Phosphorylation-dependent changes in the spatial relationship between Ca-ATPase polypeptide chains in sarcoplasmic reticulum membranes

Abstract

In order to investigate possible structural changes associated with the coupling mechanisms of the Ca-ATPase in sarcoplasmic reticulum membranes, we have utilized fluorescence resonance energy transfer between spectroscopic probes covalently bound to different domains of the ATPase. Using time-correlated single photon counting, we have directly measured the energy transfer efficiency between 5-[2-[(iodoacetyl)amino]ethyl]aminonaphthalene-1-sulfonic acid (IAEDANS), that is specifically bound to the B trypic fragment at cysteines 670 and 674 and acceptors covalently bound either near the nucleotide binding site, i.e. fluorescein 5-isothiocyanate at lysine 515, also on the B fragment, or maleimide-directed probes specifically located on the A1, tryptic fragment, i.e. 4-dimethylaminoazobenzene-4'-maleimide (DABmal) or fluorescein-5-maleimide (Fmal), probably at cysteines 344 and 364. All of these donor-acceptor pairs exhibit energy transfer both within and between Ca-ATPase molecules allowing us to investigate spatial relationships between the A1 and B domains and between different ATPase polypeptide chains. Differentiation between the intra- and intermolecular components of energy transfer was accomplished in two ways: 1) by comparing the transfer efficiencies in native membranes before and after detergent solubilization and 2) by reconstituting ATPase chains that have already been labeled with either the donor or acceptor chromophores. Using this approach, we find no significant change in the intramolecular transfer efficiency between any of these donor-acceptor pairs either upon binding of calcium to the high affinity sites or upon stabilization of the phosphoenzyme intermediate, indicating that there are no large structural changes within the B tryptic fragment or, alternatively, between the A1 and B fragments. With respect to intermolecular energy transfer, we observe no effect of calcium binding on the unliganded enzyme with either donor-acceptor pair. However, formation of the phosphoenzyme intermediate results in a measurable increase in the transfer efficiency between IAEDANS and DABmal (or Fmal); this increase is reversible upon phosphoenzyme destabilization by subsequent addition of calcium. There is no corresponding change in the intermolecular component of fluorescence resonance energy transfer between IAEDANS and fluorescein 5-isothiocyanate, indicating that the change in fluorescence resonance energy transfer probably occurs as a result of reorientation of associated ATPase polypeptide chains with respect to one another.