Oligomerization of malate synthase during glyoxysome biosynthesis

The first minutes in the life of a peroxisomal matrix protein

In the field of intracellular protein sorting, peroxisomes are most famous by their capacity to import oligomeric proteins. The data supporting this remarkable property are abundant and, understandably, have inspired a variety of hypothetical models on how newly synthesized (cytosolic) proteins reach the peroxisome matrix. However, there is also accumulating evidence suggesting that many peroxisomal oligomeric proteins actually arrive at the peroxisome still as monomers. In support of this idea, recent data suggest that PEX5, the shuttling receptor for peroxisomal matrix proteins, is also a chaperone/holdase, binding newly synthesized peroxisomal proteins in the cytosol and blocking their oligomerization. Here we review the data behind these two different perspectives and discuss their mechanistic implications on this protein sorting pathway.

Uniqueness of the mechanism of protein import into the peroxisome matrix: transport of folded, co-factor-bound and oligomeric proteins by shuttling receptors

Based on earlier suggestions that peroxisomes may have arisen from endosymbionts that later lost their DNA, it was expected that protein transport into this organelle would have parallels to systems found in other organelles of endosymbiont origin, such as mitochondria and chloroplasts. This review highlights three features of peroxisomal matrix protein import that make it unique in comparison with these other subcellular compartments - the ability of this organelle to transport folded, co-factor-bound and oligomeric proteins, the dynamics of the import receptors during the matrix protein import cycle and the existence of a peroxisomal quality-control pathway, which insures that the peroxisome membrane is cleared of cargo-free receptors.

Saccharomyces cerevisiae peroxisomal thiolase is imported as a dimer

The active conformation of native peroxisomal 3-ketoacyl-CoA thiolases (EC 2.3.1.16) is homodimeric. We have previously shown that a truncated Saccharomyces cerevisiae thiolase lacking its first 16 N-terminal amino acids fails to be translocated into peroxisomes but assembles into an enzymatically active form in the cytoplasm of a strain with a disrupted nuclear thiolase gene. We now report that when truncated thiolase is cosynthesized with full-length thiolase, approximately 50% of truncated thiolase cofractionates with the full-length thiolase to fractions enriched for peroxisomes and is translocated into peroxisomes as shown by its protection from the action of external proteases. We constructed an immunologically distinct cytosolic variant of thiolase by adding an influenza hemagglutinin epitope tag to the N terminus of the truncated thiolase. In a strain simultaneously expressing the full-length, truncated, and epitope-tagged truncated thiolases, we demonstrated that normally untargeted thiolase subunits are efficiently translocated into peroxisomes by dimerization with full-length thiolase subunits. Even though truncated and epitope-tagged truncated thiolase subunits are translocated into peroxisomes in this strain, only the full-length thiolase subunit can be coimmunoprecipitated with the epitope-tagged truncated thiolase subunit from the peroxisomal matrix. This observation suggests that interactions between thiolase subunits are not disrupted during translocation.

Malate dehydrogenase isoenzymes: cellular locations and role in the flow of metabolites between the cytoplasm and cell organelles

Malate dehydrogenases belong to the most active enzymes in glyoxysomes, mitochondria, peroxisomes, chloroplasts and the cytosol. In this review, the properties and the role of the isoenzymes in different compartments of the cell are compared, with emphasis on molecular biological aspects. Structure and function of malate dehydrogenase isoenzymes from plants, mammalian cells and ascomycetes (yeast, Neurospora) are considered. Significant information on evolutionary aspects and characterisation of functional domains of the enzymes emanates from bacterial malate and lactate dehydrogenases modified by protein engineering. The review endeavours to give up-to-date information on the biogenesis and intracellular targeting of malate dehydrogenase isoenzymes as well as enzymes cooperating with them in the flow of metabolites of a given pathway and organelle.

Import of fructose bisphosphate aldolase into the glycosomes of Trypanosoma brucei

The glycolytic enzymes of Trypanosomatids are compartmentalized within peroxisome-like microbodies called glycosomes. Fructose bisphosphate aldolase is synthesized on free polysomes and imported into glycosomes within 5 min. Peptide mapping reveals no primary structural differences between the in vivo-synthesized protein and that made in vitro from a synthetic template. However, native aldolase from glycosomes is partially protease resistant, whereas the in vitro translation product is not. Pulse-chase results indicate that aldolase in bloodstream trypanosomes has a much longer half-life than in the procyclic tsetse fly form.

Microbody transition in greening watermelon cotyledons Double immunocytochemical labeling of isocitrate lyase and hydroxypyruvate reductase

Microbody transition during the greening of watermelon cotyledons (Citrullus vulgaris Schrad.) was studied by double immunocytochemical labeling of the glyoxysomal marker enzyme isocitrate lyase and the peroxisomal marker enzyme hydroxypyruvate reductase. In order to analyze the immunocytochemistry, developmental stages representing the glyoxysomal, microbodytransition and peroxisomal stages were chosen, taking into account the time course of enzyme activity and the amounts of the respective antigens. It was shown that during microbody transition, between 83 and 91% of all the tested microbodies contained isocitrate lyase as well as hydroxypyruvate reductase, which was significantly higher than in the glyoxysomal and peroxisomal stages of development. Comprehensive controls precluded labeling artifacts. Our results support the one-population hypothesis first proposed by Trelease et al. (1971, Plant Physiol. 48, 461-465).

Import of glyoxysomal malate dehydrogenase precursor into glyoxysomes: A heterologous in-vitro system

A heterologous in-vitro system is described for the import of the precursor to glyoxysomal malate dehydrogenase from watermelon (Citrullus vulgaris Schrad., cv. Kleckey's Sweet No. 6) cotyledons into glyoxysomes from castor-bean (Ricinus communis L.) endosperm. The 41-kDa precursor is posttranslationally sequestered and correctly processed to the mature 33-kDa subunit by a crude glyoxysomal fraction or by glyoxysomes purified on a sucrose gradient. The import and the cleavage of the extrasequence is not inhibited by metal chelators such as 1,10-phenanthroline and ethylenediaminetetraacetic acid. Uncouplers (carbonylcyanide m-chlorophenylhydrazone), ionophores (valinomycin), or inhibitors of oxidative phosphorylation (oligomycin) and ATP-ADP translocation (carboxyatractyloside) do not interfere, thus indicating the independence of the process of import by the organelle from the energization of the glyoxysomal membrane.

Malate synthase: aggregation, deaggregation, and binding of phospholipids

Octameric malate synthase is located in the glyoxysomes of cucumber cotyledons. The enzyme is predominantly confined to the organelle's membrane and can be solubilized with Mg2+. Separation of cell structures in a zonal rotor afforded, besides glyoxysomes, two other zones with malate synthase activity, viz., in the gradient supernatant and in the range of the endoplasmic reticulum (ER). Malate synthases of these three fractions were purified to apparent homogeneity and classified according to their molecular weight. Differences in subunit molecular weight, however, could not be detected when malate synthases from the three fractions were compared. Mature malate synthase, as well as malate synthase prepared from fractions sedimenting similarly to the ER, exhibited the following behavior with respect to aggregation and deaggregation: at low salt concentrations and in the absence of Mg2+, the enzyme shifted to aggregated forms (approx 100 S); with 2 mM Mg2+, malate synthase deaggregated and occurred predominantly as an octamer (19 S). By changing buffer conditions, mature forms of malate synthase could be interconverted repeatedly between octameric and aggregated forms, whereas a monomeric form (5 S), prepared from soluble fractions assigned to the cytosol, did not oligomerize. The amphipathic properties of malate synthase were demonstrated by the enzyme's capacity for binding phospholipids.