Complementary DNA and derived amino acid sequence of the .beta. subunit of human complement protein C8: identification of a close structural and ancestral relationship to the .alpha. subunit and C9

Abstract

A cDNA clone encoding the .beta. subunit (Mr 64000) of the eight component of complement (C8) has been isolated from a human liver cDNA library. This clone has a cDNA inset of 1.95 kilobases (kb) and contains the entire .beta. sequence [1608 base pairs (bp)]. Analysis of total cellular RNA isolated from the hepatoma cell line HepG2 revealed the mRNA for .beta. to be .apprx.2.5 kb. This is similar to the message size for the .alpha. subunit of C8 and confirms the existence of different mRNAs for .alpha. and .beta.. This finding supports genetic evidence that .alpha. and .beta. are encoded at different loci. Analysis of the derived amino acid sequence revealed several membrane surface seeking segments that may facilitate .beta. interaction with target membranes during complement-mediated cytolysis. Determination of the carbohydrate composition indicated 1 or 2 asparagine-linked but no O-linked oligosaccharide chains. Comparison of the .beta. sequence to that reported for .alpha. in the preceding paper [Rao, A. G., Howard, O. M. Z., Ng, S. C., Whitehead, A. S., Colten, H. R. and Sodetz, J. M. (1987) Biochemistry (preceding paper in this issue)] and to that of human C9 revealed a striking homology between all three proteins. For .beta. and .alpha., the overall homology is 33% on the basis of identity and 53% when conserved substitutions are allowed. For .beta. and C9, the values are 26% and 47%, respectively. All three have a large internal domain that is nearly cysteine free and N- and C-termini that are cysteine-rich and homologous to the low-density lipoprotein receptor repeat and epidermal growth factor type sequences, respectively. The overall homology and similarities in size and structural organization are indicative of a close ancestral relationship. It is concluded that .alpha., .beta., and C9 are members of a family of structurally related proteins that are capable of interacting to produce a hydrophilic to amphiphilic transition and membrane association.