β subunit affects Na + and K + affinities of Na + /K + -ATPase: Na + and K + affinities of a hybrid Na + /K + -ATPase composed of insect α and mammalian β subunits.

The affinity for K ⁺ of silkworm Na ⁺ /K ⁺ -ATPase, which is composed of α and β subunits, is remarkably lower than that of mammalian Na ⁺ /K ⁺ -ATPase, with a slightly higher affinity for Na ⁺ . Because the α subunit had more than 70% identity to the mammalian α subunit in the amino acid sequence, whereas the β subunit, a glycosylated protein, had less than 30% identity to the mammalian β subunit, it was suggested that the β subunit was involved in the affinities for Na ⁺ and K ⁺ of Na ⁺ /K ⁺ -ATPase. To confirm this hypothesis, we examined whether replacing the silkworm β subunit with the mammalian β subunit affected the affinities for Na ⁺ and K ⁺ of Na ⁺ /K ⁺ -ATPase. Cloned silkworm α and cloned rat β1 were co-expressed in BM-N cells, a cultured silkworm ovary-derived cell lacking endogenous Na ⁺ /K ⁺ -ATPase, to construct a hybrid Na ⁺ /K ⁺ -ATPase, in which the silkworm β subunit was replaced with the rat β1 subunit. The hybrid Na ⁺ /K ⁺ -ATPase increased the affinity for K ⁺ by 4.1-fold and for Na ⁺ by 0.65-fold compared to the wild-type one. Deglycosylation of the silkworm β subunit did not affect the K ⁺ affinity. These results support the involvement of the β subunit in the Na ⁺ and K ⁺ affinities of Na ⁺ /K ⁺ -ATPase.
Competing Interests: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
(© 2022 The Authors.)