Fig. 4: Autoinhibition by ICH7 and cAMP binding primes SLC9C1 for activation.

a, Cryo-EM map of SLC9C1 in nanodiscs (coloured as in Fig. 3a) highlighting the only interaction between the cytoplasmic assembly (ICH7, purple) and the transporter module (TM7, light grey) (left). A peripherally located PA lipid next to TM7′ is also shown (blue). Top right, magnified view of the maps showing the interactions between Lys939 in ICH7′ and TM7′. Bottom right, cartoon representation of SLC9C1 with a cryo-EM mesh (grey) depicting the detailed interactions between ICH7′ (purple) and TM7′ (grey) with labelled residues (stick form). The yellow dashed lines show mostly hydrogen-bond distances of 2.4 to 4.0 Å between Lys939 and the backbone of TM7′ and the side chains Ser340 and Ser342. Further ICH7′–TM7′ interactions are formed between Asn943 and His292 and a π-cation interaction between Phe294 and Arg940. b, The SLC9C1 detergent structure with cAMP addition (coloured as in a) superimposed onto the apo SLC9C1 detergent structure (transparent grey). The cryo-EM map density was mostly absent for the CNBD with cAMP addition, as shown in Extended Data Fig. 9b, except for poor map features for one of the CNBDs (dotted ellipsoid). c, Superimposition of SLC9C1 structures in detergent without cAMP (grey) and with cAMP (coloured as in a). The binding of cAMP to the CNBD probably detaches the domain from S4 and from ICH5, which propagates with the displacement of ICH3, ICH4 and ICH5 as also shown in Extended Data Fig. 10b. The change in the position of the ICHs has altered the positioning of the VSD. Inset: magnified view showing the anti-clockwise movement of the S1–S3 helices when comparing the SLC9C1 structures obtained with (coloured) and without (grey) cAMP addition.