Inquiry Basket
Loading ......
Filter By Product Search for
C9
C9 Full Name
complement component 9
C9 Introduction
Complement component C9 is a key element in the terminal pathway of the complement system, serving as the final component of the membrane attack complex (MAC) and playing a direct role in the elimination of pathogens and target cells in humoral immunity. A notable structural feature of the C9 protein is its amphipathic nature: the N-terminal half is strongly hydrophilic, whereas the C-terminal half shows marked hydrophobicity. This unique amphipathic architecture is crucial for its subsequent pore-forming function in cell membranes. Further domain analysis reveals that C9 contains multiple conserved structural modules, such as a thrombospondin (TSR) domain, a low-density lipoprotein receptor (LDLr) class A domain, and an epidermal growth factor-like (EGF-like) domain.
The core immune function of C9 is to act as the final executor of the MAC, directly forming transmembrane pores on target cells—such as bacteria, virus-infected cells, or tumor cells—leading to cell death. Complement activation, whether through the classical, lectin, or alternative pathway, ultimately converges at the formation of C5 convertase, which cleaves C5 to generate the C5b fragment. C5b then sequentially recruits C6, C7, and C8 to form the C5b-8 complex. This complex can weakly insert into the phospholipid bilayer of the target cell, but its killing efficiency is limited. This is where C9 becomes essential. C9 molecules bind to the membrane-anchored C5b-8 complex, using it as a nucleation site to trigger rapid polymerization of multiple C9 molecules. As 10 to 18 C9 molecules are added and polymerized, they form a stable, ring-shaped transmembrane hydrophilic channel or pore—the complete MAC (C5b-9n)—on the target cell membrane. With an internal diameter reaching up to 10 nm, this pore completely disrupts the membrane's permeability barrier, preventing the cell from maintaining normal ion gradients and osmotic balance. The direct consequences are leakage of cellular contents (such as potassium ions and ATP) along with massive influx of water and external ions (e.g., sodium and calcium), ultimately leading to irreversible osmotic lysis and cell death.
Figure 1. Activation of complement system-induced formation of MAC (C5b-9). (Source: Wang YN, et al. 2022)
Hereditary C9 deficiency is a primary immunodeficiency disorder whose main clinical manifestation is a markedly increased susceptibility to invasive infections caused by Neisseria meningitidis. Beyond its role in anti-infective immunity, genetic variation in C9 has also been linked to the risk of age-related macular degeneration (AMD). Unlike C9 deficiency, the association here is not with a complete absence of C9 but rather with specific rare functional variants in the C9 gene. AMD is a complex, multifactorial degenerative eye disease, and chronic dysregulation and overactivation of the complement system are considered central to its pathophysiology. Large-scale genome-wide association studies (GWAS) and subsequent functional analyses have identified certain missense mutations in the C9 gene that are significantly associated with an increased risk of advanced AMD. These pathogenic variants are believed to potentially alter the structure of the C9 protein, affect its ability to polymerize, its stability, or its interactions with other complement components in ocular tissues such as the retinal pigment epithelium (RPE), thereby leading to excessive complement activation or impaired regulation in the local eye microenvironment.
Alternate Names for C9
C9; complement component 9; C9D; ARMD15; complement component C9;
Loading ......
