Mutation in the lumenal part of the membrane domain of HMG-CoA reductase alters its regulated degradation.

The involvement of ER lumenal domains of HMG-CoA reductase in the regulated degradation process was examined. For this purpose we studied three cell lines expressing HMG-CoA reductase molecules with introduced functional N-glycosylation sites located in the linker segments between transmembrane spans 1 and 2 (HMGal/Bins(-)), 3 and 4 (HMGal/Dins(-)) and 5 and 6 (HMGal/Fins(-)), all facing the ER lumen (Olender, E. H. and Simoni, R. D. (1992) J. Biol. Chem. 267, 4223-4235. The glycosylation insertion between spons 5 and 6 (HMGal/Fins(-)) is the only one of these mutations which eliminates regulated degradation of the enzyme. The half lives of the HMGal/Fins(-) in the presence or absence of regulatory molecules are indistinguishable. In contrast the HMGal/Bins(-) and HMGal/Dins(-) mutants show a normal pattern of regulated degradation. Tunicamycin treatment of cells expressing the HMGal/Fins(-) mutant does not significantly alter the regulation defect indicating that it is the mutation per se not the glycosylation that alters the degradation response. These results suggest that the linker segments between transmembrane spans 5 and 6 (loop F) are involved in the process of regulated degradation of HMG-CoA reductase and that the regulated degradation process may occur on the lumenal side of the ER membrane.