Fig. 1.

Domain organization of C3/C3b, and sequence coverage by mass spectrometry. (A) Individual domains in C3 and C3b are represented by different colors and labeled with the domain name abbreviation. Numbered domains signify the corresponding macroglobulin domains (MG1–MG8). Panel (B) shows the domain locations on the primary sequence of the C3 α and β-chains. Upon activation, the ANA domain is removed from C3 and allows the α’NT domain to shift from one face of the protein to the other. As a consequence, the MG8, CUB, and TED domains undergo a large relocation in relation to the protein core (MG1-6) and expose the thioester moiety (white triangle) in C3b that is essential for opsonization of foreign surfaces. A set of 82 peptides spanning 61% of the C3 sequence have been utilized for the analysis and are plotted against the domain scheme (C) and the primary sequence of C3 (D). Areas with significantly increased HD exchange in C3b (≥10%; red) were mainly identified in the β-chain, while the α-chain contained more peptides with significant decrease in HD exchange (≤−10%; blue). Yellow and black areas signify peptides with no significant change (>−10% or <10%) in (C) and (D), respectively. Dashed and solid lines represent MALDI- and ESI-derived peptides, respectively, in the sequence plot (D).