The protein phosphatases which dephosphorylate native, sarcoplasmic reticulum 
(SR)-associated phospholamban were studied in cardiac muscle extracts and in a 
Triton fraction prepared by detergent extraction of myofibrils, the latter 
fraction containing 70-80% of the SR-associated proteins present in the tissue. 
At physiological concentrations of free Mg2+ (1 mM), protein phosphatase 1 (PP1) 
accounted for approximately 70% of the total phospholamban phosphatase activity 
in these fractions towards either Ser-16 (the residue labelled by cAMP-dependent 
protein kinase, PK-A) or Thr-17 (the residue phosphorylated by an SR-associated 
Ca2+/calmodulin-dependent protein kinase). Protein phosphatase 2A (PP2A) and 
protein phosphatase 2C (PP2C) accounted for the remainder of the activity. A 
major form of cardiac PP1, present in comparable amounts in both the extract and 
Triton fraction, was similar, if not identical, to skeletal muscle protein 
phosphatase 1G (PP1G), which is composed of the PP1 catalytic (C) subunit 
complexed to a G subunit of approximately 160 kDa, responsible for targeting PP1 
to both the SR and glycogen particles of skeletal muscle. This conclusion was 
based on immunoblotting experiments using antibody to the G subunit, ability to 
bind to glycogen and the release of PP1 activity from glycogen upon incubation 
with PK-A and MgATP. PP1 accounted for approximately 90% of the phospholamban 
(Ser-16 or Thr-17) phosphatase activity in the material sedimented by 
centrifugation at 45,000 x g, a fraction prepared from cardiac extracts which is 
enriched in SR membranes. The G subunit in this fraction could be solubilised by 
Triton X-100, but not with 0.5 M NaCl or digestion with alpha-amylase, 
indicating that it is bound to membranes and not to glycogen. By analogy with 
the situation in skeletal muscle, the PK-A catalysed phosphorylation of the G 
subunit, with ensuing release of the C subunit from the SR, may prevent PP1 from 
dephosphorylating SR-bound substrates and represent one of the mechanisms by 
which adrenalin increases the phosphorylation of cardiac phospholamban (Ser-16 
and Thr-17) in vivo. Hearts left in situ post mortem lose 85-95% of their PP1 
activity within 20-30 min. This remarkable disappearance of PP1 may partly 
explain why the importance of this enzyme in cardiac muscle metabolism has not 
been recognized previously.