Complement component C9 is a multi-domain protein that contains an N-terminal type-1 TSP domain, an LDL-receptor class A repeat, a number of potential transmembrane (TM) regions and a C-terminal EGF-like domain. The membrane attack complex (MAC)/perforin-like protein complement component 9 (C9) is the major component of the MAC, a multi-protein complex that forms pores in the membrane of target pathogens. In contrast to homologous proteins such as perforin and the cholesterol-dependent cytolysins (CDCs), all of which require the membrane for oligomerisation, C9 assembles directly onto the nascent MAC from solution.
Complement component C9 deficiency is inherited as an autosomal recessive trait and it results in inability to assemble the MAC with a subsequent increased susceptiblity to infection. Serum from C9-deficient individuals is able to kill meningococci but at a slower rate than normal control serum. The risk of developing meningococcal disease among C9-deficient individuals is 700-fold greater than that in Japanese individuals with no complement deficiency.
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