We previously reported that acylphosphatase, a cytosolic enzyme present in 
skeletal and heart muscle, actively hydrolyzes the phosphoenzyme (EP) of cardiac 
sarcoplasmic reticulum (SR) Ca2+-ATPase (SERCA2a), inducing an increased 
activity of this pump. We hypothesized that acylphosphatase-induced stimulation 
of SERCA2a, in addition to enhanced EP hydrolysis, may be due to a displacement 
of phospholamban (PLN), removing its inhibitory effect. To verify this 
hypothesis co-immunoprecipitation experiments were performed by adding 
recombinant muscle acylphosphatase to solubilized heart SR vesicles, used as a 
source of SERCA2a and PLN. With anti-acylphosphatase antibodies only SERCA2a was 
co-immunoprecipitated in an amount which increased in parallel to the 
concentrations of our enzyme. Conversely, using anti-SERCA2a antibody, both PLN 
and acylphosphatase were co-immunoprecipitated with SERCA2a, and the PLN amount 
in the precipitate decreased with increasing acylphosphatase concentrations. 
SERCA2a and PLN were co-immunoprecipitated by anti-phospholamban antibodies, but 
while the amount of precipitated phospholamban increased in the presence of 
acylphosphatase, the level of SERCA2a decreased. These preliminary results 
strengthen the supposed displacement of phospholamban by acylphosphatase.