Effect of _E, β, and γ subunit mutations on Fib₄₂₀ secretion in presence of the wild-type  subunit: No secretion of mixed molecules. Cys → Ser changes were made in the fibrinogen subunits at the positions indicated above the lanes. COS cells were transfected with either wild-type or mutant _E, β, and γ cDNAs together with wild-type  cDNA in the combinations shown; _E constructs were included at a fivefold excess. Fibrinogen was immunoprecipitated from cell lysates (A) with antifibrinogen and run under reducing conditions; from the culture medium (B), fibrinogen was immunoprecipitated with anti-VI and run under nonreducing conditions. As described previously,1due to the differential proteolytic susceptibility of  and _E subunits in cell lysates, a proteolytic fragment derived only from the  chain, not from _E, appears as a band migrating below the γ chain doublet in panel A (compare also Fig 1A).