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Nielsen HN, Holm R, Sweazey R, Andersen JP, Artigas P, Vilsen B. Na +,K +-ATPase with Disrupted Na + Binding Sites I and III Binds Na + with Increased Affinity at Site II and Undergoes Na +-Activated Phosphorylation with ATP. Biomolecules 2024; 14:135. [PMID: 38275764 PMCID: PMC10812997 DOI: 10.3390/biom14010135] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/12/2023] [Revised: 01/08/2024] [Accepted: 01/10/2024] [Indexed: 01/27/2024] Open
Abstract
Na+,K+-ATPase actively extrudes three cytoplasmic Na+ ions in exchange for two extracellular K+ ions for each ATP hydrolyzed. The atomic structure with bound Na+ identifies three Na+ sites, named I, II, and III. It has been proposed that site III is the first to be occupied and site II last, when Na+ binds from the cytoplasmic side. It is usually assumed that the occupation of all three Na+ sites is obligatory for the activation of phosphoryl transfer from ATP. To obtain more insight into the individual roles of the ion-binding sites, we have analyzed a series of seven mutants with substitution of the critical ion-binding residue Ser777, which is a shared ligand between Na+ sites I and III. Surprisingly, mutants with large and bulky substituents expected to prevent or profoundly disturb Na+ access to sites I and III retain the ability to form a phosphoenzyme from ATP, even with increased apparent Na+ affinity. This indicates that Na+ binding solely at site II is sufficient to promote phosphorylation. These mutations appear to lock the membrane sector into an E1-like configuration, allowing Na+ but not K+ to bind at site II, while the cytoplasmic sector undergoes conformational changes uncoupled from the membrane sector.
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Affiliation(s)
- Hang N. Nielsen
- Department of Biomedicine, Aarhus University, DK-8000 Aarhus, Denmark
| | - Rikke Holm
- Department of Biomedicine, Aarhus University, DK-8000 Aarhus, Denmark
| | - Ryan Sweazey
- Department of Cell Physiology and Molecular Biophysics, Center for Membrane Protein Research, Texas Tech University Health Sciences Center, Lubbock, TX 79430, USA (P.A.)
| | | | - Pablo Artigas
- Department of Cell Physiology and Molecular Biophysics, Center for Membrane Protein Research, Texas Tech University Health Sciences Center, Lubbock, TX 79430, USA (P.A.)
| | - Bente Vilsen
- Department of Biomedicine, Aarhus University, DK-8000 Aarhus, Denmark
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Homareda H, Otsu M, Yamamoto S, Ushimaru M, Ito S, Fukutomi T, Jo T, Eishi Y, Hara Y. A possible mechanism for low affinity of silkworm Na +/K +-ATPase for K . J Bioenerg Biomembr 2017; 49:463-472. [PMID: 29047027 DOI: 10.1007/s10863-017-9729-5] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/05/2017] [Accepted: 09/27/2017] [Indexed: 11/26/2022]
Abstract
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.
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Affiliation(s)
- Haruo Homareda
- Department of Chemistry, Kyorin University School of Medicine, Shinkawa, Mitaka, Tokyo, 181-8611, Japan.
| | - Masahiro Otsu
- Department of Chemistry, Kyorin University School of Medicine, Shinkawa, Mitaka, Tokyo, 181-8611, Japan
| | - Sachiko Yamamoto
- Department of Chemistry, Kyorin University School of Medicine, Shinkawa, Mitaka, Tokyo, 181-8611, Japan
| | - Makoto Ushimaru
- Department of Chemistry, Kyorin University School of Medicine, Shinkawa, Mitaka, Tokyo, 181-8611, Japan
| | - Sayaka Ito
- Department of Medical Technology, School of Health Sciences, Tokyo University of Technology, Nishikamata, Ota-ku, Tokyo, 144-8535, Japan
| | - Toshiyuki Fukutomi
- Department of Pharmacology and Toxicology, Kyorin University School of Medicine, Shinkawa, Mitaka, Tokyo, 181-8611, Japan
| | - Taeho Jo
- Department of Biological Chemistry, University of Michigan, Ann Arbor, MI, 48109, USA
| | - Yoshinobu Eishi
- Department of Human Pathology, Tokyo Medical and Dental University Graduate School, Yushima, Bunkyo-ku, Tokyo, 113-8549, Japan
| | - Yukichi Hara
- Department of Human Pathology, Tokyo Medical and Dental University Graduate School, Yushima, Bunkyo-ku, Tokyo, 113-8549, Japan
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