Erythrosin 5 '-isothiocyanate labels Cys(549) as part of the low-affinity ATP binding site of Na+/K+-ATPase

The high-affinity E(1)ATP site of Na+/K+-ATPase labeled with fluorescein 5' -isothiocyanate and its E(2)ATP site labeled with erythrosin 5'-isothiocyan ate (ErITC), as was shown recently [Linnertz et al. (1998) J, Biol, Chem, 2 73, 28813-28821], reside on separate and adjacent catalytic alpha subunits, This paper provides evidence that specific labeling of the E(2)ATP binding site with ErITC resulted in a modification of the Cys(549) residue in the tryptic fragment with the sequence Val(545)-Leu-Gly-Phe-Cys(549)-His(550). Hence, Cys(549) is part of or close to the low-affinity E(2)ATP binding sit e of Na+/K+-ATPase. (C) 1998 Federation of European Biochemical Societies.