Peroxisome biogenesis and synthesis of peroxisomal enzymes in the methylotrophic yeast Hansenula polymorpha are under the strict control of glucose repression. We identified an H. polymorpha glucose catabolite repression gene (HpGCR1) that encodes a hexose transporter homologue. Deficiency in GCR1 leads to a pleiotropic phenotype that includes the constitutive presence of peroxisomes and peroxisomal enzymes in glucose-grown cells. Glucose transport and repression defects in a UV-induced gcr1-2 mutant were found to result from a missense point mutation that substitutes a serine residue (Ser 85 ) with a phenylalanine in the second predicted transmembrane segment of the Gcr1 protein. In addition to glucose, mannose and trehalose fail to repress the peroxisomal enzyme, alcohol oxidase in gcr1-2 cells. A mutant deleted for the GCR1 gene was additionally deficient in fructose repression. Ethanol, sucrose, and maltose continue to repress peroxisomes and peroxisomal enzymes normally and therefore, appear to have GCR1-independent repression mechanisms in H. polymorpha. Among proteins of the hexose transporter family of baker's yeast, Saccharomyces cerevisiae, the amino acid sequence of the H. polymorpha Gcr1 protein shares the highest similarity with a core region of Snf3p, a putative high affinity glucose sensor. Certain features of the phenotype exhibited by gcr1 mutants suggest a regulatory role for Gcr1p in a repression pathway, along with involvement in hexose transport.If provided with a mixture of carbon substrates, yeast preferentially utilizes the one that supports the fastest growth rate. This is achieved by several coordinated regulatory mechanisms of metabolic adaptation. They include: (i) the induction of enzymes involved in the metabolism of a preferred substrate and (ii) repression and/or inactivation of enzymes involved in the metabolism of less preferred carbon sources. Carbon sourcetriggered repression (or catabolite repression) generally affects expression of the corresponding target genes at the transcriptional level. Among co-repressor substrates in Saccharomyces cerevisiae, glucose is best known (for review see Refs. 1 and 2). The main targets of glucose repression in S. cerevisiae are enzymes of gluconeogenesis and the glyoxylate cycle, mitochondrial enzymes involved in oxidative phosphorylation, and enzymes involved in transport and metabolism of alternative carbon substrates, such as galactose, sucrose, and maltose. Despite extensive studies and a growing number of genes known to be involved in glucose repression in this and other species, its actual mechanism, especially in the early stages of sensing and signal transduction, is not fully understood.In methylotrophic yeasts, unique peroxisomal and cytosolic enzymes of methanol metabolism are under strict control of repression by various carbon substrates, e.g. glucose and ethanol (3, 4). Glucose and ethanol also trigger inactivation of peroxisomal enzymes by a process that involves the autophagic degradation of whole peroxisomes by vacuolar prot...

The hallmark of eukaryotic cells is compartmentalization of distinct cellular functions into speci¢c organelles. This necessitates the cells to run energetically costly mechanisms to precisely control maintenance and function of these compartments. One of these continuously controls organelle activity and abundance, a process termed homeostasis. Yeast peroxisomes are favorable model systems for studies of organelle homeostasis because both the proliferation and degradation of these organelles can be readily manipulated. Here, we highlight recent achievements in regulation of peroxisome turnover in yeast, in particular Hansenula polymorpha, with a focus on directions of future research. ß

Previously published online as an Autophagy E-publication: http://www.landesbioscience.com/journals/autophagy/abstract.php?id=1832 KEY WORDS Research PaperThe Hansenula Polymorpha ATG25 Gene Encodes a Novel Coiled-Coil Protein that is Required for Macropexophagy ABSTRACTWe have isolated the Hansenula polymorpha ATG25 gene, which is required for glucose-induced selective peroxisome degradation by macropexophagy. ATG25 represents a novel gene that encodes a 45 kDa coiled-coil protein. We show that this protein colocalizes with Atg11 on a small structure, which most likely represents the pre-autophagosomal structure (PAS).In cells of a constructed ATG25 deletion strain (atg25) peroxisomes are constitutively degraded by nonselective microautophagy, a process that in WT H. polymorpha is only observed at nitrogen limitation conditions. This suggests that nonselective microautophagy is deregulated in H. polymorpha atg25 cells.

Hansenula polymorpha PDD genes are involved in the selective degradation of peroxisomes via macropexophagy. We have isolated various novel pdd mutants by a gene‐tagging method. Here we describe the isolation and characterisation of PDD7, which encodes a protein with high sequence similarity (40% identity) to Saccharomyces cerevisiae Apg1p/Aut3p, previously described to be involved in random autophagy and the cytoplasm‐to‐vacuole targeting pathway. Our data indicate that HpPdd7p is essential for two processes that degrade peroxisomes, namely the highly selective process of macropexophagy and microautophagy, which occurs in H. polymorpha upon nitrogen starvation.

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