Fig. 7. Various substitutions for the two residues in FSHr that gave a gain of function towards hCG. Concentration–action curves for FSHr mutantated in position K104 or K179, under stimulation by increasing concentrations of hCG. COS-7 cells transfected with the various constructs were handled as described in the legend to Figure 3. (A) Only the K104N mutant gave a drastic gain of sensitivity towards hCG. Loss of a basic residue (K104A) or charge reversal (K104D) gave only a very slight gain of function towards hCG. (B) The loss of basic character in position 179 is important; but full gain of function is observed only if a small neutral residue (Gly or Ala) is substitued for K179. (C) Short table presenting expression and EC₅₀ values for hCG stimulation (designed as in Figure 3).