From expressed sequence tags to peroxisome biogenesis disorder genes

The roles of FgPEX2 and FgPEX12 in virulence and lipid metabolism in Fusarium graminearum

Fusarium head blight (FHB) is a wheat disease with a worldwide prevalence, caused by Fusarium graminearum. Peroxisomes are ubiquitous in eukaryotic cells and are involved in various biochemical phenomena. FgPEX2 and FgPEX12 encode RING-finger peroxins PEX2 and PEX12 in F. graminearum. This study aimed to functionally characterize FgPEX2 and FgPEX12 in F. graminearum. We constructed deletion mutants of FgPEX2 and FgPEX12 via homologous recombination. The ΔPEX2 and ΔPEX12 mutants displayed defects in sexual and asexual development, virulence, cell wall integrity (CWI), and lipid metabolism. Deletion of FgPEX2 and FgPEX12 significantly decreased deoxynivalenol production. Furthermore, fluorescence microscopic analysis of the subcellular localization of GFP-PMP70 and GFP-HEX1 revealed that FgPEX2 and FgPEX12 maintain Woronin bodies. These results show that FgPEX2 and FgPEX12 are required for growth, conidiation, virulence, cell wall integrity, and lipid metabolism in F. graminearum and do not influence their peroxisomes.

MoPex19, which is essential for maintenance of peroxisomal structure and woronin bodies, is required for metabolism and development in the rice blast fungus

Peroxisomes are present ubiquitously and make important contributions to cellular metabolism in eukaryotes. They play crucial roles in pathogenicity of plant fungal pathogens. The peroxisomal matrix proteins and peroxisomal membrane proteins (PMPs) are synthesized in the cytosol and imported post-translationally. Although the peroxisomal import machineries are generally conserved, some species-specific features were found in different types of organisms. In phytopathogenic fungi, the pathways of the matrix proteins have been elucidated, while the import machinery of PMPs remains obscure. Here, we report that MoPEX19, an ortholog of ScPEX19, was required for PMPs import and peroxisomal maintenance, and played crucial roles in metabolism and pathogenicity of the rice blast fungus Magnaporthe oryzae. MoPEX19 was expressed in a low level and Mopex19p was distributed in the cytoplasm and newly formed peroxisomes. MoPEX19 deletion led to mislocalization of peroxisomal membrane proteins (PMPs), as well peroxisomal matrix proteins. Peroxisomal structures were totally absent in Δmopex19 mutants and woronin bodies also vanished. Δmopex19 exhibited metabolic deficiency typical in peroxisomal disorders and also abnormality in glyoxylate cycle which was undetected in the known mopex mutants. The Δmopex19 mutants performed multiple disorders in fungal development and pathogenicity-related morphogenesis, and lost completely the pathogenicity on its hosts. These data demonstrate that MoPEX19 plays crucial roles in maintenance of peroxisomal and peroxisome-derived structures and makes more contributions to fungal development and pathogenicity than the known MoPEX genes in the rice blast fungus.

PEROXISOMEBIOGENESISDISORDERS

The peroxisome biogenesis disorders (PBDs) comprise 12 autosomal recessive complementation groups (CGs). The multisystem clinical phenotype varies widely in severity and results from disturbances in both development and metabolic homeostasis. Progress over the last several years has lead to identification of the genes responsible for all of these disorders and to a much improved understanding of the biogenesis and function of the peroxisome. Increasing availability of mouse models for these disorders offers hope for a better understanding of their pathophysiology and for development of therapies that might especially benefit patients at the milder end of the clinical phenotype.

The genetics of peroxisome biogenesis

The segregation of metabolic functions within discrete organelles is a hallmark of eukaryotic cells. These compartments allow for the concentration of related metabolic functions, the separation of competing metabolic functions, and the formation of unique chemical microenvironments. However, such organization is not spontaneous and requires an array of genes that are dedicated to the assembly and maintenance of these structures. In this review we focus on the genetics of peroxisome biogenesis and on how defects in this process cause human disease.

Import of peroxisomal matrix and membrane proteins

This review summarizes the progress made in our understanding of peroxisome biogenesis in the last few years, during which the functional roles of many of the 23 peroxins (proteins involved in peroxisomal protein import and peroxisome biogenesis) have become clearer. Previous reviews in the field have focussed on the metabolic functions of peroxisomes, aspects of import/biogenesis the role of peroxins in human disease, and involvement of the endoplasmic reticulum in peroxisome membrane biogenesis as well as the degradation of this organelle. This review refers to some of the earlier work for the sake of introduction and continuity but deals primarily with the more recent progress. The principal areas of progress are the identification of new peroxins, definition of protein-protein interactions among peroxins leading to the recognition of complexes involved in peroxisomal protein import, insight into the biogenesis of peroxisomal membrane proteins, and, of most importance, the elucidation of the role of many conserved peroxins in human disease. Given the rapid progress in the field, this review also highlights some of the unanswered questions that remain to be tackled.

Identification and characterization of the human peroxin PEX3

The biogenesis of peroxisomes requires the interaction of several peroxins, encoded by PEX genes and is well conserved between yeast and humans. We have cloned the human cDNA of PEX3 based on its homology to different yeast PEX3 genes. The deduced peroxin HsPEX3 is a peroxisomal membrane protein with a calculated molecular mass of 42.1 kDa. We created N- and C-terminal tagged PEX3 to assay its topology at the peroxisomal membrane by immunofluorescence microscopy. Our results and the one predicted transmembrane spanning region are in line with the assumption that H sPEX3 is an integral peroxisomal membrane protein with the N-terminus inside the peroxisome and the C-terminus facing the cytoplasm. The farnesylated peroxisomal membrane protein PEX19 interacts with HsPEX3 in a mammalian two-hybrid assay in human fibroblasts. The physical interaction could be confirmed by coimmunoprecipitation of the two in vitro transcribed and translated proteins. To address the targeting of PEX3 to the peroxisomal membrane, the expression of different N- and C-terminal PEX3 truncations fused to green fluorescent protein (GFP) was investigated in human fibroblasts. The N-terminal 33 amino acids of PEX3 were necessary and sufficient to direct the reporter protein GFP to peroxisomes and seemed to be integrated into the peroxisomal membrane. The expression of a 1-16 PEX3-GFP fusion protein did not result in a peroxisomal localization, but interestingly, this and several other truncated PEX3 fusion proteins were also localized to tubular and/or vesicular structures representing mitochondria.

Peroxisome synthesis in the absence of preexisting peroxisomes

Zellweger syndrome and related diseases are caused by defective import of peroxisomal matrix proteins. In all previously reported Zellweger syndrome cell lines the defect could be assigned to the matrix protein import pathway since peroxisome membranes were present, and import of integral peroxisomal membrane proteins was normal. However, we report here a Zellweger syndrome patient (PBD061) with an unusual cellular phenotype, an inability to import peroxisomal membrane proteins. We also identified human PEX16, a novel integral peroxisomal membrane protein, and found that PBD061 had inactivating mutations in the PEX16 gene. Previous studies have suggested that peroxisomes arise from preexisting peroxisomes but we find that expression of PEX16 restores the formation of new peroxisomes in PBD061 cells. Peroxisome synthesis and peroxisomal membrane protein import could be detected within 2-3 h of PEX16 injection and was followed by matrix protein import. These results demonstrate that peroxisomes do not necessarily arise from division of preexisting peroxisomes. We propose that peroxisomes may form by either of two pathways: one that involves PEX11-mediated division of preexisting peroxisomes, and another that involves PEX16-mediated formation of peroxisomes in the absence of preexisting peroxisomes.

Components involved in peroxisome import, biogenesis, proliferation, turnover, and movement

In the decade that has elapsed since the discovery of the first peroxisomal targeting signal (PTS), considerable information has been obtained regarding the mechanism of protein import into peroxisomes. The PTSs responsible for the import of matrix and membrane proteins to peroxisomes, the receptors for several of these PTSs, and docking proteins for the PTS1 and PTS2 receptors are known. Many peroxins involved in peroxisomal protein import and biogenesis have been characterized genetically and biochemically. These studies have revealed important new insights regarding the mechanism of protein translocation across the peroxisomal membrane, the conservation of PEX genes through evolution, the role of peroxins in fatal human peroxisomal disorders, and the biogenesis of the organelle. It is clear that peroxisomal protein import and biogenesis have many features unique to this organelle alone. More recent studies on peroxisome degradation, division, and movement highlight newer aspects of the biology of this organelle that promise to be just as exciting and interesting as import and biogenesis.

Mutations in PDX1, the human lipoyl-containing component X of the pyruvate dehydrogenase-complex gene on chromosome 11p1, in congenital lactic acidosis

We have identified and sequenced a cDNA that encodes an apparent human orthologue of a yeast protein-X component (ScPDX1) of pyruvate dehydrogenase multienzyme complexes. The new human cDNA that has been referred to as "HsPDX1" cDNA was cloned by use of the "database cloning" strategy and had a 1,506-bp open reading frame. The amino acid sequence of the protein encoded by the cDNA was 20% identical with that encoded by the yeast PDX1 gene and 40% identical with that encoded by the lipoate acetyltransferase component of the pyruvate dehydrogenase and included a lipoyl-bearing domain that is conserved in some dehydrogenase enzyme complexes. Northern blot analysis demonstrated that the major HsPDX1 mRNA was 2.5 kb in length and was expressed mainly in human skeletal and cardiac muscles but was also present, at low levels, in other tissues. FISH analysis performed with a P1-derived artificial chromosome (PAC)-containing HsPDX1 gene sublocalized the gene to 11p1.3. Molecular investigation of PDX1 deficiency in four patients with neonatal lactic acidemias revealed mutations 78del85 and 965del59 in a homozygous state, and one other patient had no PDX1 mRNA expression.

Identification of a fourth half ABC transporter in the human peroxisomal membrane

Three half ATP-binding cassette transporters (ALDP, ALDR, PMP70) are known to be present in the human peroxisome membrane. Mutations in the gene encoding ALDP cause X-linked adrenoleukodystrophy; the role of ALDR and PMP70 in human disease is unclear. We report the cloning and characterization of a fourth human gene encoding a peroxisomal half ABC transporter. The gene, designated P70R, maps to chromosome 14q24, encodes a 73 kDa transporter most similar to PMP70, and is expressed in all human tissues examined. Because half ABC transporters heterodimerize to form functional transporters, the identification of a fourth member of this family in the peroxisome membrane has implications for our understanding of mammalian peroxisomes and the genetic disorders of peroxisomal function.

Human PEX7 encodes the peroxisomal PTS2 receptor and is responsible for rhizomelic chondrodysplasia punctata

Rhizomelic chondrodysplasia punctata (RCDP) is a rare autosomal recessive phenotype that comprises complementation group 11 of the peroxisome biogenesis disorders (PBD). PEX7, a candidate gene for RCDP identified in yeast, encodes the receptor for peroxisomal matrix proteins with the type-2 peroxisome targeting signal (PTS2). By homology probing we identified human and murine PEX7 genes and found that expression of either corrects the PTS2-import defect characteristic of RCDP cells. In a collection of 36 RCDP probands, we found two inactivating PEX7 mutations: one, L292ter, was present in 26 of the probands, all with a severe phenotype; the second, A218V, was present in three probands, including two with a milder phenotype. A third mutation, G217R, whose functional significance is yet to be determined, was present in five probands, all compound heterozygotes with L292ter. We conclude that PEX7 is responsible for RCDP (PBD CG11) and suggest a founder effect may explain the high frequency of L292ter.