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.