Transfected cDNA directs expression of hemolytically active murine C4 in cultured mouse and monkey cells

Previously cloned and sequenced full-length cDNAs for murine C4 and the closely related sex-limited protein (Slp) have been placed into an eucaryotic expression vector. Transfer of these DNA constructs transiently into monkey COS cells or stably into mouse L cells results in the expression and secretion of hemolytically active mouse C4 and mature Slp. We estimate from hemolytic activities that COS and L cells secrete 0.04 and 3%, respectively, of the C4 level found in mouse plasma. Slp expression is consistently only 10-20% that of C4 although the identical expression system is used for both. Our results show subtle but reproducible cell-type-specific differences in C4 maturation; they also indicate that surprisingly large shifts in electrophoretic mobility on SDS-polyacrylamide gels are induced by a small number of amino acid substitutions. The expression of C4 from cDNAs of known sequence provides a starting point for studies of structure/function relationships in C4 employing site-specific mutagenesis and gene transfer.