Synthesis and secretion of _E- and -fibrinogen by transfected COS cells. Cells were transfected with stoichiometric proportions of pBC-vectors containing fibrinogen subunit cDNAs6 12 in combination as indicated below the lanes. When all four subunit cDNAs were cotransfected, _E cDNA was included at a twofold excess. Fibrinogen was immunoprecipitated from cell lysates (A) and culture medium (B) with either antifibrinogen (f) or anti-VI (VI) as indicated above the lanes, and subunits were separated under reducing conditions. Migration of the γ chain as a doublet in cell extracts (see also Figs 5A and 6A) may reflect the presence of the nonglycosylated precursor.