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Cryptic peroxisomal targeting via alternative splicing and stop codon read-through in fungi


Peroxisomes are eukaryotic organelles important for the metabolism of long-chain fatty acids1,2. Here we show that in numerous fungal species, several core enzymes of glycolysis, including glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and 3-phosphoglycerate kinase (PGK), reside in both the cytoplasm and peroxisomes. We detected in these enzymes cryptic type 1 peroxisomal targeting signals (PTS1)3, which are activated by post-transcriptional processes. Notably, the molecular mechanisms that generate the peroxisomal isoforms vary considerably among different species. In the basidiomycete plant pathogen Ustilago maydis, peroxisomal targeting of Pgk1 results from ribosomal read-through, whereas alternative splicing generates the PTS1 of Gapdh. In the filamentous ascomycete Aspergillus nidulans, peroxisomal targeting of these enzymes is achieved by exactly the opposite mechanisms. We also detected PTS1 motifs in the glycolytic enzymes triose-phosphate isomerase and fructose-bisphosphate aldolase. U. maydis mutants lacking the peroxisomal isoforms of Gapdh or Pgk1 showed reduced virulence. In addition, mutational analysis suggests that GAPDH, together with other peroxisomal NADH-dependent dehydrogenases, has a role in redox homeostasis. Owing to its hidden nature, partial peroxisomal targeting of well-studied cytoplasmic enzymes has remained undetected. Thus, we anticipate that further bona fide cytoplasmic proteins exhibit similar dual targeting.

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Figure 1: Cryptic peroxisomal targeting signals in Gapdh and Pgk1 in U. maydis.
Figure 2: Dual targeting of GAPDH and PGK occurs in many fungi.
Figure 3: Peroxisomal localization of glycolytic enzymes is important for virulence and growth in U. maydis.

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We are indebted to J. Heitman and W. K. Holloman for reading of the manuscript. We thank R. Fischer and G. Barth for providing strains and DNA. We acknowledge technical assistance by M. Piscator. We thank D. Lanver and T. Stehlik for discussions. This work was supported by a grant from the Deutsche Forschungsgemeinschaft (DFG-GK1216). J.A. received a fellowship from the Marburg University Research Academy (MARA).

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M.B. and J.F. designed this study. J.F. and J.A. performed the experiments. All authors contributed to data analysis. M.B. and J.F. wrote the manuscript.

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Correspondence to Michael Bölker.

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The authors declare no competing financial interests.

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Freitag, J., Ast, J. & Bölker, M. Cryptic peroxisomal targeting via alternative splicing and stop codon read-through in fungi. Nature 485, 522–525 (2012).

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