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Volume 271, Number 43, Issue of October 25, 1996 pp. 26602-26608
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.

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Kinetic Interconversion of Rat and Bovine Homologs of the  Subunit of an Amiloride-sensitive Na⁺ Channel by C-terminal Truncation of the Bovine Subunit

(Received for publication, May 21, 1996, and in revised form, July 17, 1996)

From the Department of Physiology and Biophysics, University of Alabama at Birmingham, Birmingham, Alabama 35294

We have recently cloned the  subunit of a bovine amiloride-sensitive Na⁺ channel (bENaC). This subunit shares extensive homology with both rat and human ENaC subunits but shows marked divergence at the C terminus beginning at amino acid 584 of the 697-residue sequence. When incorporated into planar lipid bilayers, bENaC almost exclusively exhibits a main transition to 39 picosiemens (pS) with very rare 13 pS step transitions to one of two subconductance states (26 and 13 pS). In contrast, the  subunit of the rat renal homolog of ENaC (rENaC) has a main transition step to 13 pS that is almost constituitively open, with a second stepwise transition of 26 to 39 pS. A deletion mutant of bENaC, encompassing the entire C-terminal region (R567X), converts the kinetic behavior of bENaC to that of rENaC, i.e. a transition to 13 pS followed by a second 26 pS transition to 39 pS. Chemical cross-linking of R567X restores the wild-type bENaC gating pattern, whereas treatment with the reducing agent dithiothreitol produced only 13 pS transitions. In contrast, an equivalent C-terminal truncation of rENaC (R613X) had no effect on the gating pattern of rENaC. These results are consistent with the hypothesis that interactions between the C termini of bENaC account for the different kinetic behavior of this member of the ENaC family of Na⁺ channels.

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