In the reaction cycle of the Na,K-pump, the cation sites are proposed to occlude Na+- and K+-ions in a ping pong sequence. For understanding the pump mechanism it is important to identify" amino acid residues engaged in coordination of Na+ or K+. To identify amino acid side chains contributing to occlusion of K+ in the E2[2K]-form, direct measurements of S6Rb+ and 24T1+ occlusion were done on recombinant Na,K-ATPase from yeast. The wild type enzyme was capable of occluding two S6Rb+ or 24Tl+-ions per ouabain binding site or al3l unit with high apparent affinity, like the purified Na,KATPase from pig kidney. Mutations to Gluaar(Gln,Asp), AspS4(Asn,Glu), AspSS(Asn,Glu) and the substitution of Glurr9 for Asp almost abolished the occlusion capacity. The substitution of Glurr9 for Gtn reduced the occlusion capacity to one 24T1+ ion per al/31 unit. Reduced phosphorylation levels or affinities for Na+ in presence of oligomycin indicate that Glua2r, Asps4 and Aspss also contribute to coordination of Na+ in the EIP[3Na] form. This demonstration of alternate interactions of Na+ or K+ with Glua2r in TM4 and Asps4 and Aspss in TM6 support the notion of cation binding in a ping-pong sequence in catalytic models of Na,K-pumping. This work was supported by the Danish Research Council, the Novo Nordic Foundation and the Carlsberg Foundation.