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Arg352 Is a Major Determinant of Charge Selectivity in the Cystic Fibrosis Transmembrane Conductance Regulator Chloride Channel
 

Summary: Arg352 Is a Major Determinant of Charge Selectivity in the Cystic Fibrosis
Transmembrane Conductance Regulator Chloride Channel
Romain Guinamard and Myles H. Akabas*,,
Center for Molecular Recognition and Departments of Physiology & Cellular Biophysics and Medicine,
Columbia UniVersity College of Physicians and Surgeons, 630 West 168th Street, New York, New York 10032
ReceiVed January 21, 1999; ReVised Manuscript ReceiVed March 5, 1999
ABSTRACT: The cystic fibrosis transmembrane conductance regulator forms an anion-selective channel.
We previously showed that charge selectivity, the ability to discriminate between anions and cations,
occurs near the cytoplasmic end of the channel. The molecular determinants of charge selectivity, however,
are unknown. We investigated the role of Arg352, a residue flanking the predicted cytoplasmic end of the
M6 segment, in the mechanism of charge selectivity. We determined the Cl- to Na+ permeability ratio
(PCl/PNa) from the reversal potential measured in a 10-fold NaCl gradient. For the wild type, PCl/PNa was
36 (range of 28-51). For the R352H mutant, PCl/PNa was dependent on cytoplasmic pH. At pH 5.4, the
PCl/PNa was 33 (range of 27-41), similar to that of the wild type, but at pH 7.2, where the histidine
should be largely uncharged, PCl/PNa was 3 (range of 2.9-3.1). For the R352C and R352Q mutants,
PCl/PNa was 7 (range of 6-8) and 4 (range of 3.5-4.4), respectively. Furthermore, Na+ which does not
carry a significant fraction of the current through the wild type is measurably conducted through R352Q.
Thus, the charge of the side chain at position 352 is a strong determinant of charge selectivity. In the wild
type, the positive charge on Arg352 contributes to an electrostatic potential in the channel that forms a
barrier to cation permeation. Mutation of Arg352 did not alter the halide selectivity sequence. Selectivity

  

Source: Akabas, Myles - Department of Physiology and Biophysics, Albert Einstein College of Medicine, Yeshiva University

 

Collections: Biology and Medicine