Peroxisomal protein targeting and identification of peroxisomal targeting signals in cholesterol biosynthetic enzymes.

At least three different subcellular compartments, including peroxisomes, are involved in cholesterol synthesis. Recently, it has been demonstrated that peroxisomes contain a number of enzymes involved in cholesterol biogenesis that previously were considered to be cytosolic or located in the endoplasmic reticulum. Peroxisomes have been shown to contain acetoacetyl-CoA thiolase, HMG-CoA synthase, HMG-CoA reductase, mevalonate kinase, phosphomevalonate kinase, phosphomevalonate decarboxylase, isopentenyl diphosphate isomerase and FPP synthase. Moreover, the activities of these enzymes are also significantly decreased in liver tissue and fibroblast cells obtained from patients with peroxisomal deficiency diseases. In addition, the cholesterol biosynthetic capacity is severely impaired in cultured skin fibroblasts obtained from patients with peroxisomal deficiency diseases. These findings support the proposal that peroxisomes play an essential role in isoprenoid biosynthesis. This paper presents a review of peroxisomal protein targeting and of recent studies demonstrating the localization of cholesterol biosynthetic enzymes in peroxisomes and the identification of peroxisomal targeting signals in these proteins.

Identification of peroxisomal targeting signals in cholesterol biosynthetic enzymes. AA-CoA thiolase, hmg-coa synthase, MPPD, and FPP synthase.

At least three different subcellular compartments, including peroxisomes, are involved in cholesterol synthesis. The peroxisomal targeting signals for phosphomevalonate kinase and isopentenyl diphosphate isomerase have been identified. In the current study we identify the peroxisomal targeting signals required for four other enzymes of the cholesterol biosynthetic pathway: acetoacetyl-CoA (AA-CoA) thiolase, 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) synthase, mevalonate diphosphate decarboxylase (MPPD), and farnesyl diphosphate (FPP) synthase. Data are presented that demonstrate that mitochondrial AA-CoA thiolase contains both a mitochondrial targeting signal at the amino terminus and a peroxisomal targeting signal (PTS-1) at the carboxy terminus. We also analyze a new variation of PTS-2 sequences required to target HMG-CoA synthase and MPPD to peroxisomes. In addition, we show that FPP synthase import into peroxisomes is dependent on the PTS-2 receptor and identify at the amino terminus of the protein a 20-amino acid region that is required for the peroxisomal localization of the enzyme. These data provide further support for the conclusion that peroxisomes play a critical role in cholesterol biosynthesis.

Characterization of phosphomevalonate kinase: chromosomal localization, regulation, and subcellular targeting.

Phosphomevalonate kinase catalyzes the conversion of mevalonate-5-phosphate to mevalonate-5-diphosphate and was originally believed to be a cytosolic enzyme. In this study we have localized the phosphomevalonate kinase gene to chromosome 1p13-1q22-23 and present a genomic map indicating that the gene spans more than 8.4 kb in the human genome. Furthermore, we show that message levels and enzyme activity of rat liver phosphomevalonate kinase are regulated in response to dietary sterol levels and that this regulation is coordinate with 3-hydroxy-3-methylglutaryl coenzyme A reductase, the rate-limiting enzyme of cholesterol biosynthesis. In addition, we demonstrate that phosphomevalonate kinase is a peroxisomal protein which requires the C-terminal peroxisomal targeting signal, Ser-Arg-Leu, for localization to the organelle.