Functional consequences of alterations to Pro 328 and Leu 332 located in the 4th transmembrane segment of the α-subunit of the rat kidney Na +,K +-ATPase

Abstract

Site-specific mutagenesis was used to analyse the functional roles of the residues Pro 328 and Leu 332 located in the conserved PEGLL motif of the predicted transmembrane helix M4 in the α 1-subunit of the ouabain resistant rat kidney Na +,K +-ATPase. cDNAs encoding either of the Na +,K +-ATPase mutants Pro 328→Ala and Leu 332→Ala, and wild type, were cloned into the expression vector pMT2 and transfected into COS-1 cells. Ouabain-resistant clones growing in the presence of 10 μM ouabain were isolated, and the Na +, K +, ATP and pH dependencies of the Na +,K +-ATPase activity measured in the presence of 10 μM ouabain were analysed. Under these conditions the exogenous expressed Na +,K +-ATPase contributed more than 95% of the Na +,K +-ATPase activity. The Pro 328→Ala mutant displayed a reduced apparent affinity for Na + ( K 0.5 (Na +) 13.04 mM), relative to the wild type ( K 0.5 (Na +) 7.13 mM). By contrast, the apparent affinity for Na + displayed by the Leu 332→Ala mutant was increased ( K 0.5 (Na +) 3.92 mM). Either of the mutants exhibited lower apparent affinity for K + relative to the wild type ( K 0.5 (K +) 2.46 mM for Pro 328→Ala and 1.97 mM for Leu 332→Ala, compared with 0.78 mM for wild type). Both mutants exhibited higher apparent affinity for ATP than the wild type ( K 0.5 (ATP) 0.086 mM for Pro 328→Ala and 0.042 mM for Leu 332→Ala, compared with 0.287 mM for wild type). The influence of pH was in accordance with an acceleration of the E2 (K)→E1 transition in the mutants relative to the wild type. These data are consistent with a role of Pro 328 and Leu 332 in the stabilization of the E2 form and of Pro 328 in Na + binding. The possible role of the mutated residues in K + binding is discussed.