Structure at 1.44 angstrom resolution of an N-terminally truncated form ofthe rat serum complement C3d fragment

Complement component C3 plays a key role in the complement-mediated immune defence, and occupies a central position within the complement cascade syst em. One of its degradation products, C3dg, was purified from rat serum and crystallised in two different crystal forms as N-terminally truncated fragm ent. Despite the truncation and the lack of a significant portion of the N- terminus as compared to C3d, the structure of the fragment is highly simila r to that of recombinant human C3d (Nagar et al., Science 280 (1998) 1277-1 281). Structural details of the reactive site have been obtained, suggestin g a possible mode of thioester bond formation between Cys-1010 and Gin-1013 and thioester bond cleavage in the transacylation reaction involving His-1 126. The truncation at the N-terminus of C3d leads to the exposure of a sur face of the molecule that favours dimerisation, so that in both crystal for ms, the fragment is present as a dimer, with monomers related by a two-fold axis. (C) 2000 Elsevier Science B.V. All rights reserved.