A possible mechanism for low affinity of silkworm Na+/K+-ATPase for K

The affinity for K + of silkworm nerve Na + /K + -ATPase is markedly lower than that of mammalian Na + /K + -ATPase (Homareda 2010 ). In order to obtain clues on the molecular basis of the difference in K + affinities, we cloned cDNAs of silkworm ( Bombyx mori ) nerve Na + /K + -ATPase α and β subunits, and analyzed the deduced amino acid sequences. The molecular masses of the α and β subunits were presumed to be 111.5 kDa with ten transmembrane segments and 37.7 kDa with a single transmembrane segment, respectively. The α subunit showed 75% identity and 93% homology with the pig Na + /K + -ATPase α1 subunit. On the other hand, the amino acid identity of the β subunit with mammalian counterparts was as low as 30%. Cloned α and β cDNAs were co-expressed in cultured silkworm ovary-derived cells, BM-N cells, which lack endogenous Na + /K + -ATPase. Na + /K + -ATPase expressed in the cultured cells showed a low affinity for K + and a high affinity for Na + , characteristic of the silkworm nerve Na + /K + -ATPase. These results suggest that the β subunit is responsible for the affinity for K + of Na + /K + -ATPase.