Selectivity of externally facing ion-binding sites in the Na/K pump to alkali metals and organic cations
RIS ID
38758
Abstract
The Na/K pump is a P-type ATPase that exchanges three intracel- lular Na+ ions for two extracellular K+ ions through the plasma- lemma of nearly all animal cells. The mechanisms involved in cation selection by the pump’s ion-binding sites (site I and site II bind either Na+ or K+; site III binds only Na+) are poorly understood. We studied cation selectivity by outward-facing sites (high K+ af- finity) of Na/K pumps expressed in Xenopus oocytes, under voltage clamp. Guanidinium+, methylguanidinium+, and aminoguanidi- nium+ produced two phenomena possibly reflecting actions at site III: (i) voltage-dependent inhibition (VDI) of outwardly di- rected pump current at saturating K+, and (ii) induction of pump- mediated, guanidinium-derivative–carried inward current at nega- tive potentials without Na+ and K+. In contrast, formamidinium+ and acetamidinium+ induced K+-like outward currents. Measure- ment of ouabain-sensitive ATPase activity and radiolabeled cation uptake confirmed that these cations are external K+ congeners. Molecular dynamics simulations indicate that bound organic cati- ons induce minor distortion of the binding sites. Among tested metals, only Li+ induced Na+-like VDI, whereas all metals tested except Na+ induced K+-like outward currents. Pump-mediated K+- like organic cation transport challenges the concept of rigid struc- tural models in which ion specificity at site I and site II arises from a precise and unique arrangement of coordinating ligands. Further- more, actions by guanidinium+ derivatives suggest that Na+ binds to site III in a hydrated form and that the inward current observed without external Na+ and K+ represents cation transport when nor- mal occlusion at sites I and II is impaired. These results provide insights on external ion selectivity at the three binding sites.
Publication Details
Ratheal, I. M., Virgin, G. K., Yu, H., Roux, B., Gatto, C. & Artigas, P. (2010). Selectivity of externally facing ion-binding sites in the Na/K pump to alkali metals and organic cations. National Academy of Sciences. Proceedings, 107 (43), 18718-18723.