THE PLASMA-MEMBRANE CA2-]ARGININE RETAINS SOME ACTIVITY, BUT IS STILLINACTIVATED BY FLUORESCEIN ISOTHIOCYANATE( PUMP MUTANT LYSINE(591))

Inactivation of the wild-type human plasma membrane Ca2+ pump (isoform 4b) by fluorescein isothiocyanate is accompanied by covalent modifica tion of Lys(591). The mutation of Lys(591) to arginine reduced the Ca2 + transport activity to 35%, of the wildtype, and diminished the amoun t of acylphosphate formed from ATP by a corresponding amount. When thi s mutant was treated with fluorescein isothiocyanate, the enzyme was s till irreversibly inactivated, even though no reactive residue was ava ilable at position 591. The results show that, although Ca2+ pump func tion is sensitive to the residue at position 591, Lys(591) is not esse ntial for enzyme activity. They also demonstrate that irreversible inh ibition of the plasma membrane Ca2+ pump by fluorescein isothiocyanate does not require the covalent modification of Lys(591). This indicate s that fluorescein isothiocyanate reacts with lysine residues at other positions in addition to Lys(591).