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      Molecular Requirements for Peroxisomal Targeting of Alanine-Glyoxylate Aminotransferase as an Essential Determinant in Primary Hyperoxaluria Type 1

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          The crystal structure of the peroxisome enzyme alanine-glyoxylate aminotransferase bound to its targeting receptor Pex5p explains why even minor fold defects prevent targeting of the enzyme and cause kidney disease.


          Alanine-glyoxylate aminotransferase is a peroxisomal enzyme, of which various missense mutations lead to irreversible kidney damage via primary hyperoxaluria type 1, in part caused by improper peroxisomal targeting. To unravel the molecular mechanism of its recognition by the peroxisomal receptor Pex5p, we have determined the crystal structure of the respective cargo–receptor complex. It shows an extensive protein/protein interface, with contributions from residues of the peroxisomal targeting signal 1 and additional loops of the C-terminal domain of the cargo. Sequence segments that are crucial for receptor recognition and hydrophobic core interactions within alanine-glyoxylate aminotransferase are overlapping, explaining why receptor recognition highly depends on a properly folded protein. We subsequently characterized several enzyme variants in vitro and in vivo and show that even minor protein fold perturbations are sufficient to impair Pex5p receptor recognition. We discuss how the knowledge of the molecular parameters for alanine-glyoxylate aminotransferase required for peroxisomal translocation could become useful for improved hyperoxaluria type 1 treatment.

          Author Summary

          Peroxisomes are cell organelles contain proteins involved in various aspects of metabolism. Peroxisome proteins translocate from their site of synthesis in the cytoplasm across the organelle membrane in a fully folded and functional form. One such protein is the enzyme alanine–glyoxylate aminotransferase (AGT). It contains a targeting signal in its C-terminus that is recognized by a receptor protein, Pex5p, in the cytoplasm, which allows its subsequent translocation into the peroxisome. Mutations in AGT cause a disease known as primary hyperoxaluria type 1, in which patients suffer irreversible kidney damage; this disease results, in many cases, from improper targeting of AGT into peroxisomes. To understand better the mechanism of AGT import into peroxisomes and the molecular basis of this disease, we have determined the crystal structure of the complex between AGT and its receptor Pex5p. The structure reveals how overlapping segments of the protein sequence are crucial for both receptor recognition and maintaining the folded structure of the enzyme. Subsequently, we created and studied several mutants of the enzyme, including mutants that are known to cause disease, and found that even minor folding defects in the enzyme prevent its recognition by Pexp5 and its import into peroxisomes. Our data thus provide novel insights into the consequences of mutations in AGT on the catalytic activity of the enzyme, as well as into the mechanisms that cause primary hyperoxaluria type 1.

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          Most cited references 41

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          The peroxisomal importomer constitutes a large and highly dynamic pore.

          The peroxisomal protein import machinery differs fundamentally from known translocons (endoplasmic reticulum, mitochondria, chloroplasts, bacteria) as it allows membrane passage of folded, even oligomerized proteins. However, the mechanistic principles of protein translocation across the peroxisomal membrane remain unknown. There are various models that consider membrane invagination events, vesicle fusion or the existence of large import pores. Current data show that a proteinaceous peroxisomal importomer enables docking of the cytosolic cargo-loaded receptors, cargo translocation and receptor recycling. Remarkably, the cycling import receptor Pex5p changes its topology from a soluble cytosolic form to an integral membrane-bound form. According to the transient pore hypothesis, the membrane-bound receptor is proposed to form the core component of the peroxisomal import pore. Here, we demonstrate that the membrane-associated import receptor Pex5p together with its docking partner Pex14p forms a gated ion-conducting channel which can be opened to a diameter of about 9 nm by the cytosolic receptor-cargo complex. The newly identified pore shows striking dynamics, as expected for an import machinery translocating proteins of variable sizes.
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            Pex2 and pex12 function as protein-ubiquitin ligases in peroxisomal protein import.

            The PTS1-dependent peroxisomal matrix protein import is facilitated by the receptor protein Pex5 and can be divided into cargo recognition in the cytosol, membrane docking of the cargo-receptor complex, cargo release, and recycling of the receptor. The final step is controlled by the ubiquitination status of Pex5. While polyubiquitinated Pex5 is degraded by the proteasome, monoubiquitinated Pex5 is destined for a new round of the receptor cycle. Recently, the ubiquitin-conjugating enzymes involved in Pex5 ubiquitination were identified as Ubc4 and Pex4 (Ubc10), whereas the identity of the corresponding protein-ubiquitin ligases remained unknown. Here we report on the identification of the protein-ubiquitin ligases that are responsible for the ubiquitination of the peroxisomal protein import receptor Pex5. It is demonstrated that each of the three RING peroxins Pex2, Pex10, and Pex12 exhibits ubiquitin-protein isopeptide ligase activity. Our results show that Pex2 mediates the Ubc4-dependent polyubiquitination whereas Pex12 facilitates the Pex4-dependent monoubiquitination of Pex5.
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              The CCP4 suite: programs for protein crystallography


                Author and article information

                Role: Academic Editor
                PLoS Biol
                PLoS Biol
                PLoS Biology
                Public Library of Science (San Francisco, USA )
                April 2012
                April 2012
                17 April 2012
                : 10
                : 4
                [1 ]European Molecular Biology Laboratory Hamburg, Hamburg, Germany
                [2 ]Department of Systems Biology, Faculty of Medicine, Institute for Physiological Chemistry, Ruhr University of Bochum, Bochum, Germany
                Princeton University, United States of America
                Author notes

                The author(s) have made the following declarations about their contributions: Conceived and designed the experiments: KF WS MW. Performed the experiments: KF JW. Analyzed the data: KF JW WS MW. Contributed reagents/materials/analysis tools: WS RE MW. Wrote the paper: KF WS MW.

                Fodor et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
                Page count
                Pages: 12
                Research Article
                Biomacromolecule-Ligand Interactions
                Macromolecular Assemblies
                Protein Folding
                Gene Function
                Genetic Mutation
                Genetics of Disease
                Human Genetics
                Molecular Cell Biology
                Membranes and Sorting
                Clinical Genetics
                Autosomal Recessive
                Chromosomal Disorders
                Mitochondrial Diseases

                Life sciences


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