Complement C1q subcomponent subunit B
1_MMMKI 6_ PWGSI 11_ PVLML 16_ LLLLG 21_ LIDIS 26_ QAQLS 31_ CTGPP 36_ AIPGI 41_ PGIPG 46_ TPGPD 51_ GQPGT 56_ PGIKG 61_ EKGLP 66_ GLAGD 71_ HGEFG 76_ EKGDP 81_ GIPGN 86_ PGKVG 91_ PKGPM 96_ GPKGG 101_ PGAPG 106_ APGPK 111_ GESGD 116_ YKATQ 121_ KIAFS 126_ ATRTI 131_ NVPLR 136_ RDQTI 141_ RFDHV 146_ ITNMN 151_ NNYEP 156_ RSGKF 161_ TCKVP 166_ GLYYF 171_ TYHAS 176_ SRGNL 181_ CVNLM 186_ RGRER 191_ AQKVV 196_ TFCDY 201_ AYNTF 206_ QVTTG 211_ GMVLK 216_ LEQGE 221_ NVFLQ 226_ ATDKN 231_ SLLGM 236_ EGANS 241_ IFSGF 246_LLFPD
1: Core component of the complement C1 complex, a multiprotein complex that initiates the classical pathway of the complement system, a cascade of proteins that leads to phagocytosis and breakdown of pathogens and signaling that strengthens the adaptive immune system (PubMed:12847249, PubMed:19006321, PubMed:24626930, PubMed:29449492, PubMed:3258649, PubMed:34155115, PubMed:6249812, PubMed:6776418). The classical complement pathway is initiated by the C1Q subcomplex of the C1 complex, which specifically binds IgG or IgM immunoglobulins complexed with antigens, forming antigen-antibody complexes on the surface of pathogens: C1QA, together with C1QB and C1QC, specifically recognizes and binds the Fc regions of IgG or IgM via its C1q domain (PubMed:12847249, PubMed:19006321, PubMed:24626930, PubMed:29449492, PubMed:3258649, PubMed:6776418). Immunoglobulin-binding activates the proenzyme C1R, which cleaves C1S, initiating the proteolytic cascade of the complement system (PubMed:29449492). The C1Q subcomplex is activated by a hexamer of IgG complexed with antigens, while it is activated by a pentameric IgM (PubMed:19706439, PubMed:24626930, PubMed:29449492). The C1Q subcomplex also recognizes and binds phosphatidylserine exposed on the surface of cells undergoing programmed cell death, possibly promoting activation of the complement system (PubMed:18250442)