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C3 receptors on macrophages
1Department of Biochemistry, University of Oxford, UK.
Insights
Complement receptors CR1 and CR3 on macrophages bind opsonized targets. CR1 recognizes C3b, while CR3 recognizes iC3b, highlighting their roles in immune cell adhesion and phagocytosis.
Area of Science:
- Immunology
- Molecular Biology
- Cell Biology
Background:
- Macrophage complement receptors mediate binding and ingestion of opsonized particles.
- Key receptors include CR1 (recognizes C3b) and CR3 (recognizes iC3b).
- iC3b is a product of C3b cleavage by factor I.
Purpose of the Study:
- To describe the structural and functional characteristics of macrophage complement receptors CR1 and CR3.
- To elucidate the molecular basis of complement-mediated cellular interactions.
Main Methods:
- Structural analysis of complement receptors CR1 and CR3.
- Sequence homology comparisons with related adhesion molecules.
- Functional characterization of iC3b binding activity.
Main Results:
- CR1 comprises 30 short consensus repeat (SCR) units, a transmembrane segment, and a cytoplasmic domain.
- CR3 is a heterodimer with alpha and beta subunits; the beta subunit is shared with LFA-1 and p150,95.
- The beta subunit's homology to fibronectin and vitronectin binding proteins suggests a role in adhesion molecules.
Conclusions:
- CR1 and CR3 are critical for macrophage phagocytosis and immune surveillance.
- Structural similarities indicate CR3 and related molecules form an extended family of surface adhesion proteins.
- Understanding these receptors is vital for immune system function and potential therapeutic targeting.
Abstract:
The complement receptors on macrophage are responsible for their binding and ingestion of opsonized targets. The two established receptors are CR1, which recognizes C3b, and CR3, which recognizes iC3b, the natural product of C3b from cleavage by the complement control protein factor I and its cofactors. CR1 belongs to a group of proteins that contain a structural element characterized by its size of 60-65 amino acids, and four conservatively positioned cysteines, which engage in a self-contained 1-3, 2-4 disulphide arrangement. This structural unit is called SCR (short consensus repeat) and is found in the complement proteins C1r, C1s, C2, factor B, factor H, C4BP, DAF, MCP and CR2, each of which interacts with some cleavage products of C3 and/or C4. CR1 has 30 SCR units accounting for its entire extracellular structure. It has a transmembrane segment and a small cytoplasmic domain. CR3 is a heterodimer containing an alpha and beta subunit held together by non-covalent forces. The beta subunit is also found in the two leukocyte antigens, LFA-1 and p150,95, which have alpha subunits distinct from that of CR3. The beta subunit contains 56 cysteine residues, 42 of which lie in a span of 256 residues immediately adjacent to the transmembrane segment. It shares extensive sequence homology with subunits of membrane protein complexes that bind fibronectin and vitronectin, implicating that they all belong to an extended set of surface adhesion molecules not restricted to the immune system. p150,95 is also expressed on macrophages and it has iC3b binding activity. It also shares some functional properties with CR3 as an adhesion surface molecule.