An AMP-activated protein kinase has been reported to phosphorylate rodent 3-hydroxy-3-methylglutaryl-coenzyme A reductase [HMG-CoA reductase; (S)-mevalonate:-NAD+ oxidoreductase (CoA-acylating), EC 1.1.1.88] at Ser-871, thereby lowering its catalytic activity [Clarke, P. R. & Hardie, D. G. (1990) EMBO J. 9, 2439-2446]. To explore the physiologic role of this reaction, we prepared a cDNA encoding a mutant form of hamster HMG-CoA reductase with alanine substituted for serine at residue 871. When overexpressed in transfected cells, the wild-type enzyme, but not the Ser-871 to Ala mutant, was labeled with [32P]phosphate, confirming Ser-871 as the site of phosphorylation. The wild-type enzyme, but not the mutant enzyme, showed reduced activity when the cells were harvested with the phosphatase inhibitor KF, confirming phosphorylation as a mechanism for inactivation within the cell. Despite the lack of phosphorylation, the posttranscriptional feedback regulation of the mutant enzyme was normal, as indicated by reduced activity when cells were incubated with mevalonate, 25-hydroxycholesterol, or low density lipoprotein. Moreover, the mutant enzyme showed a normal acceleration of degradation when the transfected cells were incubated with sterols. Cells expressing the wild-type enzyme showed a decreased incorporation of [14C]pyruvate into sterols when ATP was depleted by incubation with 2-deoxy-D-glucose. No such reduction was seen in cells expressing the Ser-871 to Ala mutant enzyme. We conclude that the AMP-activated protein kinase does not play a role in end-product feedback regulation of HMG-CoA reductase, but rather it comes into play when cellular ATP levels are depleted, thereby lowering the rate of cholesterol synthesis and preserving the energy stores of the cell.