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C3 is central to the activation of all 3 pathways of complement activation. Initiation of each pathway generates proteolytic enzyme complexes which binds the target surface. These enzymes cleave a peptide bond in C3 releasing the anaphylatoxin C3a and activating C3b. Most of the C3 activated during complement activation never attaches to the surface due to its thioester reaction with water forming fluid phase C3b which is rapidly inactivated by factors H and I forming iC3b. Surface-bound C3b is necessary in all 3 pathways for efficient activation of C5 and formation of C5b-9 complexes that lyse the target cell membrane.

CFI cleaves and inactivates C3b and C4b. CFI is inactive without a cofactor such as the soluble factor H and C4b binding protein. CFI cleaves the alpha-peptide chain of C3b and C4b when these binds1 of the cofactors. This cleavage inactivates all of the complement activating functions of these proteins producing iC3b and iC4b.CFI cleaves the alpha chain of C3b twice and this releases a small fragment called C3f. CFI can cleave iC3b releasing C3c from C3dg in the presence of CR1.C4b is cleaved rapidly at 2 sites separating C4c from C4d.

C2 forms the proteolytic subunit of the C3 and C5 convertase of both pathways. Initiation of each pathway generates proteolytic enzyme complexes which binds to the target surface. C1s and MASP in these complexes activate both C4 and C2. They cleave a peptide bond in C4 depositing C4b on the surface and also cleave C2 into 2 fragments.

C1QTNF2 can be found in many different organism parts such as the testis, cervix. C1QTNF2 is located in the extracellular space and has various roles such as activation of MAPK activity, positive regulation of fatty acid oxidation, positive regulation of glycogen biosynthetic process, and positive regulation of glucose import.

Complement C1q Tumor Necrosis Factor-Related Protein 5 (C1QTNF5) encodes a short-chain collagen which is expressed mainly in sub-retinal pigment epithelium, ciliary epithelium and adipose tissue. C1QTNF5 is increased in mtDNA-depleted myocytes and stimulates the phosphorylation of AMP activated protein kinase. C1QTNF5 takespart in the adhesion of the retinal pigment epithelium (RPE) to the Bruch Membrane. Mutations in C1QTNF5 have been associated with late-onset retinal degeneration.

C1QTNF7 is a collagen type-IV specific for basement membranes.

C1QTNF3 also called Cartducin is a novel angiogenic factor in the formation of neointima following angioplasty. C1QTNF3 a paralog of Acrp30 (adiponectin). C1QTNF3 is a secretory protein produced by chondrogenic precursors & proliferating chondrocytes, and belongs to a novel C1q family of proteins. Cartducin promotes the growth of mesenchymal chondroprogenitor cells & chondrosarcoma-derived chondrocytic cells in vitro. Cartducin stimulates mesenchymal chondroprogenitor cell proliferation through extracellular signal-regulated kinase and phosphatidylinositol 3-kinase/Akt pathways. C1QTNF3 promotes proliferation & the migration of endothelial cells.

Plasma protease C1 inhibitor (SERPING1) is a part of the serpin superfamily of serine protease inhibitors. SERPING1 plays an important role in regulating activation of both the complement and contact systems. SERPING1 is also activating C1 by coupling with the active catalytic site at the light chains of C1r and C1s. SERPING1 insufficiency results in hereditary angioedema, which is characterized by recurrent episodes of localized angioedema of the skin, gastrointestinal mucosa or upper respiratory mucosa.

CD93, is a receptor or else an element of a larger receptor complex for C1q, MBL2-mannose-binding lectin and SPA-pulmonary surfactant protein A. CD93 mediates the enhancement of phagocytosis in monocytes as well as macrophages upon interaction with soluble defense collagens. CD93 takes part in the intercellular adhesion. Furthermore, CD93 was expressed on (pre) plasmablasts/plasma cells, including long-lived plasma cells which demonstrated decreased cell cycle activity, high levels of isotype-switched Ig secretion, as well as modification of the transcriptional network. CD93 is vital for the maintenance of plasma cells in bone marrow niches.

Plasma protease C1 inhibitor (SERPING1) is a part of the serpin superfamily of serine protease inhibitors. SERPING1 takes part in regulating activation of both the complement and contact systems. That isdue to the fact that SERPING1 regulates the activation of complement factor C1 in addition to the activity of activated C1 by coupling with the active catalytic site at the light chains of C1r and C1s. Lack of SERPING1 results in hereditary angioedema, which is characterized by recurrent episodes of localized angioedema of the skin, gastrointestinal mucosa or upper respiratory mucosa.

CD93, is a receptor or else an element of a larger receptor complex for C1q, MBL2-mannose-binding lectin and SPA-pulmonary surfactant protein A. CD93 mediates the enhancement of phagocytosis in monocytes as well as macrophages upon interaction with soluble defense collagens. CD93 takes part in the intercellular adhesion. Furthermore, CD93 was expressed on (pre) plasmablasts/plasma cells, including long-lived plasma cells which demonstrated decreased cell cycle activity, high levels of isotype-switched Ig secretion, as well as modification of the transcriptional network. CD93 is vital for the maintenance of plasma cells in bone marrow niches.

Factor B is a protein, build from a single chain that circulates in the blood. Complement factor D cleaves and activates factor B, thus creating a catalytic subunit Bb and noncatalytic chain Ba. Subunit Bb is a serine protease that can bind to C3b and forms alternative pathway C3 convertase. Subunit Ba acts as an inhibitor for proliferation of lymphocytes, when Bb is part of the lymphocytes’ proliferation. The gene that codes for this protein is located on chromosome six.

C1QBP binds to the globular "heads" of c1q thus inhibiting c1 activation. C1QBP interacts with a wide range of ligands and is implicated in cell signaling. C1QBP associates with C1r and C1s in order to yield the first component of the serum complement system. C1QBP protein has been identified as the p32 subunit of pre-mRNA splicing factor SF2, as well as a hyaluronic acid-binding protein.
C1QBP is a new marker of tumor cells and tumor-associated macrophages/myeloid cells in hypoxic/metabolically deprived areas of tumors.
Mitochondrial C1QBP is a critical mediator of p14ARF-induced apoptosis.
C1QBP functions as a chemotactic factor for immature dendritic cells, and migration is mediated through ligation of both C1QBP and cC1qR/CR.
C1QBP overexpression successfully blocks mRNA accumulation from the adenovirus major late transcription unit (MLTU) and stimulates RNA polymerase II carboxy-terminal domain phosphorylation in virus-infected cells.
C1QBP binds with Hepacivirus core protein on CD8+ and CD4+ positive t-cells and inactivates lck and akt.

CD21 is expressed strongly on mature B cells, follicular dentritic cells and weakly on immature thymocytes and T lymphocytes. In B-cell ontogeny, CD21 appears after the pre-B-stage, is maintained during peripheral B-cell development and is lost upon terminal differentiation into plasma cells. CD21 expression is also gradually lost after stimulation of B cells in vitro. CD21 functions as receptor for C3d, C3dg and iC3b Complement components, for EBV and for IFNalpha. CD21 binds to CD23 and associates with CD19, CD81 and Leu13 to form a large signal-transduction complex involved in B cell activation.

CD18 (ITGB2 or integrin beta-2) is a receptor for ICAM1, ICAM2, ICAM3 and ICAM4. CD18 may be a member of an extended family of cell surface molecules including the fibronectin binding protein. In addition, CD18 is a complement receptor type 3 (CR3). CD18 triggers neutrophil transmigration during lung injury through PTK2B/PYK2-mediated activation.

ASCC1 is a subunit of the triggering signal cointegrator 1 (ASC-1) complex which is a transcriptional coactivator. ASC-1 complex takes a vital part in gene transactivation using various transcription factors such as activating protein 1 (AP-1), nuclear factor kappa-B (NF-kB) and serum response factor (SRF). ASCC1 has an N-terminal KH-type RNA-binding motif, essential for AP-1 transactivation by the ASC-1 complex. Alterations in ASCC1 result in Barrett esophagus and esophageal adenocarcinoma.

Exosome component 8 and transcription factor 4 (EXOSC8) is a part of the exosome complex. In the cytoplasm, the RNA exosome complex is engaged in general mRNA turnover and specifically degrades naturally unstable mRNAs containing AU-rich elements (AREs) within their 3' untranslated regions, and in RNA inspection pathways, preventing translation of aberrant mRNAs. EXOSC8 appears to be involved in degradation of histone mRNA. EXOSC8 attaches to ARE-containing RNAs. The EXOSC8 gene encodes a 3'-5' exoribonuclease, which specifically interacts with mRNAs containing AU-rich elements. The EXOSC8 protein is part of the exosome complex, which is significant for the degradation of many RNA species.

EXOSC1 is a non-catalytic component of the exosome which has 3'->5' exoribonuclease activity and participates in many cellular RNA processing and degradation occasions. EXOSC1 associates with the exosome by protein-protein interactions with ribosomal RNA-processing protein 42 and ribosomal RNA-processing protein 46.

CD55 belongs to the RCA (regulators of complement activation) family characterized by 4-30 SCRs (short consensus repeats) in their plasma-exposed regions. CD55 is widely expressed in malignant tumors and nevertheless functions as an inhibitor of the complement system.

Exosome Component 3 (EXOSC3) is a non-catalytic component of the human exosome, a complex with 3'-5' exoribonuclease activity. EXOSC3 takes part in various RNA processing and degradation activities including degradation of histone mRNA. Linked pseudogenes of EXOSC3 are located on chromosome 19 and 21.

XRCC2 belongs to the RecA/Rad51-related protein family which takes part in homologous recombination to repair DNA damage and preserve chromosome stability. XRCC2 gene participates in the mending of DNA double-strand disruptions by homologous recombination. Additionally, XRCC2 functionally complements Chinese hamster irs1, a repair-deficient mutant which displays hypersensitivity to diverse DNA-damaging agents.

Intercellular Adhesion Molecule-5 or ICAM-5 is a glycoprotein, part of the immunoglobulin family and located in the cell membrane. We can find ICAM-5 in telencephalic neurons, in a couple of adhesion activity, firstly heterophilic binding amid neurons and leukocytes and secondly, homophilic binding amid neurons. ICAM-5 is expressed exclusively in telencephalic areas of the central nervous system. Similar to other ICAMs molecules, ICAM-5 adhere to LFA-1 which is a leukocyte integrin.