Nitric Oxide (NO) Scaffolds the Peroxisomal Protein-Protein Interaction Network in Higher Plants

The peroxisome is a single-membrane subcellular compartment present in almost all eukaryotic cells from simple protists and fungi to complex organisms such as higher plants and animals. Historically, the name of the peroxisome came from a subcellular structure that contained high levels of hydrogen peroxide (H2O2) and the antioxidant enzyme catalase, which indicated that this organelle had basically an oxidative metabolism. During the last 20 years, it has been shown that plant peroxisomes also contain nitric oxide (NO), a radical molecule than leads to a family of derived molecules designated as reactive nitrogen species (RNS). These reactive species can mediate post-translational modifications (PTMs) of proteins, such as S-nitrosation and tyrosine nitration, thus affecting their function. This review aims to provide a comprehensive overview of how NO could affect peroxisomal metabolism and its internal protein-protein interactions (PPIs). Remarkably, many of the identified NO-target proteins in plant peroxisomes are involved in the metabolism of reactive oxygen species (ROS), either in its generation or its scavenging. Therefore, it is proposed that NO is a molecule with signaling properties with the capacity to modulate the peroxisomal protein-protein network and consequently the peroxisomal functions, especially under adverse environmental conditions.

Pex24 and Pex32 tether peroxisomes to the ER for organelle biogenesis, positioning and segregation

We analyzed all four Pex23 family proteins of the yeast Hansenula polymorpha, which localize to the ER. Of these Pex24 and Pex32, but not Pex23 and Pex29, accumulate at peroxisome-ER contacts, where they are important for normal peroxisome biogenesis and proliferation and contribute to organelle positioning and segregation.Upon deletion of PEX24 and PEX32 - and to a lesser extent of PEX23 and PEX29 - peroxisome-ER contacts are disrupted, concomitant with peroxisomal defects. These defects are suppressed upon introduction of an artificial peroxisome-ER tether.Accumulation of Pex32 at peroxisomes-ER contacts is lost in the absence of the peroxisomal membrane protein Pex11. At the same time peroxisome-ER contacts are disrupted, indicating that Pex11 contributes to Pex32-dependent peroxisome-ER contact formation.Summarizing, our data indicate that H. polymorpha Pex24 and Pex32 are tethers at peroxisome-ER contacts that are important for normal peroxisome biogenesis and dynamics.SummaryTwo Hansenula polymorpha ER proteins, Pex24 and Pex32, are tethers at peroxisome-ER contacts and function together with the peroxisomal protein Pex11. Their absence disturbs these contacts leading to multiple peroxisomal defects, which can be restored by an artificial tether.

Functional Analyses of a Putative, Membrane-Bound, Peroxisomal Protein Import Mechanism from the Apicomplexan Protozoan Toxoplasma gondii

Peroxisomes are central to eukaryotic metabolism, including the oxidation of fatty acids—which subsequently provide an important source of metabolic energy—and in the biosynthesis of cholesterol and plasmalogens. However, the presence and nature of peroxisomes in the parasitic apicomplexan protozoa remains controversial. A survey of the available genomes revealed that genes encoding peroxisome biogenesis factors, so-called peroxins (Pex), are only present in a subset of these parasites, the coccidia. The basic principle of peroxisomal protein import is evolutionarily conserved, proteins harbouring a peroxisomal-targeting signal 1 (PTS1) interact in the cytosol with the shuttling receptor Pex5 and are then imported into the peroxisome via the membrane-bound protein complex formed by Pex13 and Pex14. Surprisingly, whilst Pex5 is clearly identifiable, Pex13 and, perhaps, Pex14 are apparently absent from the coccidian genomes. To investigate the functionality of the PTS1 import mechanism in these parasites, expression of Pex5 from the model coccidian Toxoplasma gondii was shown to rescue the import defect of Pex5-deleted Saccharomyces cerevisiae. In support of these data, green fluorescent protein (GFP) bearing the enhanced (e)PTS1 known to efficiently localise to peroxisomes in yeast, localised to peroxisome-like bodies when expressed in Toxoplasma. Furthermore, the PTS1-binding domain of Pex5 and a PTS1 ligand from the putatively peroxisome-localised Toxoplasma sterol carrier protein (SCP2) were shown to interact in vitro. Taken together, these data demonstrate that the Pex5–PTS1 interaction is functional in the coccidia and indicate that a nonconventional peroxisomal import mechanism may operate in the absence of Pex13 and Pex14.