Targetting of protein phosphatase 1 to the sarcoplasmic reticulum of rabbit skeletal muscle by a protein that is very similar or identical to the G subunit that directs the enzyme to glycogen.
The amount of protein phosphatase 1 (PP1) activity in rabbit skeletal muscle associated with membranes (predominantly sarcoplasmic reticulum) is similar to that bound to glycogen-protein particles. Membrane-vesicle-associated (sarcovesicular) PP1 can be solubilised with 0.5% Triton X-100 (but not 0.5M NaCl) and is complexed to a protein that is structurally and functionally very similar or identical to the G subunit which targets PP1 to glycogen-protein particles. This conclusion is based on immunoblotting and immunotitration experiments using two different preparations of G-subunit-specific antibodies, binding of Triton-solubilised sarcovesicular enzyme to glycogen, stimulation of phosphorylase phosphatase activity by glycogen, phosphorylation of the same tryptic peptides by cyclic-AMP-dependent protein kinase (A-kinase) and release of catalytic subunit following phosphorylation by A-kinase. Membrane-association is not mediated via glycogen because sarcovesicular PP1 is (1) not released by digestion with alpha-amylase or at dilutions which fully dissociate the glycogen-bound enzyme, and (2) is solubilised by Triton X-100 (whereas glycogen-associated PP1 is not). These findings demonstrate that sarcovesicular PP1 is highly homologous to, or the same as, glycogen-associated PP1G and raises the possibility that a common targetting subunit may direct PP1 to different subcellular locations.
Hubbard
M J
MJ
Department of Biochemistry, University of Dundee, Scotland.
Dent
P
P
Smythe
C
C
Cohen
P
P
D000818:Animals; D002850:Chromatography, Gel; D019204:GTP-Binding Proteins / Q000378:metabolism*; D006003:Glycogen / Q000378:metabolism* / Q000494:pharmacology; D007700:Kinetics; D046911:Macromolecular Substances; D009132:Muscles / Q000201:enzymology*; D010748:Phosphopeptides / Q000302:isolation & purification; D010749:Phosphoprotein Phosphatases / Q000302:isolation & purification / Q000378:metabolism*; D010766:Phosphorylation; D011189:Potassium Chloride / Q000494:pharmacology; D011485:Protein Binding; D054645:Protein Phosphatase 1; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*; D014357:Trypsin
D046911:Macromolecular Substances; D010748:Phosphopeptides; D011189:Potassium Chloride; D006003:Glycogen; D010749:Phosphoprotein Phosphatases; D054645:Protein Phosphatase 1; D014357:Trypsin; D019204:GTP-Binding Proteins