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Epithelial Na⁺ Channel Subunit Stoichiometry

Alexander Staruschenko ^*, Emily Adams ^*, Rachell E. Booth  and James D. Stockand ^*

^* Department of Physiology, University of Texas Health Science Center at San Antonio, San Antonio, Texas; and Department of Chemistry and Biochemistry, Texas State University, San Marcos, Texas

Correspondence: Address reprint requests to Alexander Staruschenko, University of Texas Health Science Center at San Antonio, Dept. of Physiology, 7756 7703 Floyd Curl Dr., San Antonio, TX 78229-3900. Tel.: 210-567-4360; Fax: 210-567-4410; E-mail: starushchenk{at}uthscsa.edu.

Ion channels, including the epithelial Na⁺ channel (ENaC), are intrinsic membrane proteins comprised of component subunits. Proper subunit assembly and stoichiometry are essential for normal physiological function of the channel protein. ENaC comprises three subunits, , ß, and , that have common tertiary structures and much amino acid sequence identity. For maximal ENaC activity, each subunit is required. The subunit stoichiometry of functional ENaC within the membrane remains uncertain. We combined a biophysical approach, fluorescence intensity ratio analysis, used to assess relative subunit stoichiometry with total internal reflection fluorescence microscopy, which enables isolation of plasma membrane fluorescence signals, to determine the limiting subunit stoichiometry of ENaC within the plasma membrane. Our results demonstrate that membrane ENaC contains equal numbers of each type of subunit and that at steady state, subunit stoichiometry is fixed. Moreover, we find that when all three ENaC subunits are coexpressed, heteromeric channel formation is favored over homomeric channels. Electrophysiological results testing effects of ENaC subunit dose on channel activity were consistent with total internal reflection fluorescence/fluorescence intensity ratio findings and confirmed preferential formation of heteromeric channels containing equal numbers of each subunit.

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