intermolecular disulfide bond. Inactivation of C3b by a specific serine protease factor I in the presence of cofactors such as factor H, CD35, or CD46 results in the first-step cleavage of the C3b α′ subunit into 68- and 43-kDa fragments with the release of a 2-kDa fragment called C3f. The resulting C3b component is called iC3b. The closed balloon indicates the position of the thioester bond, and the solid vertical arrows indicate the site of first-order cleavage by factor I. (B) Western blot analysis of the peak fractions to identify the species of C3b associated with CHIKV. Blots containing the peak fractions 6 to 8 (lanes 3 to 5), purified C3b (lane 1), and iC3b (lane 2) were probed with anti-C3 antibody. The samples in lanes 3, 4, and 5 lacked the α′-subunit of C3b (note lane 1) and corresponded to iC3b (note lane 2) characterized by the presence of the 68- and 43-kDa fragments, suggesting the association of inactive C3b (iC3b). The image is representative of 3 independent experiments.