Membrane Processing and Steady-State Regulation of the Alternative Peroxisomal Import Receptor Pex9p.

Markus Rudowitz,Ralf Erdmann,Wolfgang Schliebs

doi:10.3389/fcell.2020.566321

Markus Rudowitz, Ralf Erdmann + Show 1 more

Open Access

https://doi.org/10.3389/fcell.2020.566321

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Abstract

Import of peroxisomal matrix proteins with a type 1 peroxisomal targeting signal (PTS1) in Saccharomyces cerevisiae is facilitated by cytosolic import receptors Pex5p and Pex9p. While Pex5p has a broad specificity for all PTS1 proteins independent of the growth conditions, Pex9p is only expressed in fatty-acid containing media to mediate peroxisomal import of the two malate synthases, Mls1p and Mls2p, as well as the glutathione transferase Gto1p. Pex5p-cargo complexes dock at the peroxisomal membrane, translocate their cargo-protein via a transient pore and are recycled into the cytosol for a further round of import. The processing of Pex5p has been shown to require a complex network of interactions with other membrane-bound peroxins, as well as decoration with ubiquitin as signal for its ATP-dependent release and recycling. Here, we show that the alternative receptor Pex9p requires the same set of interacting peroxins to mediate peroxisomal import of Mls1p. However, while Pex5p is rather stable, Pex9p is rapidly degraded during its normal life cycle. The steady-state regulation of Pex9p, combining oleate-induced expression with high turnover rates resembles that of Pex18p, one of the two co-receptors of the PTS2-dependent targeting pathway into peroxisomes. Both Pex9p- and Pex18p-dependent import routes serve the fast metabolic adaptation to changes of carbon sources in baker’s yeast. By sequence similarities, we identified another Pex9p homolog in the human pathogenic fungus Candida glabrata, in which similar metabolic reprogramming strategies are crucial for survival of the pathogen.

Highlights

Peroxisomes are ubiquitous cell organelles with variable content of enzymes depending on cell types and environmental or developmental conditions
Several double deletion strains deficient in PEX5 and another PEX-gene of interest were analyzed for functional Pex9p-dependent import of GFPMls1p and the Pex5p-dependent import of GFP-peroxisomal targeting signal type 1 (PTS1) into peroxisomes
While the double mutant pex5 pex9 showed no import of GFP-Mls1p, the single deletion mutant pex5 and all mutants affected in the PTS2 import pathway exhibited discrete puncta, indicating that these peroxins are dispensable for GFP-Mls1p targeting to peroxisomes

Summary

Introduction

Peroxisomes are ubiquitous cell organelles with variable content of enzymes depending on cell types and environmental or developmental conditions. Peroxisomal enzymes are posttranslationally imported as fully folded and oligomerized proteins and assisted by cytosolic receptor proteins. PTS2-containing proteins are targeted to peroxisomes by the receptor Pex7p in conjunction with co-factors, like Pex18p or Pex21p in the yeast Saccharomyces cerevisiae (Marzioch et al, 1994; Zhang and Lazarow, 1996; Purdue et al, 1998; Kunze et al, 2011). Upon interaction between the cytosolic receptor-cargo complexes with the docking complex at the peroxisomal membrane, Pex5p or Pex18p associate Pex14p to assemble the respective protein conducting channels of the PTS1- and PTS2-pathway (Albertini et al, 1997; Bottger et al, 2000; Meinecke et al, 2010; Montilla-Martinez et al, 2015). While the non-labeled receptor escapes proteasomal degradation, some poorly understood quality control pathways can lead to lysine-conjugated polyubiquitination of the membrane-bound receptor, leading to rapid degradation by the proteasome (Platta et al, 2005)

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