Evidence for N-glycosylation and ubiquitination of the prolactin receptor expressed in a baculovirus-insect cell system.

The molecular mass of the rabbit prolactin receptor (rbPRLR) deduced from cDNA cloning is 66 kDa. However, the molecular mass of the full-length receptor expressed in the insect Sf9 cells was found to be 94 kDa. In order to explain this discrepancy, we analyzed the possible post-translational modifications of the PRLR. Sf9 cells were infected with recombinant baculoviruses in the presence of tunicamycin, an inhibitor of N-glycosylation. Results showed that an additional approximately 9 kDa of the extracellular domain could be attributed to the N-glycosylation and another additional approximately 20 kDa covalent modification occurred in the cytoplasmic part of the receptor. Western blot analysis, using anti-ubiquitin antibodies, revealed that the rbPRLR was ubiquitinated in its cytoplasmic domain.