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.