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Self-association and domain rearrangements between complement C3 and C3u provide insight into the activation mechanism of C3

Abstract : Component C3 is the central protein of the complement system. During complement activation, the thioester group in C3 is slowly hydrolysed to form C3u, then the presence of C3u enables the rapid conversion of C3 to functionally-active C3b. C3u shows functional similarities to C3b. To clarify this mechanism, the self-association properties and solution structures of C3 and C3u were determined using analytical ultracentrifugation and X-ray scattering. Sedimentation coefficients identified two different dimerisation events in both proteins. A fast dimerisation was observed in 50 mM NaCl that was removed in 137 mM NaCl. Low amounts of a slow dimerisation was observed for C3u and C3 in both buffers. The X-ray radius of gyration RG values were unchanged for both C3 and C3u in 137 mM NaCl, but depend on concentration in 50 mM NaCl. The C3 crystal structure gave good X-ray fits for C3 in 137 mM NaCl. By randomisation of the TED/CUB domains in the C3b crystal structure, X-ray fits showed that the TED/CUB domains in C3u are extended and differ from the more compact arrangement of C3b. This TED/CUB conformation is intermediate between those of C3 and C3b. The greater exposure of the TED domain in C3u (which possesses the hydrolysed reactive thioester) accounts for the greater self-association of C3u in low salt. This conformational variability of the TED/CUB domains would facilitate their interactions with a broad range of antigenic surfaces. The second dimerisation of C3 and C3u may correspond to a dimer observed in one of the crystal structures of C3b.
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Submitted on : Tuesday, September 14, 2010 - 2:51:44 AM
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Keying Li, Jayesh Gor, Stephen J Perkins. Self-association and domain rearrangements between complement C3 and C3u provide insight into the activation mechanism of C3. Biochemical Journal, Portland Press, 2010, 431 (1), pp.63-72. ⟨10.1042/BJ20100759⟩. ⟨hal-00517253⟩

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