The 19-amino acid cassette of cyclooxygenase-2 mediates entry of the protein into the endoplasmic reticulum-associated degradation system.

Cyclooxygenase (COX) isoforms catalyze the committed step in prostaglandin biosynthesis. The primary structures of COX-1 and COX-2 are very similar except that COX-2 has a 19-amino acid (19-AA) segment of unknown function located just inside its C terminus. Here we provide evidence that the major role of the 19-AA cassette is to mediate entry of COX-2 into the ER-associated degradation system that transports ER proteins to the cytoplasm. COX-1 is constitutively expressed in many cells, whereas COX-2 is usually expressed inducibly and transiently. In murine NIH/3T3 fibroblasts, we find that COX-2 protein is degraded with a half-life (t(1/2)) of about 2 h, whereas COX-1 is reasonably stable (t(1/2) > 12 h); COX-2 degradation is retarded by 26 S proteasome inhibitors. Similarly, COX-1 expressed heterologously in HEK293 cells is quite stable (t(1/2) > 24 h), whereas COX-2 expressed heterologously is degraded with a t(1/2) of approximately 5 h, and its degradation is slowed by proteasome inhibitors. A deletion mutant of COX-2 was prepared lacking 18 residues of the 19-AA cassette. This mutant retains native COX-2 activity but, unlike native COX-2, is stable in HEK293 cells. Conversely, inserting the COX-2 19-AA cassette near the C terminus of COX-1 yields a mutant ins594-612 COX-1 that is unstable (t(1/2) approximately 3 h). Mutation of Asn-594, an N-glycosylation site at the beginning of the 19-AA cassette, stabilizes both COX-2 and ins594-612 COX-1; nonetheless, COX mutants that are glycosylated at Asn-594 but lack the remainder of the 19-amino acid cassette (i.e. del597-612 COX-2 and ins594-596 COX-1) are stable. Thus, although glycosylation of Asn-594 is necessary for COX-2 degradation, at least part of the remainder of the 19-AA insert is also required. Finally, kifunensine, a mannosidase inhibitor that can block entry of ER proteins into the ER-associated degradation system, retards COX-2 degradation.