Acts as a cofactor for complement factor I, a serine protease which protects autologous cells against complement-mediated injury by cleaving C3b and C4b deposited on host tissue. May be involved in the fusion of the spermatozoa with the oocyte during fertilization. Also acts as a costimulatory factor for T-cells which induces the differentiation of CD4+ into T-regulatory 1 cells. T-regulatory 1 cells suppress immune responses by secreting interleukin-10, and therefore are thought to prevent autoimmunity. (Microbial infection) A number of viral and bacterial pathogens seem to bind MCP in order to exploit its immune regulation property and directly induce an immunosuppressive phenotype in T-cells. (Microbial infection) Acts as a receptor for Adenovirus subgroup B2 and Ad3. (Microbial infection) Acts as a receptor for cultured Measles virus. (Microbial infection) Acts as a receptor for Herpesvirus 6/HHV-6. (Microbial infection) May act as a receptor for pathogenic bacteria Neisseria and Streptococcus pyogenes (PubMed:7708671, PubMed:9379894, PubMed:11260136, PubMed:11971006). Interacts with C3b (PubMed:3260937, PubMed:1717583). Interacts with C4b (PubMed:1717583). Interacts with moesin/MSN (PubMed:7884872). (Microbial infection) Interacts with measles virus H protein. (Microbial infection) Interacts with human herpesvirus 6 GH protein (PubMed:12663806, PubMed:12724329). (Microbial infection) Interacts with human adenovirus B/D fiber protein (PubMed:12915534, PubMed:14566335, PubMed:15047806, PubMed:15078926, PubMed:15919905, PubMed:16254377). (Microbial infection) Binds to Streptococcus pyogenes M protein and to type IV pili from Neisseria (PubMed:7708671, PubMed:9379894, PubMed:11260136, PubMed:11971006). Inner acrosomal membrane of spermatozoa. Internalized upon binding of Measles virus, Herpesvirus 6 or Neisseria gonorrhoeae, which results in an increased susceptibility of infected cells to complement-mediated injury. In cancer cells or cells infected by Neisseria, shedding leads to a soluble peptide. Additional isoforms seem to exist. The complete sequences of the isoforms are not known. Isoforms are classified as alpha (isoform C and isoform D), beta (isoform E and isoform F), gamma (isoform A and isoform B) and delta (isoform N). Isoforms gamma are preferentially expressed in EBV-B cells and leukemic cells. Isoforms alpha (66 kDa) and isoforms beta (56 kDa) are found in all tissues except sperm. Isoform delta is expressed in spermatozoa. The exon 9 is specifically deleted in some placentae isoforms. All tissues differentially splice exon 13. Expressed by all cells except erythrocytes. Sushi domains 1 and 2 are required for interaction with human adenovirus B PIV/FIBER protein and with Measles virus H protein. Sushi domains 2 and 3 are required for Herpesvirus 6 binding. Sushi domain 3 is required for Neisseria binding. Sushi domains 3 and 4 are required for interaction with Streptococcus pyogenes M protein and are the most important for interaction with C3b and C4b. N-glycosylated on Asn-83; Asn-114 and Asn-273 in most tissues, but probably less N-glycosylated in testis. N-glycosylation on Asn-114 and Asn-273 is required for cytoprotective function. N-glycosylation on Asn-114 is required for Measles virus binding. N-glycosylation on Asn-273 is required for Neisseria binding. N-glycosylation is not required for human adenovirus binding. Extensively O-glycosylated in the Ser/Thr-rich domain. O-glycosylation is required for Neisseria binding but not for Measles virus or human adenovirus binding. In epithelial cells, isoforms B/D/F/H/J/L/3 are phosphorylated by YES1 in response to infection by Neisseria gonorrhoeae; which promotes infectivity. In T-cells, these isoforms may be phosphorylated by LCK. Disease susceptibility is associated with variants affecting the gene represented in this entry. Other genes may play a role in modifying the phenotype. Patients with CD46 mutations seem to have an overall better prognosis compared to patients carrying CFH mutations.