Domain organization of sGC subunits Each sGC subunit, α1 (grey) and β1 (tan), is composed of four modular domains. The H-NOX domain of the β1 subunit contains the heme co-factor (red). Amino acid numberings approximate the boundaries of the β1 domains. The H-NOX structure is modeled from a standalone H-NOX from Nostoc sp. PCC 7120 (33% identity, PDB: 2o09). The representative PAS domain is from Nostoc punctiforme histidine kinase (35% identity, PDB: 2P04). The helical domain (PDB: 3HLS) and catalytic domain (PDB: 3UVJ) are crystallized truncations of sGC from Rattus norvegicus and Homo sapiens, respectively.