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Figure 5.
(a) Model of the cytoplasmic domains in a hypothetical
dimeric arrangement, with the transmembrane domain viewed from
the intracellular side. Gray ribbon, structure of E. coli ClC
dimer (gray ribbon), which serves as a model for the
transmembrane domains; green ribbon, R-helix; green spheres,
bound ions; blue and red ribbons, the two domains, in
arrangement observed in a homologous bacterial protein. ATP
molecules are shown as CPK models. (b) Alternative model, with
domain dimers in the conformation observed in the ClC-5 domain
crystal form. View is from within the membrane; coloring scheme
is similar to a. (c) Schematic model of a possible
conformational change in ClC-5 induced by ATP binding. Left,
model of the ClC-5 mutant E211A. ATP is bound to the cytoplasmic
domain, stabilizing a conformation that allows Cl^- ions to flow
equally well in both directions. Right, model of a mutant with
compromised nucleotide-binding properties. In the absence of
bound nucleotides, the cytoplasmic domains induce a
conformational change in the ion-binding site via a regulatory
helix of the transmembrane domain (R- helix, green) that
diminishes Cl^- flow from the cytoplasm. The two subunits are
colored in red and blue, respectively.
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