Nucleotide specificity of cardiac sarcoplasmic reticulum. Inhibition of GTPase activity by ATP analogue in fluorescein isothiocyanate-modified calcium ATPase

Phospholamban modulates the functional coupling between nucleotide domains in Ca-ATPase oligomeric complexes in cardiac sarcoplasmic reticulum

Oligomeric interactions between Ca-ATPase polypeptide chains and their modulation by phospholamban (PLB) were measured in native cardiac sarcoplasmic reticulum (SR) microsomes. Progressive modification of Lys(514) with fluorescein 5-isothiocyanate (FITC), which physically blocks access to the nucleotide binding site by ATP, demonstrates that Ca-ATPase active sites function independently of one another prior to the phosphorylation of PLB. However, upon cAMP-dependent protein kinase (PKA) phosphorylation of PLB, a second-order dependence between residual enzyme activity and the fraction of active sites is observed, consistent with a dimeric functional complex. Complementary distance measurements were made using FITC or 5-iodoacetamidofluorescein (IAF) bound to Cys(674) within the N- or P-domains, respectively, to detect structural coupling within oligomeric complexes. Accompanying the phosphorylation of PLB, neighboring Ca-ATPase polypeptide chains exhibit a 4 +/- 2 A decrease in the proximity between FITC sites within the N-domain and a 9 +/- 3 A increase in the proximity between IAF sites within P-domains. Thus, the phosphorylation of PLB induces spatial rearrangements between the N- and P-domain elements of proximal Ca-ATPase polypeptide chains which restore functional interactions between neighboring polypeptide chains and, in turn, result in increased rates of catalytic turnover. These results are interpreted in terms of a structural model, calculated through optimization of shape complementarity, desolvation, and electrostatic energies, which suggests a dimeric arrangement of Ca-ATPase polypeptide chains through the proximal association of N-domains that accommodates interaction with PLB. We suggest that the phosphorylation of PLB acts to release constraints involving interdomain subunit interactions that enhance catalytically important N-domain motions.

Inhibition of the red blood cell calcium pump by eosin and other fluorescein analogues

This paper addresses the mechanism of inhibition of the plasma membrane Ca pump by fluorescein analogues and their isothiocyanate derivatives. Eosin (i.e., tetrabromofluorescein) was found to be one of the most potent reversible inhibitors of the erythrocyte Ca pump [half-maximal inhibitory concentration (IC50) < 0.2 microM]; fluorescein itself was about four orders of magnitude less potent (IC50 approximately 1,000 microM). Eosin decreased the maximum influx and thus did not compete with ATP for the Ca pump. Irreversible inhibition produced by the isothiocyanate analogues of eosin and fluorescein [eosin 5-isothiocyanate (EITC) and fluorescein 5-isothiocyanate (FITC), respectively] was also studied. While EITC bound reversibly at the eosin site, two results suggest that EITC does not react covalently at this site: 1) eosin did not alter the time course of the EITC irreversible reaction, and 2) the concentration dependence for reversible EITC inhibition was different from the concentration dependence for irreversible EITC inhibition. ATP did slow the rate of inactivation of both EITC and FITC consistent with the idea that EITC and FITC bind to the ATP site. Our results are consistent with eosin and ATP binding to separate sites and EITC reacting covalently at the ATP site, but not the eosin site.

The MgATPase activity of rat cardiac sarcoplasmic reticulum is a function of the calcium ATPase protein

Magnesium-dependent ATPase (MgATPase) activity is associated with many E1-E2 or P-type transport ATPases including the sarcoplasmic reticulum (SR) calcium ATPase. The SR isolated from rat heart has a MgATPase activity which is 6-12 times faster than the MgATPase activity of the SR isolated from dog heart. To determine the origin of the high MgATPase activity of rat heart SR, we compared and contrasted cardiac SR isolated from both species. The preparations were similar in the following ways: (i) contamination by other organelles; (ii) the comigration of MgATPase activity with calcium-dependent ATPase (CaATPase) activity through a sucrose gradient; (iii) a similar ATPase activity sensitivity to pH and ATP concentration; (iv) the high and similar of sensitivity of ATPase activity to detergent; and (v) a similar protein profile. In both preparations, a single protein in the 105,000-Da region of polyacrylamide gels was phosphorylated by ATP, and the phosphorylated species was an acylphosphate formed in the presence and absence of calcium. Dimethyl sulfoxide, which slows acylphosphoenzyme breakdown, markedly inhibited both CaATPase and MgATPase activities of both preparations but not other enzyme activities. Importantly, the specific inhibitor of the SR calcium pump, thapsigargin, completely inhibited the CaATPase activity with an I50 of 6-7 nM; however, a higher concentration (I50 of 2 microM) was required to inhibit the MgATPase activity of the rat cardiac SR. These results provide evidence that the MgATPase activity of rat cardiac SR is part of the enzyme cycle of the calcium ATPase protein.