Characterization of tryptic fragments of human complement factor C3

Abstract

C3c and C3d fragments were prepared in pure form from trypsin-digested human C3, and the individual chains of tryptic C3c were isolated by gel filtration on Sepharose 4B in 6 M guanidinium hydrochloride. No low mol. wt ( M r ) fragments were identified. The polypeptide chains were characterized with regard to M r , amino acid composition and N-terminal amino acid sequence. Tryptic C3c consisted of one fragment from the β-chain ( M r 64,000) and two from the α'-chain ( M r 40,000 and 23,000). The β-chain fragment was derived from the C-terminal part of the chain, and the 23,000- M r component constituted the amino terminal end of the α-chain. The 40,000- M tr fragment emanated from the C-terminal end of the α-chain. Tryptic C3d displayed microheterogeneity on polyacrylamide gel electrophoresis in sodium dodecyl sulfate, but possessed a homogeneous N-terminal, identical to that described by Tack et al. (1980) ( proc. natn. Acad. Sci. U.S.A. 77, 5764–5768). By utilization of antisera against subunits of C3 and C3c in immunoblotting a degradation scheme for C3 by trypsin was proposed and the positions of the fragments in the intact molecule indicated.