cyclic AMP Regulation and Its Command in the Pacemaker Channel HCN4

Figures

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  Figure 1. (A) Left, ribbon representation (side view) of two opposite subunits of HCN1 (PDB_ID: 5U6O) colored in blue and light blue. The four cytoplasmic C-terminal regions of HCN1 are shown and colored in blue, brown, light blue, and yellow. Right, ribbon representation (side view) of one subunit of HCN1. The domains of the protein are labeled: The Cytoplasmic N-terminal hyperpolarization-activated cyclic nucleotide-gated domain (HCND) in orange, the voltage sensor domain (VSD) in magenta, the pore domain (PD) in green, the cytoplasmic C-terminal C-linker in light blue, and the cyclic nucleotide binding domain (CNBD) in red. (B) Representative whole-cell currents of HCN4 recorded at the indicated voltages in control solution (left) and in the presence of saturating cyclic AMP [cAMP; right; adapted from Porro et al. (2019) with permission]. Scale bar is 200 pA × 500 ms. (C) Mean activation curves of HCN4 in control solution (black) and in the presence of saturating cAMP [red; adapted from Porro et al. (2019) with permission]. Values of the activation curves are shown as mean ± SEM.

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  Figure 2. (A) Superimposition of the two structures of HCN2 CNBD, represented as ribbons, solved in the absence (gray, PDB_ID: 2MPF) and in the presence of cAMP (red, PDB_ID: 3U10). cAMP, represented as red sticks, is shown within the beta roll. The structural elements involved in the principal movements induced by cAMP are labeled. The black arrows indicate the direction of such movements. (B) Ribbon representation of HCN2 CNBD (gray) in complex with TRIP8bnano peptide [green; adapted from Saponaro et al. (2018) with permission]. The structural elements of both proteins are labeled. The N-bundle loop of the CNBD is colored in yellow. The crucial residues involved in the interaction are shown as sticks and labeled. In particular, the negative residues of TRIP8bnano are colored in red, while in blue the positive residues of HCN2 CNBD. (C) Mean activation curves of HCN4 in control solution (black), in the presence of cAMP (red) and in the presence of cAMP and TRIP8bnano [blue; adapted from Saponaro et al. (2018) with permission]. (D) Ribbon representation of HCN4 C-linker/CNBD (green) solved in the presence of cGMP (PDB_ID: 4KL1). The two cyclic GMP (cGMP) molecules, represented as sticks,…