Protein kinase A regulates phosphorylation of UBE2J1 at serine residue S266 in response to glucagon signalling
The ubiquitin conjugating enzyme UBE2J1/Ubc6e localizes to the endoplasmic reticulum where it mediates the ubiquitination and proteasomal degradation of terminally misfolded proteins. Although the protein is known to undergo phosphorylation at serine S184, we have considered modification at an additional site and used a bespoke anti-phospho antibody to confirm phosphorylation also at serine residue S266. Despite the well-described role of UBE2J1 in ER associated degradation (ERAD), we found no evidence for regulation at S266 during Unfolded Protein Response (UPR) induction by thapsigargin. Instead, our studies suggest that phosphorylation occurs independently at the S184 and S266 sites, with mutation at one site failing to disrupt basal phosphorylation at the second. We identified several contexts in which these two phosphorylations were differentially regulated. For example, ER localization, which is important for phosphorylation at S184, was not required for modification at S266, and sensitivity to proteasome inhibitors, which is regarded as a distinguishing feature of the S184 phospho-variant, was unaltered by the S266A mutation. Regarding regulation at S266 on the other hand, we found that pharmacological activation of protein kinase A resulted in rapid phosphorylation, with differential use of phospho-specific antibodies confirming that phosphorylation at S184 was unchanged by this treatment. Hormonal stimulation by glucagon resulted in a similar pattern of UBE2J1 phosphorylation, which occurred exclusively at S266 and could be inhibited by H89. The differential regulation demonstrated in these studies extends our understanding of the UBE2J1 enzyme, and may indicate a role in the integration of energy metabolism with environmental stress conditions.