Stress exposure results in increased peroxisomal levels of yeast Pnc1 and Gpd1, which are imported via a piggy-backing mechanism.

Saccharomyces cerevisiae glycerol phosphate dehydrogenase 1 (Gpd1) and nicotinamidase (Pnc1) are two stress-induced enzymes. Both enzymes are predominantly localised to peroxisomes at normal growth conditions, but were reported to localise to the cytosol and nucleus upon exposure of cells to stress. Import of both proteins into peroxisomes depends on the peroxisomal targeting signal 2 (PTS2) receptor Pex7. Gpd1 contains a PTS2, however, Pnc1 lacks this sequence. Here we show that Pnc1 physically interacts with Gpd1, which is required for piggy-back import of Pnc1 into peroxisomes. Quantitative fluorescence microscopy analyses revealed that the levels of both proteins increased in peroxisomes and in the cytosol upon exposure of cells to stress. However, upon exposure of cells to stress we also observed enhanced cytosolic levels of the control PTS2 protein thiolase, when produced under control of the GPD1 promoter. This suggests that these conditions cause a partial defect in PTS2 protein import, probably because the PTS2 import pathway is easily saturated.

The Peroxisomal Targeting Signal 1 in sterol carrier protein 2 is autonomous and essential for receptor recognition.

BACKGROUND: The majority of peroxisomal matrix proteins destined for translocation into the peroxisomal lumen are recognised via a C-terminal Peroxisomal Target Signal type 1 by the cycling receptor Pex5p. The only structure to date of Pex5p in complex with a cargo protein is that of the C-terminal cargo-binding domain of the receptor with sterol carrier protein 2, a small, model peroxisomal protein. In this study, we have tested the contribution of a second, ancillary receptor-cargo binding site, which was found in addition to the characterised Peroxisomal Target Signal type 1. RESULTS: To investigate the function of this secondary interface we have mutated two key residues from the ancillary binding site and analyzed the level of binding first by a yeast-two-hybrid assay, followed by quantitative measurement of the binding affinity and kinetics of purified protein components and finally, by in vivo measurements, to determine translocation capability. While a moderate but significant reduction of the interaction was found in binding assays, we were not able to measure any significant defects in vivo. CONCLUSIONS: Our data therefore suggest that at least in the case of sterol carrier protein 2 the contribution of the second binding site is not essential for peroxisomal import. At this stage, however, we cannot rule out that other cargo proteins may require this ancillary binding site.

Target and identify: triazene linker helps identify azidation sites of labelled proteins via click and cleave strategy.

A method for identifying probe modification of proteins via tandem mass spectrometry was developed. Azide bearing molecules are immobilized on functionalised sepharose beads via copper catalysed Huisgen-type click chemistry and selectively released under acidic conditions by chemical cleavage of the triazene linkage. We applied this method to identify the modification site of targeted-diazotransfer on BirA.

Label-Free and Real-Time Detection of Protein Ubiquitination with a Biological Nanopore.

The covalent addition of ubiquitin to target proteins is a key post-translational modification that is linked to a myriad of biological processes. Here, we report a fast, single-molecule, and label-free method to probe the ubiquitination of proteins employing an engineered Cytolysin A (ClyA) nanopore. We show that ionic currents can be used to recognize mono- and polyubiquitinated forms of native proteins under physiological conditions. Using defined conjugates, we also show that isomeric monoubiquitinated proteins can be discriminated. The nanopore approach allows following the ubiquitination reaction in real time, which will accelerate the understanding of fundamental mechanisms linked to protein ubiquitination.

Phosphorylation of Pex11p does not regulate peroxisomal fission in the yeast Hansenula polymorpha.

Pex11p plays a crucial role in peroxisomal fission. Studies in Saccharomyces cerevisiae and Pichia pastoris indicated that Pex11p is activated by phosphorylation, which results in enhanced peroxisome proliferation. In S. cerevisiae but not in P. pastoris, Pex11p phosphorylation was shown to regulate the protein's trafficking to peroxisomes. However, phosphorylation of PpPex11p was proposed to influence its interaction with Fis1p, another component of the organellar fission machinery. Here, we have examined the role of Pex11p phosphorylation in the yeast Hansenula polymorpha. Employing mass spectrometry, we demonstrate that HpPex11p is also phosphorylated on a Serine residue present at a similar position to that of ScPex11p and PpPex11p. Furthermore, through the use of mutants designed to mimic both phosphorylated and unphosphorylated forms of HpPex11p, we have investigated the role of this post-translational modification. Our data demonstrate that mutations to the phosphorylation site do not disturb the function of Pex11p in peroxisomal fission, nor do they alter the localization of Pex11p. Also, no effect on peroxisome inheritance was observed. Taken together, these data lead us to conclude that peroxisomal fission in H. polymorpha is not modulated by phosphorylation of Pex11p.

Peroxin 5: a cycling receptor for protein translocation into peroxisomes.

Peroxins are proteins that regulate the biogenesis of peroxisomes-small vesicular subcellular organelles essential for human life and health. A key peroxin - to date the best studied - is peroxin 5. Structurally, peroxin 5 is a bi-domain protein of about 70 kDa containing both globular and non-globular segments and displaying conformational flexibility. Functionally, it is a cycling receptor for importing essential enzymes into the peroxisome lumen, facilitated by highly promiscuous interactions with numerous proteins and possibly lipids. Peroxin 5 has medical significance in that (i) congenital defects can lead to fatal peroxisome biogenesis disorders, (ii) inefficient peroxisome targeting is linked to disease and aging and (iii) differences between human peroxin 5 and homologues in pathogens may be exploited in the development of therapeutics.