The plasma membrane Ca2+ pump mutant lysine(591)->arginine retains some activity, but is still inactivated by fluorescein isothiocyanate

Inactivation of the wild-type human plasma membrane Ca2+ pump (isoform 4b) by fluorescein isothiocyanate is accompanied by covalent modification of Lys(591). The mutation of Lys(591) to arginine reduced the Ca2+ transport activity to 35%, of the wildtype, and diminished the amount of acylphosphate formed from ATP by a corresponding amount. When this mutant was treated with fluorescein isothiocyanate, the enzyme was still irreversibly inactivated, even though no reactive residue was available at position 591. The results show that, although Ca2+ pump function is sensitive to the residue at position 591, Lys(591) is not essential for enzyme activity. They also demonstrate that irreversible inhibition of the plasma membrane Ca2+ pump by fluorescein isothiocyanate does not require the covalent modification of Lys(591). This indicates that fluorescein isothiocyanate reacts with lysine residues at other positions in addition to Lys(591).