Microbe-Specific C3b Deposition in the Horseshoe Crab Complement System in a C2/Factor B-Dependent or -Independent Manner
Figure 9
Proposed mechanism for deposition of TtC3b on the surfaces of microbes.
On the surface of Gram-negative bacteria, the LPS-sensitive protease factor C is activated through the interaction with LPS. Activated factor C then acts as a C3 convertase, and the resulting TtC3b is deposited. On the Gram-positive bacteria, Ca2+-dependent lectins such as TtCRP-1 (Tachypleus tridentatus C-reactive protein-1) and TL-5A (Tachylectin-5A) recruit the complex (C3-B), TtC3-TtC2/Bf-1, or TtC3-TtC2/Bf-2. The second C3 convertase (C3b-Bb), TtC3b-TtC2/Bf-1b or TtC3b-TtC2/Bf-2b complex, may be formed by an unidentified protease on the surfaces of Gram-positive bacteria and fungi. TL-1 (Tachylectin-1) and possibly TPL-1 (Tachypleus plasma lectin-1) also promote TtC3b deposition on the three types of microbes, especially on fungi.