Conformational change of the extracellular parts of the CFTR protein during channel gating

Molecular dynamics study of Cl- permeation through cystic fibrosis transmembrane conductance regulator (CFTR)

The recent elucidation of atomistic structures of Cl^- channel CFTR provides opportunities for understanding the molecular basis of cystic fibrosis. Despite having been activated through phosphorylation and provided with ATP ligands, several near-atomistic cryo-EM structures of CFTR are in a closed state, as inferred from the lack of a continuous passage through a hydrophobic bottleneck region located in the extracellular portion of the pore. Here, we present repeated, microsecond-long molecular dynamics simulations of human CFTR solvated in a lipid bilayer and aqueous NaCl. At equilibrium, Cl^- ions enter the channel through a lateral intracellular portal and bind to two distinct cationic sites inside the channel pore but do not traverse the narrow, de-wetted bottleneck. Simulations conducted in the presence of a strong hyperpolarizing electric field led to spontaneous Cl^- translocation events through the bottleneck region of the channel, suggesting that the protein relaxed to a functionally open state. Conformational changes of small magnitude involving transmembrane helices 1 and 6 preceded ion permeation through diverging exit routes at the extracellular end of the pore. The pore bottleneck undergoes wetting prior to Cl^- translocation, suggesting that it acts as a hydrophobic gate. Although permeating Cl^- ions remain mostly hydrated, partial dehydration occurs at the binding sites and in the bottleneck. The observed Cl^- pathway is largely consistent with the loci of mutations that alter channel conductance, anion binding, and ion selectivity, supporting the model of the open state of CFTR obtained in the present study.

Conserved amino acids in the region connecting membrane spanning domain 1 to nucleotide binding domain 1 are essential for expression of the MRP1 (ABCC1) transporter

Multidrug resistance protein 1 (MRP1) (gene symbol ABCC1) is an ATP-binding cassette (ABC) transporter which effluxes xeno- and endobiotic organic anions including estradiol glucuronide and the pro-inflammatory leukotriene C₄. MRP1 also confers multidrug resistance by reducing intracellular drug accumulation through active efflux. MRP1 has three membrane spanning domains (MSD), and two nucleotide binding domains (NBD). MSD1 and MSD2 are linked to NBD1 and NBD2 by connecting regions (CR) 1 and CR2, respectively. Here we targeted four residues in CR1 (Ser⁶¹², Arg⁶¹⁵, His⁶²², Glu⁶²⁴) for alanine substitution and unexpectedly, found that cellular levels of three mutants (S612A, R615A, E624A) in transfected HEK cells were substantially lower than wild-type MRP1. Whereas CR1-H622A properly trafficked to the plasma membrane and exhibited organic anion transport activity comparable to wild-type MRP1, the poorly expressing R615A and E624A (and to a lesser extent S612A) mutant proteins were retained intracellularly. Analyses of cryogenic electron microscopic and atomic homology models of MRP1 indicated that Arg⁶¹⁵ and Glu⁶²⁴ might participate in bonding interactions with nearby residues to stabilize expression of the transporter. However, this was not supported by double exchange mutations E624K/K406E, R615D/D430R and R615F/F619R which failed to improve MRP1 levels. Nevertheless, these experiments revealed that the highly conserved CR1-Phe⁶¹⁹ and distal Lys⁴⁰⁶ in the first cytoplasmic loop of MSD1 are also essential for expression of MRP1 protein. This study is the first to demonstrate that CR1 contains several highly conserved residues critical for plasma membrane expression of MRP1 but thus far, currently available structures and models do not provide any insights into the underlying mechanism(s). Additional structures with rigorous biochemical validation data are needed to fully understand the bonding interactions critical to stable expression of this clinically important ABC transporter.

Contribution of the eighth transmembrane segment to the function of the CFTR chloride channel pore

Our molecular understanding of the cystic fibrosis transmembrane conductance regulator (CFTR)-the chloride channel that is mutated in cystic fibrosis-has been greatly enhanced by a number of recent atomic-level structures of the protein in different conformations. One surprising aspect of these structures was the finding that the eighth of CFTR's 12 membrane-spanning segments (TM8) appeared close to the channel pore. Although functional evidence supports a role for other TMs in forming the pore, such a role for TM8 has not previously been reported. Here, we use patch-clamp recording to investigate the functional role of TM8. Using substituted cysteine accessibility mutagenesis, we find that three amino acid side-chains in TM8 (Y913, Y914, and Y917) are exposed to the extracellular, but not the intracellular, solution. Cysteine cross-linking experiments suggest that Y914 and Y917 are in close proximity to L102 (TM1) and F337 (TM6), respectively, suggesting that TM8 contributes to the narrow selectivity filter region of the pore. Different amino acid substitutions suggest that Y914, and to a lesser extent Y917, play important roles in controlling anion flux through the open channel. Furthermore, substitutions that reduce side-chain volume at Y917 severely affect channel gating, resulting in a channel with an extremely unstable open state. Our results suggest that pore-lining TM8 is among the most important TMs controlling the permeation phenotype of the CFTR channel, and also that movement of TM8 may be critically involved in channel gating.

Structural mechanisms of CFTR function and dysfunction

Hwang et al. integrate new structural insights with prior functional studies to reveal the functional anatomy of CFTR chloride channels.

Cystic fibrosis (CF) transmembrane conductance regulator (CFTR) chloride channel plays a critical role in regulating transepithelial movement of water and electrolyte in exocrine tissues. Malfunction of the channel because of mutations of the cftr gene results in CF, the most prevalent lethal genetic disease among Caucasians. Recently, the publication of atomic structures of CFTR in two distinct conformations provides, for the first time, a clear overview of the protein. However, given the highly dynamic nature of the interactions among CFTR’s various domains, better understanding of the functional significance of these structures requires an integration of these new structural insights with previously established biochemical/biophysical studies, which is the goal of this review.