Phosphorylation by CK2 and MAPK enhances calnexin association with ribosomes
Open Access
- 1 July 1999
- journal article
- research article
- Published by Springer Nature in The EMBO Journal
- Vol. 18 (13) , 3655-3666
- https://doi.org/10.1093/emboj/18.13.3655
Abstract
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.Keywords
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