Na +,K +-ATPase of the photoreceptor: Selective expression of α3 and β2 isoforms

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In photoreceptors, Na +,K +-ATPase maintains the ion gradients which power the dark current that sustains the response to light. The enzyme is composed of at least two polypeptides: α (the catalytic subunit) and β. Three different isoforms of the α subunit and two isoforms of the β subunit have been identified in rat. In some tissues, the isozymes have been shown to be differentially expressed during development or in response to varying physiological conditions. RNAs prepared from isolated photoreceptors and from whole retina were analyzed on blots that were hybridized with cDNA probes for the α1, α2, α3, β1 and β2 isoforms. The predominant α and β subunit mRNAs present in the photoreceptor preparation were those encoding the α3 and β2 isoforms, accounting for 85% of the total α signal and 79% of the total β signal, respectively. Proportions of each mRNA were similar in retina, but very different from those observed in two control tissues, brain and kidney. To confirm that the α-subunit mRNA species detected were translated, membranes prepared from isolated photoreceptors and whole retina were examined by immunoblotting. The antibodies detected a pattern of α isoform distribution in these tissues and in kidney and brain controls that agreed remarkably well with the pattern of mRNA expression in the same tissues. Moreover, the α3 isoform was detectable in the inner segment plasma membrane of the photoreceptor by electron microscopic immunocytochemistry. These results indicate that α3 and β2 are the predominant isoforms of Na +, K +-ATPase expressed in photoreceptors and retina.