NTR/C345C domain, complement C3 subfamily; The NTR domain found in complement C3 is also known as the C345C domain because it occurs at the C-terminus of complement C3, C4 and C5. Complement C3 plays a pivotal role in the activation of the complement systems, as all pathways (classical, alternative, and lectin) result in the processing of C3 by C3 convertase. The larger fragment, activated C3b, contains the NTR/C345C domain and binds covalently, via a reactive thioester, to cell surface carbohydrates including components of bacterial cell walls and immune aggregates. The smaller cleavage product, C3a, acts independently as a diffusible signal to mediate local inflammatory processes. The structure of C3 shows that the NTR/C345C domain is located in an exposed position relative to the rest of the molecule. The function of the domain in complement C3 is poorly understood.
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