Functional effects of endogenous cross-linking of double-Cys mutants. In pWT-HRV, Cys was substituted for each of the four residues flanking S0 (R17C, G18C, Q19C, and R20C, as indicated on the abscissas). These were paired with Cys substituted for residues flanking (A and B) S1 (P137C), (C and D) S2 (F144C), and (E–H) S3–S4 (L199C and R201C), as indicated in the top right of each panel. The conditions were as in Fig. 4. ΔV₅₀ and k_MUT/k_pWT-HRV were corrected for the effects of the single Cys mutations and were extrapolated to 100% cross-linking, taking into account the actual extent of cross-linking, as described in Materials and methods. The errors were propagated as in Fig. 4 (n > 4 in all cases). (I) Binned effects of cross-linking on V₅₀, represented by color-coded lines connecting the cross-linked residues, superimposed on the extracellular ends of S0–S4, taken from Fig. 3 F. (J) Binned effects of cross-linking on k_ACT,MUT/k_ACT,pWT-HRV or k_DEACT,MUT/k_{DEACT,pWT-HRV}, whichever was greater.