Mammalian cells typically contain hundreds of peroxisomes but can increase peroxisome abundance further in response to extracellular stimuli. We report here the identification and characterization of two novel human peroxisomal membrane proteins, PEX11␣ and PEX11␤. Overexpression of the human PEX11␤ gene alone was sufficient to induce peroxisome proliferation, demonstrating that proliferation can occur in the absence of extracellular stimuli and may be mediated by a single gene. Time course studies indicated that PEX11␤ induces peroxisome proliferation through a multistep process involving peroxisome elongation and segregation of PEX11␤ from other peroxisomal membrane proteins, followed by peroxisome division. Overexpression of PEX11␣ also induced peroxisome proliferation but at a much lower frequency than PEX11␤ in our experimental system. The patterns of PEX11␣ and PEX11␤ expression were examined in the rat, the animal in which peroxisome proliferation has been examined most extensively. Levels of PEX11␤ mRNA were similar in all tissues examined and were unaffected by peroxisomeproliferating agents. Conversely, PEX11␣ mRNA levels varied widely among different tissues, were highest in tissues that are sensitive to peroxisome-proliferating agents, and were induced more than 10-fold in response to the peroxisome proliferators clofibrate and di(2-ethylhexyl) phthalate. Taken together, these data implicate PEX11␤ in the constitutive control of peroxisome abundance and suggest that PEX11␣ may regulate peroxisome abundance in response to extracellular stimuli.Peroxisomes are ubiquitous components of eukaryotic cells, absent only from mature erythrocytes and certain primitive unicellular eukaryotes. One of the more intriguing aspects of peroxisome biogenesis is how cells control the abundance of this organelle. Mammalian cells contain hundreds of peroxisomes under normal growth conditions, suggesting that there are constitutive mechanisms for raising peroxisome abundance above one per cell. In addition, peroxisome abundance may change in response to extracellular stimuli, indicating the existence of a signal transduction pathway that exerts additional control over peroxisome abundance. Inducers of peroxisome proliferation include both hypolipidemic drugs (e.g. clofibrate) and plasticizing agents (e.g. di(2-ethylhexyl) phthalate (DEHP) 1 ), which act through PPAR␣, the ␣ isoform of the peroxisome proliferator-activated receptor (1-3). PPAR␣ is a member of the nuclear hormone receptor superfamily and functions as a heterodimer with retinoid X receptor (RXR), another nuclear hormone receptor. The activated PPAR␣⅐RXR heterodimer binds peroxisome proliferator-responsive elements (PPREs) and mediates transcriptional activation of a large array of PPRE-containing genes in a drug-dependent manner (4). However, the pathway between altered gene expression and peroxisome proliferation remains to be elucidated.Peroxisome proliferation has also been observed in lower eukaryotes. In the yeast Saccharomyces cerevisiae, fatty acid ox...

The peroxisome biogenesis disorders (PBDs) are a group of lethal autosomal-recessive diseases caused by defects in peroxisomal matrix protein import, with the concomitant loss of multiple peroxisomal enzyme activities. Ten complementation groups (CGs) have been identified for the PBDs, with CG1 accounting for 51% of all PBD patients. We identified the human orthologue of yeast PEX1, a gene required for peroxisomal matrix protein import. Expression of human PEX1 restored peroxisomal protein import in fibroblasts from 30 CG1 patients, and PEX1 mutations were detected in multiple CG1 probands. A common PEX1 allele, G843D, is present in approximately half of CG1 patients and has a deleterious effect on PEX1 activity. Phenotypic analysis of PEX1-deficient cells revealed severe defects in peroxisomal matrix protein import and destabilization of PEX5, the receptor for the type-1 peroxisomal targetting signal, even though peroxisomes were present in these cells and capable of importing peroxisomal membrane proteins. These data demonstrate an important role for PEX1 in peroxisome biogenesis and suggest that mutations in this gene are the most common cause of the PBDs.

Peroxisomal matrix protein import requires the action of two AAA ATPases, PEX1 and PEX6. Mutations in either the PEX1 or PEX6 gene are the most common cause of the lethal neurologic disorders Zellweger syndrome, neonatal adrenoleukodystrophy, and infantile Refsum disease and account for disease in 80% of all such patients. We report here that overexpression of PEX6 can suppress the phenotypes of certain PEX1-deficient cells, that overexpression of PEX1 can suppress the phenotypes of certain PEX6-deficient cells, and that these instances of suppression are allele-specific and require partial activity of the mutated gene. In addition to genetic evidence for interaction between PEX1 and PEX6, we find that the PEX1 and PEX6 proteins interact in the yeast two-hybrid assay and physically associate with one another in vitro. We previously identified a missense mutation in PEX1, G843D, which attenuates PEX1 function and is the most common cause of these diseases, present in one-third of all such patients. The G843D mutation attenuates the interaction between PEX1 and PEX6 in both the two-hybrid system and in vitro and appears to be suppressed by overexpression of PEX6. We conclude that PEX1 and PEX6 form a complex of central importance to peroxisome biogenesis and that mutations affecting this complex constitute the most common cause of the Zellweger syndrome spectrum of diseases.