| Abstract: | Lipid metabolism in a concanavalin A-resistant, glycosylation-defective mutant cell line was investigated by comparing growth properties, lipid composition, and lipid biosynthesis in wild-type (WT), mutant (CR-7), and revertant (RCR-7) cells. In contrast to WT and RCR-7, the mutant was auxotrophic for cholesterol, but mevalonolactone did not restore growth on lipoprotein-deficient medium. The use of R-2-14C]mevalonolactone revealed that CR-7 was deficient in the conversion of lanosterol to cholesterol. Total lipid and phospholipid content and composition were similar in all three cell lines, but CR-7 displayed subnormal content and biosynthesis of cholesterol and unsaturated fatty acids. The mutant was hypersensitive to compactin and was unable to upregulate either 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activity or the binding and internalization of 125I-labeled low-density lipoprotein (LDL) in response to lipoprotein deprivation. HMG-CoA reductase activity in all three cell lines showed similar kinetics and phosphorylation status, and the binding kinetics and degradation of 125I-LDL were also similar, suggesting that CR-7 possesses kinetically normal reductase and LDL binding sites, but is deficient in their coordinate regulation. Tunicamycin (1-2 micrograms/ml) strongly and reversibly suppressed reductase activity in WT and RCR-7. CR-7 was resistant to this inhibitor. In WT cells this suppressive effect was accompanied by inhibition of 3H-labeled mannose incorporation into cellular protein, but 3H-labeled leucine incorporation was unaffected. Immunotitration of HMG-CoA reductase activity in extracts of WT cells, cultured in the presence and absence of tunicamycin, showed that suppression of reductase activity reflected the presence of reduced amounts of reductase protein, implying that glycosylation plays an important role in the coordinate regulation of HMG-CoA reductase activity and LDL binding. |
