RESUMO
Peroxisomes are eukaryotic organelles that function in numerous metabolic pathways and defects in peroxisome function can cause serious developmental brain disorders such as adrenoleukodystrophy (ALD). Peroxisomal membrane proteins (PMPs) play a crucial role in regulating peroxisome function. Therefore, PMP homeostasis is vital for peroxisome function. Recently, we established that certain PMPs are degraded by the Ubiquitin Proteasome System yet little is known about how faulty/non-functional PMPs undergo quality control. Here we have investigated the degradation of Pxa1p, a fatty acid transporter in the yeast Saccharomyces cerevisiae. Pxa1p is a homologue of the human protein ALDP and mutations in ALDP result in the severe disorder ALD. By introducing two corresponding ALDP mutations into Pxa1p (Pxa1MUT), fused to mGFP, we show that Pxa1MUT-mGFP is rapidly degraded from peroxisomes in a proteasome-dependent manner, while wild type Pxa1-mGFP remains relatively stable. Furthermore, we identify a role for the ubiquitin ligase Ufd4p in Pxa1MUT-mGFP degradation. Finally, we establish that inhibiting Pxa1MUT-mGFP degradation results in a partial rescue of Pxa1p activity in cells. Together, our data demonstrate that faulty PMPs can undergo proteasome-dependent quality control. Furthermore, our observations may provide new insights into the role of ALDP degradation in ALD.
Assuntos
Membro 1 da Subfamília D de Transportadores de Cassetes de Ligação de ATP/genética , Transportadores de Cassetes de Ligação de ATP/genética , Adrenoleucodistrofia/genética , Proteínas de Saccharomyces cerevisiae/genética , Ubiquitina-Proteína Ligases/genética , Adrenoleucodistrofia/patologia , Humanos , Proteínas de Membrana/genética , Redes e Vias Metabólicas/genética , Mutação/genética , Peroxissomos/genética , Complexo de Endopeptidases do Proteassoma/genética , Proteólise , Saccharomyces cerevisiae/genéticaRESUMO
Peroxisomes are essential subcellular organelles involved in a variety of metabolic processes. Their importance is underlined by the identification of a large group of inherited diseases in humans in which one or more of the peroxisomal functions are impaired. The yeast Saccharomyces cerevisiae has been used as a model organism to study the functions of peroxisomes. Efficient oxidation of fatty acids does not only require the participation of peroxisomal enzymes but also the active involvement of other gene products. One group of important gene products in this respect includes peroxisomal membrane proteins involved in metabolite transport. This overview discusses the various aspects of fatty acid beta-oxidation in S. cerevisiae. Addressed are the various enzymes and their particular functions as well as the various transport mechanisms to take up fatty acids into peroxisomes or to export the beta-oxidation products out of the peroxisome to mitochondria for full oxidation to CO2 and H2O.
Assuntos
Ácidos Graxos/metabolismo , Peroxissomos/metabolismo , Saccharomyces cerevisiae/metabolismo , Trifosfato de Adenosina/metabolismo , Transporte Biológico/fisiologia , Ácidos Graxos/química , Humanos , Proteínas de Membrana/metabolismo , Mitocôndrias/metabolismo , Estrutura Molecular , Oxirredução , Peroxissomos/enzimologia , Saccharomyces cerevisiae/genéticaRESUMO
In recent years much has been learned about the essential role of peroxisomes in cellular metabolism. Much less, however, is known about the permeability properties of peroxisomes although it is well established now that peroxisomes are impermeable to small molecules which implies the existence of transporters in the peroxisomal membrane. In this paper we report the identification of PMP34, a peroxisomal membrane protein belonging to the mitochondrial solute carrier family, as an adenine nucleotide transporter. This is concluded from different experimental findings including rescue of the defect in medium-chain fatty acid oxidation in Saccharomyces cerevisiae cells in which the ANT1 gene coding for Ant1p, the peroxisomal adenine nucleotide carrier, was disrupted. Furthermore, we have purified PMP34, reconstituted the protein in proteoliposomes, and provide direct proof that PMP34 is an adenine nucleotide transporter.
Assuntos
Proteínas de Membrana/metabolismo , Peroxissomos/metabolismo , Translocador 1 do Nucleotídeo Adenina/genética , Translocador 1 do Nucleotídeo Adenina/metabolismo , Trifosfato de Adenosina/metabolismo , Fracionamento Celular , Proteínas Fúngicas/genética , Proteínas Fúngicas/metabolismo , Humanos , Proteínas de Membrana/genética , Oxirredução , Saccharomyces cerevisiae/fisiologiaRESUMO
In this report, we reinvestigate the only patient ever reported with a deficiency of peroxisomal 3-ketoacyl-CoA thiolase (THIO). At the time when they were described, the abnormalities in this patient, which included accumulation of very-long-chain fatty acids and the bile-acid intermediate trihydroxycholestanoic acid, were believed to be the logical consequence of a deficiency of the peroxisomal beta-oxidation enzyme THIO. In light of the current knowledge of the peroxisomal beta-oxidation system, however, the reported biochemical aberrations can no longer be explained by a deficiency of this thiolase. In this study, we show that the true defect in this patient is at the level of d-bifunctional protein (DBP). Immunoblot analysis revealed the absence of DBP in postmortem brain of the patient, whereas THIO was normally present. In addition, we found that the patient had a homozygous deletion of part of exon 3 and intron 3 of the DBP gene, resulting in skipping of exon 3 at the cDNA level. Our findings imply that the group of single-peroxisomal beta-oxidation-enzyme deficiencies is limited to straight-chain acyl-CoA oxidase, DBP, and alpha-methylacyl-CoA racemase deficiency and that there is no longer evidence for the existence of THIO deficiency as a distinct clinical entity.
