Amino acid sequence around the thiol and reactive acyl groups of human complement component C4

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Abstract
Activation of the 4th component of complement (C4) by C.hivin.1s results in the generation of a reactive acyl group, able to react with putrescine, and in the release of a free thiol group that cannot be detected in the native hemolytically active molecule. Both the reactive acyl group and the free thiol group reside in C4d, a fragment of the .alpha.''-chain of C4b derived from digestion of the molecule with the control proteins C3b inactivator and C4-binding protein. Peptides derived from CNBr digestion of [1,4-14C]putrescine-labeled and iodo[2-14C]acetic acid-labeled C4d were obtained and used to establish a continuous sequence of 88 residues from the N-terminus of the molecule. The thiol and reactive acyl groups are contained in an octapeptide that shows near identity with the equivalent sequences reported for .alpha.2-macroglobulin and C3. Other adjacent short sections also show homology of sequence between the 3 proteins, and they probably contribute to the overall structure that gives a unique reactivity to the thiol ester bond postulated to exist in the native forms of the 3 proteins.