Properties of complement receptors

The extracellular domain also possesses at least four tandem repeats of 450 amino acids, each comprised of seven SCRs. These structures are 70-99% homologous with each other and form the basis for the allelic differences in CR1 structure. For example, it has been proposed that the 160-kDa form possesses four tandem repeats, the 190-kDa form five, the 220-kDa form six and the 250-kDa form seven of these repeats. 

CR3 comprises two polypeptides, an a-subunit of 185 kDa and a -sub-unit of 95 kDa, which are covalently linked into an a fl structure. Both chains are exposed at the cell surface, whilst the a-chain forms the major portion of the ligand-binding site. The /J-subunit is common to the other adhesins, LFA-1 and pi50,95. These have an antigenically-distinct a-chain but a common /J-chain (see Fig. 3.6). Of these three receptors with common j3-chains, only CR3 binds significant amounts of C3bi. 

CR3 is present on neutrophils, monocytes and macrophages. Macrophages possess more CR3 than do monocytes, and macrophages from all 

Similarly, activation of neutrophils with PMA can also increase CR1 expression by a factor of 2-3, whereas CRl-dependent phagocytosis is enhanced 20-30-fold. This, again, may be due to activation of receptor function as well as to increasing receptor number on the cell surface. In addition 

In some circumstances CR1 and CR3 may efficiently bind their ligands, but phagocytosis may not be initiated. As described above, pretreatment with PMA may increase the ability of neutrophils to ingest these bound particles. Several naturally-occurring molecules can also promote the ingestion of coated particles presumably, this is achieved via alterations in the efficiency of coupling of receptor occupancy to intracellular signalling systems. 

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