a, Side view of soluble guanylate cyclase in the inactive state, highlighting key interfaces (grey rectangles). Each domain is coloured as in Fig. 1a. The surface of sGC is shown in transparency. b, The interface between the α1 H-NOX domain and the PAS domains boxed in a. c, The interface between the PAS domains boxed in a. d, The interface between β1 H-NOX and adjacent domains boxed in a. e, A 180° rotated view compared to d. f, The structure of the transducer module boxed in a. The side chains of α1 L425 and β1 L365 that are in close proximity are shown as spheres. g, The interface between the transducer module and the catalytic module boxed in a. h, A 90° rotated top view compared to g. i, End-point activity of the less-Cys construct (sGCLC, α1LC + β1LC) compared to the wild-type sGC with CGFP. Mean ± s.d., n = 3 biologically independent samples. j, SDS–PAGE of the in vitro disulfide bond cross-linking experiment of α1LC (L275C) with β1LC(A316C) mutants under reducing and non-reducing conditions. The in-gel GFP fluorescence of the α1 subunit is shown in black on a white background. The position of cross-linked heterodimer is indicated with a red asterisk. Oxidative cross-linking happened only when the cysteine mutants, α1(L275C) and β1(A316C), were present in both subunits simultaneously. The experiments were repeated independently three times with similar results. For gel source data, see Supplementary Fig. 1. k, SDS–PAGE of the in vitro disulfide bond cross-linking experiment of α1LC(L425C) with β1LC(L365C) under reducing and non-reducing conditions. Oxidative cross-linking happened only when the cysteine mutants, α1(L425C) and β1(L365C), were present in both subunit simultaneously. For gel source data, see Supplementary Fig. 1. The experiments were repeated independently three times with similar results. l, SDS–PAGE of the in vitro disulfide bond cross-linking experiment of α1LC(L275C) with β1LC(L365C) and α1LC(L425C) with β1LC(A316C) under reducing and non-reducing conditions. In contrast to α1(L275C) with β1(A316C) and α1(L425C) with β1(L365C), α1(L275C) did not crosslink with β1(L365C), and α1(L425C) did not crosslink with β1(A316C), owing to their long spatial distance. For gel sourcfe data, see Supplementary Fig. 1. The experiments were repeated independently twice with similar results. Source Data