Magnesium. An update on physiological, clinical and analytical aspects.

beta-oxidation of fatty acids in mitochondria, peroxisomes, and bacteria: a century of continued progress.

Isolation of the Pichia pastoris glyceraldehyde-3-phosphate dehydrogenase gene and regulation and use of its promoter.

The mechanism of translocation of peroxisomal proteins from the cytoplasm into the matrix is largely unknown. We have been studying this problem in yeast. We show that the peroxisomal targeting sequences SKL or AKL, with or without a spacer of nine glycines (G9), are sufficient to target chloramphenicol acetyltransferase (CAT) to peroxisomes of Saccharomyces cerevisiae in vivo. The mature form of CAT is a homotrimer, and complete trimerization of CAT was found to occur within a few minutes of synthesis. In contrast, import, measured by immunoelectron microscopy and organellar fractionation, occurred over several hours. To confirm that import of preassembled CAT trimers was occurring, we co-expressed CAT-G9-AKL with CAT lacking a peroxisomal targeting sequence but containing a hemagglutinin-derived epitope tag (HA-CAT). We found that HA-CAT was not imported unless it was co-expressed with CAT-G9-AKL. Both proteins were released from the organelles under mild conditions (pH 8.5) that released other matrix proteins, indicating that import had occurred. These results strongly suggested that HA-CAT was imported as a heterotrimer with CAT-G9-AKL. The process of oligomeric import also occurs in animal cells. When HA-CAT was co-expressed with CAT-G9-AKL in CV-1 cells, HA-CAT co-localized with peroxisomes but was cytoplasmic when expressed alone. It is not clear whether the import of globular proteins into peroxisomes occurs through peroxisomal membrane pores or involves membrane internalization. Both possibilities are discussed.

https://rupress.org/jcb/article-pdf/127/5/1245/1264110/1245.pdf

An oligomeric protein is imported into peroxisomes in vivo

PEX5 encodes the type-1 peroxisomal targeting signal (PTS1) receptor, one of at least 15 peroxins required for peroxisome biogenesis. Pex5p has a bimodal distribution within the cell, mostly cytosolic with a small amount bound to peroxisomes. This distribution indicates that Pex5p may function as a cycling receptor, a mode of action likely to require interaction with additional peroxins. Loss of peroxins required for protein translocation into the peroxisome (PEX2 or PEX12) resulted in accumulation of Pex5p at docking sites on the peroxisome surface. Pex5p also accumulated on peroxisomes in normal cells under conditions which inhibit protein translocation into peroxisomes (low temperature or ATP depletion), returned to the cytoplasm when translocation was restored, and reaccumulated on peroxisomes when translocation was again inhibited. Translocation inhibiting conditions did not result in Pex5p redistribution in cells that lack detectable peroxisomes. Thus, it appears that Pex5p can cycle repeatedly between the cytoplasm and peroxisome. Altered activity of the peroxin defective in CG7 cells leads to accumulation of Pex5p within the peroxisome, indicating that Pex5p may actually enter the peroxisome lumen at one point in its cycle. In addition, we found that the PTS1 receptor was extremely unstable in the peroxin-deficient CG1, CG4, and CG8 cells. Altered distribution or stability of the PTS1 receptor in all cells with a defect in PTS1 protein import implies that the genes mutated in these cell lines encode proteins with a direct role in peroxisomal protein import.

/pdf/multiple-pex-genes-are-required-for-proper-subcellular-2t1807adyb.pdf

Multiple PEX genes are required for proper subcellular distribution and stability of Pex5p, the PTS1 receptor: evidence that PTS1 protein import is mediated by a cycling receptor.

/pdf/import-of-proteins-into-peroxisomes-and-other-microbodies-12m4jw80yz.pdf

Import of proteins into peroxisomes and other microbodies.

Amine oxidase from the yeast Hansenula polymorpha is a peroxisomal protein. The signal for routing of the protein into peroxisomes has not been identified yet. Expression of a mutant amine oxidase in H. polymorpha has revealed that the C-terminal sequence, which possesses an internal SRL tripeptide, is not involved in targeting (Faber et al., unpublished). We have explored heterologous expression of the amine oxidase gene (AMO) in Saccharomyces cerevisiae to investigate the conservation of peroxisomal targeting pathways between yeasts. Surprisingly, wild-type amine oxidase is not recognized as a peroxisomal protein by S. cerevisiae. The enzyme, which was fully active and accumulated to levels similar to those found in H. polymorpha, stayed entirely in the cytosol. However, fusing a SKL or a SRL sequence to the C-terminus forced the protein at least partially into peroxisomes of the heterologous host. These data suggest that the functional targeting sequence of amine oxidase may differ from the C-terminal peroxisomal targeting signal S/C/A-K/R/H-L (Gould et al., 1989).Contrary to the established tripeptide motif, the amine oxidase targeting signal appears not to be conserved between the different yeast species.

The peroxisomal import signal of amine oxidase from the yeast Hansenula polymorpha is not universal

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1016/0014-5793%2891%2981306-S

Overexpression of alcohol oxidase in Pichia pastoris.

To study the conservation of peroxisomal targeting signals, we have determined the intracellular localization of human peroxisomal catalase when expressed in yeast. Using immunofluorescence, differential centrifugation and immunoelectron microscopy, we show that the protein is targeted to the peroxisomes of the heterologous cell and assembled in its active tetrameric form. These data show the conservation of the catalase targeting signal and import specificity between human and yeast peroxisomes.

M. J. De Hoop

Papers

Import of proteins into peroxisomes and other microbodies.

The peroxisomal import signal of amine oxidase from the yeast Hansenula polymorpha is not universal

Overexpression of alcohol oxidase in Pichia pastoris.

Human catalase is imported and assembled in peroxisomes of Saccharomyces cerevisiae.