Phosphorylation by CK2 and MAPK enhances calnexin association with ribosomes.
Calnexin was initially identified as an endoplasmic reticulum (ER) type I integral membrane protein, phosphorylated on its cytosolic domain by ER-associated protein kinases. Although the role of the ER luminal domain of calnexin has been established as a constituent of the molecular chaperone machinery of the ER, less is known about the role of the cytosolic phosphorylation of calnexin. Analysis by two-dimensional phosphopeptide maps revealed that calnexin was in vitro phosphorylated in isolated microsomes by casein kinase 2 (CK2) and extracellular-signal regulated kinase-1 (ERK-1) at sites corresponding to those for in vivo phosphorylation. In canine pancreatic microsomes, synergistic phosphorylation by CK2 and ERK-1 led to increased association of calnexin with membrane-bound ribosomes. In vivo, calnexin-associated ERK-1 activity was identified by co-immunoprecipitation. This activity was abolished in cells expressing a dominant-negative MEK-1. Activation of ERK-1 in cells by addition of serum led to a 4-fold increase in ribosome-associated calnexin over unstimulated cells. Taken together with studies revealing calnexin association with CK2 and ERK-1, a model is proposed whereby phosphorylation of calnexin leads to a potential increase in glycoprotein folding close to the translocon.
Chevet
E
E
Department of Anatomy and Cell Biology, McGill University, Montreal, Quebec, H3A 2B2.
Wong
H N
HN
Gerber
D
D
Cochet
C
C
Fazel
A
A
Cameron
P H
PH
Gushue
J N
JN
Thomas
D Y
DY
Bergeron
J J
JJ
D000818:Animals; D001798:Blood Proteins / Q000494:pharmacology; D002135:Calcium-Binding Proteins / Q000378:metabolism*; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000378:metabolism*; D037281:Calnexin; D047390:Casein Kinase II; D002460:Cell Line; D003600:Cytosol / Q000378:metabolism; D004285:Dogs; D018870:Endoplasmic Reticulum, Rough / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism; D004789:Enzyme Activation / Q000187:drug effects; D007425:Intracellular Membranes / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism; D048369:MAP Kinase Kinase 1; D008861:Microsomes / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism; D048052:Mitogen-Activated Protein Kinase 3; D020929:Mitogen-Activated Protein Kinase Kinases; D020928:Mitogen-Activated Protein Kinases; D010179:Pancreas / Q000166:cytology; D010766:Phosphorylation; D011233:Precipitin Tests; D011485:Protein Binding / Q000187:drug effects; D017346:Protein Serine-Threonine Kinases / Q000235:genetics / Q000378:metabolism*; D011505:Protein-Tyrosine Kinases / Q000235:genetics / Q000378:metabolism; D051381:Rats; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D012270:Ribosomes / Q000187:drug effects / Q000378:metabolism*; D012694:Serine / Q000378:metabolism
D001798:Blood Proteins; D002135:Calcium-Binding Proteins; D011993:Recombinant Fusion Proteins; D037281:Calnexin; D012694:Serine; D011505:Protein-Tyrosine Kinases; D047390:Casein Kinase II; D017346:Protein Serine-Threonine Kinases; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D048052:Mitogen-Activated Protein Kinase 3; D020928:Mitogen-Activated Protein Kinases; D048369:MAP Kinase Kinase 1; D020929:Mitogen-Activated Protein Kinase Kinases