Cvt9/Gsa9 functions in sequestering selective cytosolic cargo destined for the vacuole

Three overlapping pathways mediate the transport of cytoplasmic material to the vacuole in Saccharomyces cerevisiae. The cytoplasm to vacuole targeting (Cvt) pathway transports the vacuolar hydrolase, aminopeptidase I (API), whereas pexophagy mediates the delivery of excess peroxisomes for degradation. Both the Cvt and pexophagy pathways are selective processes that specifically recognize their cargo. In contrast, macroautophagy nonselectively transports bulk cytosol to the vacuole for recycling. Most of the import machinery characterized thus far is required for all three modes of transport. However, unique features of each pathway dictate the requirement for additional components that differentiate these pathways from one another, including at the step of specific cargo selection.We have identified Cvt9 and its Pichia pastoris counterpart Gsa9. In S. cerevisiae, Cvt9 is required for the selective delivery of precursor API (prAPI) to the vacuole by the Cvt pathway and the targeted degradation of peroxisomes by pexophagy. In P. pastoris, Gsa9 is required for glucose-induced pexophagy. Significantly, neither Cvt9 nor Gsa9 is required for starvation-induced nonselective transport of bulk cytoplasmic cargo by macroautophagy. The deletion of CVT9 destabilizes the binding of prAPI to the membrane and analysis of a cvt9 temperature-sensitive mutant supports a direct role of Cvt9 in transport vesicle formation. Cvt9 oligomers peripherally associate with a novel, perivacuolar membrane compartment and interact with Apg1, a Ser/Thr kinase essential for both the Cvt pathway and autophagy. In P. pastoris Gsa9 is recruited to concentrated regions on the vacuole membrane that contact peroxisomes in the process of being engulfed by pexophagy. These biochemical and morphological results demonstrate that Cvt9 and the P. pastoris homologue Gsa9 may function at the step of selective cargo sequestration.     

CCZ1, MON1 and YPT7 genes are involved in pexophagy, the Cvt pathway and non-specific macroautophagy in the methylotrophic yeast Pichia pastoris

Orthologues of Saccharomyces cerevisiae CCZ1, MON1 and YPT7 genes in the methylotrophic yeast, Pichia pastoris, have been identified. These genes encode proteins, which act as a complex, being involved in degradation of oleate-induced peroxisomes, Cvt (cytoplasm to vacuole targeting) pathway and non-specific macroautophagy in S. cerevisiae. CCZ1, MON1 and YPT7 gene orthologues are essential for multiple delivery pathways in P. pastoris. Strains with deletion of either of these genes displayed complete deficiency in pexophagy, non-specific macroautophagy and the biosynthetic Cvt pathway. The data suggest that CCZ1, MON1 and YPT7 genes are involved in degradation of both small oleate-induced and large methanol-induced peroxisomes. The data suggest conservative functions of CCZ1, MON1 and YPT7 genes among yeast species.     

Autophagy-related pathways and specific role of sterol glucoside in yeasts

Recently, we showed that the requirement of sterol glucoside (SG) during pexophagy in yeasts is dependent on the species and the nature of peroxisome inducers. Atg26, the enzyme that converts sterol to SG, is essential for degradation of very large methanol-induced peroxisomes, but only partly required for degradation of smaller-sized oleate- and amine-induced peroxisomes in Pichia pastoris. Moreover, oleate- and amine-induced peroxisomes of another yeast, Yarrowia lipolytica, are degraded by an Atg26-independent mechanism. The same is true for degradation of oleate-induced peroxisomes in Saccharomyces cerevisiae. Here, we review our findings on the specificity of Atg26 function in pexophagy and extend our observations to the role of SG in the cytoplasm to vacuole targeting (Cvt) pathway and bulk autophagy. The results presented here and elsewhere indicate that Atg26 might increase the efficacy of all autophagy-related pathways in P. pastoris, but not in other yeasts. Recently, it was shown that P. pastoris Atg26 (PpAtg26) is required for elongation of the pre-autophagosomal structure (PAS) into the micropexophagic membrane apparatus (MIPA) during micropexophagy. Therefore, we speculate that SG might facilitate elongation of any double membrane from the PAS and this enhancer function of SG becomes essential when extremely large double membranes are formed.     

Atg26 is not involved in autophagy-related pathways in Saccharomyces cerevisiae

Autophagy is a degradative pathway conserved among eukaryotes. It is a major route for degradation of long-lived proteins and entire organelles, such as peroxisomes. Atg26, a sterol glucosyltransferase, is specifically required for micro- and macropexophagy, but not for starvation-induced bulk autophagy in Pichia pastoris. Here we study the requirement of Saccharomyces cerevisiae Atg26 in the Cvt pathway, nonspecific autophagy and pexophagy. Our results show that the S. cerevisiae atg26Delta strain is not defective in prApe1 maturation, macroautophagy or peroxisome degradation, in contrast to the situation seen in Pichia pastoris. These studies highlight the importance of examining mutants in multiple organisms.     

A cytoplasm to vacuole targeting pathway in P. pastoris

The cytoplasm-to-vacuole targeting (Cvt) pathway of Saccharomyces cerevisiae delivers aminopeptidase I (Ape1) from the cytosol to the vacuole, bypassing the normal secretory route. The Cvt pathway, although well-studied, was known only in S. cerevisiae. We demonstrate its existence in the methylotrophic yeast, Pichia pastoris, where it also delivers P. pastoris Ape1 (PpApe1) to the vacuole. Most proteins known to be required for the Cvt pathway in S. cerevisiae were, to the extent we found orthologs, also required in P. pastoris. The P. pastoris Cvt pathway differs, however, from that in S. cerevisiae, in that new proteins, such as PpAtg28 and PpAtg26, are involved. The discovery of a Cvt pathway in P. pastoris makes it an excellent model system for the dissection of autophagy-related pathways in a single organism and for the discovery of new Cvt pathway components.     

GSA11 encodes a unique 208-kDa protein required for pexophagy and autophagy in Pichia pastoris

Cells are capable of adapting to changes in their environment by synthesizing needed proteins and degrading superfluous ones. Pichia pastoris synthesizes peroxisomal enzymes to grow in methanol medium. Upon adapting from methanol medium to one containing glucose, this yeast rapidly and selectively degrades peroxisomes by an autophagic process referred to as pexophagy. In this study, we have utilized a novel approach to identify genes required for this degradative pathway. Our approach involves the random integration of a vector containing the Zeocin resistance gene into the yeast genome by restriction enzyme-mediated integration. Cells unable to degrade peroxisomes during glucose adaptation were isolated, and the genes that were disrupted by the insertion of the vector were determined by sequencing. By using this approach, we have identified a number of genes required for glucose-induced selective autophagy of peroxisomes (GSA genes). We report here the characterization of Gsa11, a unique 208-kDa protein. We found that this protein is required for glucose-induced pexophagy and starvation-induced autophagy. Gsa11 is a cytosolic protein that becomes associated with one or more structures situated near the vacuole during glucose adaptation. The punctate localization of Gsa11 was not observed in gsa10, gsa12, gsa14, and gsa19 mutants. We have previously shown that Gsa9 appears to relocate from a compartment at the vacuole surface to regions between the vacuole and the peroxisomes being sequestered. In the gsa11 mutants, the vacuole only partially surrounded the peroxisomes, but Gsa9 was still distributed around the peroxisome cluster. This suggests that Gsa9 binds to the peroxisomes independent of the vacuole. The data also indicate that Gsa11 is not necessary for Gsa9 to interact with peroxisomes but acts at an intermediate event required for the vacuole to engulf the peroxisomes.     

Pex3-anchored Atg36 tags peroxisomes for degradation in Saccharomyces cerevisiae

Peroxisomes undergo rapid, selective autophagic degradation (pexophagy) when the metabolic pathways they contain are no longer required for cellular metabolism. Pex3 is central to the formation of peroxisomes and their segregation because it recruits factors specific for these functions. Here, we describe a novel Saccharomyces cerevisiae protein that interacts with Pex3 at the peroxisomal membrane. We name this protein Atg36 as its absence blocks pexophagy, and its overexpression induces pexophagy. We have isolated pex3 alleles blocked specifically in pexophagy that cannot recruit Atg36 to peroxisomes. Atg36 is recruited to mitochondria if Pex3 is redirected there, where it restores mitophagy in cells lacking the mitophagy receptor Atg32. Furthermore, Atg36 binds Atg8 and the adaptor Atg11 that links receptors for selective types of autophagy to the core autophagy machinery. Atg36 delivers peroxisomes to the preautophagosomal structure before being internalised into the vacuole with peroxisomes. We conclude that Pex3 recruits the pexophagy receptor Atg36. This reinforces the pivotal role played by Pex3 in coordinating the size of the peroxisome pool, and establishes its role in pexophagy in S. cerevisiae.     

Apg2 is a novel protein required for the cytoplasm to vacuole targeting, autophagy, and pexophagy pathways

To survive starvation conditions, eukaryotes have developed an evolutionarily conserved process, termed autophagy, by which the vacuole/lysosome mediates the turnover and recycling of non-essential intracellular material for re-use in critical biosynthetic reactions. Morphological and biochemical studies in Saccharomyces cerevisiae have elucidated the basic steps and mechanisms of the autophagy pathway. Although it is a degradative process, autophagy shows substantial overlap with the biosynthetic cytoplasm to vacuole targeting (Cvt) pathway that delivers resident hydrolases to the vacuole. Recent molecular genetics analyses of mutants defective in autophagy and the Cvt pathway, apg, aut, and cvt, have begun to identify the protein machinery and provide a molecular resolution of the sequestration and import mechanism that are characteristic of these pathways. In this study, we have identified a novel protein, termed Apg2, required for both the Cvt and autophagy pathways as well as the specific degradation of peroxisomes. Apg2 is required for the formation and/or completion of cytosolic sequestering vesicles that are needed for vacuolar import through both the Cvt pathway and autophagy. Biochemical studies revealed that Apg2 is a peripheral membrane protein. Apg2 localizes to the previously identified perivacuolar compartment that contains Apg9, the only characterized integral membrane protein that is required for autophagosome/Cvt vesicle formation.     

ATG genes involved in non-selective autophagy are conserved from yeast to man, but the selective Cvt and pexophagy pathways also require organism-specific genes

ATG genes encode proteins that are required for macroautophagy, the Cvt pathway and/or pexophagy. Using the published Atg protein sequences, we have screened protein and DNA databases to identify putative functional homologs (orthologs) in 21 fungal species (yeast and filamentous fungi) of which the genome sequences were available. For comparison with Atg proteins in higher eukaryotes, also an analysis of Arabidopsis thaliana and Homo sapiens databases was included. This analysis demonstrated that Atg proteins required for non-selective macroautophagy are conserved from yeast to man, stressing the importance of this process in cell survival and viability. The A. thaliana and human genomes encode multiple proteins highly similar to specific fungal Atg proteins (paralogs), possibly representing cell type-specific isoforms. The Atg proteins specifically involved in the Cvt pathway and/or pexophagy showed poor conservation, and were generally not present in A. thaliana and man. Furthermore, Atg19, the receptor of Cvt cargo, was only detected in Saccharomyces cerevisiae. Nevertheless, Atg11, a protein that links receptor-bound cargo (peroxisomes, the Cvt complex) to the autophagic machinery was identified in all yeast species and filamentous fungi under study. This suggests that in fungi an organism-specific form of selective autophagy may occur, for which specialized Atg proteins have evolved.     

Atg26 is Not Involved in Autophagy-Related Pathways in Saccharomyces cerevisiae

Autophagy is a degradative pathway conserved among eukaryotes. It is a major route for degradation of long-lived proteins and entire organelles, such as peroxisomes. Atg26, a sterol glucosyltransferase, is specifically required for micro- and macropexophagy, but not for starvation-induced bulk autophagy in Pichia pastoris. Here we study the requirement of Saccharomyces cerevisiae Atg26 in the Cvt pathway, nonspecific autophagy and pexophagy. Our results show that the S. cerevisiae atg26? strain is not defective in prApe1 maturation, macroautophagy or peroxisome degradation, in contrast to the situation seen in Pichia pastoris. These studies highlight the importance of examining mutants in multiple organisms.     

Atg23 is essential for the cytoplasm to vacuole targeting pathway and efficient autophagy but not pexophagy

Cells must regulate both biosynthesis and degradation to ensure proper homeostasis of cellular organelles and proteins. This balance is demonstrated in a unique way in the yeast Saccharomyces cerevisiae, which possesses two distinct, yet mechanistically related trafficking routes mediating the delivery of proteins from the cytoplasm to the vacuole: the biosynthetic cytoplasm to vacuole targeting (Cvt) and the degradative autophagy pathways. Several components employed by these two transport routes have been identified, but their mechanistic interactions remain largely unknown. Here we report a novel gene involved in these pathways, which we have named ATG23. Atg23 localizes to the pre-auto-phagosomal structure but also to other cytosolic punctate compartments. Our characterization of the Atg23 protein indicates that it is required for the Cvt pathway and efficient autophagy but not pexophagy. In the absence of Atg23, cargo molecules such as prApe1 are correctly recruited to a pre-autophagosomal structure that is unable to give rise to Cvt vesicles. We also demonstrate that Atg23 is a peripheral membrane protein that requires the presence of Atg9/Apg9 to be specifically targeted to lipid bilayers. Atg9 transiently interacts with Atg23 suggesting that it participates in the recruitment of this protein.     

The relevance of the phosphatidylinositolphosphat-binding motif FRRGT of Atg18 and Atg21 for the Cvt pathway and autophagy

Atg18 and Atg21 are homologous S. cerevisiae autophagy proteins. Atg18 is essential for biogenesis of Cvt vesicles and autophagosomes, while Atg21 is only essential for Cvt vesicle formation. We found that mutated Atg18-(FTTGT), which lost almost completely its binding to PtdIns3P and PtdIns(3,5)P(2), is non-functional during the Cvt pathway but active during autophagy and pexophagy. Since the Cvt pathway does not depend on PtdIns(3,5)P(2), we conclude that the Cvt pathway requires binding of Atg18 to PtdIns3P. Mutated Atg21-(FTTGT) is inactive during the Cvt pathway but showed only partly reduced binding to PtdIns-phosphates, suggesting further lipid binding domains in Atg21. GFP-Atg18-(FTTGT) and Atg21-(FTTGT)-GFP are released from vacuolar punctae to the cytosol.     

Early and late molecular events of glucose-induced pexophagy in Pichia pastoris require Vac8

We have identified the Pichia pastoris Vac8 homolog, a 60-64 kDa armadillo repeat protein, and have examined the role of PpVac8 in the degradative pathways involving the yeast vacuole. We report here that PpVac8 is required for glucose-induced pexophagy, but not ethanol-induced pexophagy or starvation-induced autophagy. This has been demonstrated by the persistence of peroxisomal alcohol oxidase activity in mutants lacking PpVac8 during glucose adaptation. During glucose-induced micropexophagy, in the absence of PpVac8, the vacuole was invaginated with arm-like "segmented" extensions that almost completely surrounded the adjacent peroxisomes. Vac8-GFP was found at the vacuolar membrane and concentrated at the base of the arm-like protrusions that extend from the vacuole to sequester the peroxisomes. The localization of Vac8-GFP to the vacuolar membrane occurred independent of PpAtg1, PpAtg9 or PpAtg11. Mutagenesis of the palmitoylated cysteines to alanines or deletion of the myristoylation and palmitoylation sites of PpVac8 resulted in decreased protein stability, impaired vacuolar association and reduced degradation of peroxisomal alcohol oxidase. Deletion of the central armadillo repeat domains of the PpVac8 did not alter its association with the vacuolar membrane, but resulted in a non-functional protein that suppressed the formation of the arm-like extensions from the vacuole to engulf the peroxisomes. PpVac8 is essential for the trafficking of PpAtg11, but not PpAtg1 or PpAtg18, to the vacuole membrane. Together, our results support a role for PpVac8 in early (formation of sequestering membranes) and late (post-MIPA membrane fusion) molecular events of glucose-induced pexophagy.     

Role of Vac8 in Cellular Degradation Pathways in Pichia pastoris

We have identified the Pichia pastoris Vac8 homolog, a 60-64 kDa armadillo repeat protein, and have examined the role of PpVac8 in the degradative pathways involving the yeast vacuole. We report here that PpVac8 is required for glucose-induced pexophagy and mitophagy, but not ethanol-induced pexophagy or starvation-induced autophagy. This has been demonstrated by the persistence of peroxisomal alcohol oxidase activity and GFP-labeled mitochondria in mutants lacking PpVac8 during glucose adaptation. During glucose-induced micropexophagy, in the absence of PpVac8, the vacuole was invaginated with arm-like “segmented” extensions that almost completely surrounded the adjacent peroxisomes. PpVac8-GFP was found at the vacuolar membrane and concentrated at the base of the sequestering membranes that extend from the vacuole to engulf the peroxisomes. The localization of PpVac8-GFP to the vacuolar membrane occurred independent of PpAtg1, PpAtg9 or PpAtg11. Mutagenesis of the palmitoylated cysteines to alanines or deletion of the myristoylation and palmitoylation sites of PpVac8, resulted in an impaired vacuolar association and decreased degradation of alcohol oxidase. Deletion of the central armadillo repeat domains of the PpVac8 did not alter its association with the vacuolar membrane, but resulted in a nonfunctional protein that suppressed the formation of the arm-like extensions from the vacuole to engulf the peroxisomes. PpVac8 is essential for the trafficking of PpAtg11, but not PpAtg1 or PpAtg18, to the vacuole membrane. Together, our results support a role for PpVac8 in early (formation of sequestering membranes) and late (post-MIPA membrane fusion) molecular events of glucose-induced pexophagy.     

Autophagy-Related Pathways and Specific Role of Sterol Glucoside in Yeasts

Recently, we showed that the requirement of sterol glucoside (SG) during pexophagy in yeasts is dependent on the species and the nature of peroxisome inducers. Atg26, the enzyme that converts sterol to SG, is essential for degradation of very large methanol-induced peroxisomes, but only partly required for degradation of smaller-sized oleate- and amine-induced peroxisomes in Pichia pastoris. Moreover, oleate- and amine-induced peroxisomes of another yeast, Yarrowia lipolytica, are degraded by an Atg26-independent mechanism. The same is true for degradation of oleate-induced peroxisomes in Saccharomyces cerevisiae. Here, we review our findings on the specificity of Atg26 function in pexophagy and extend our observations to the role of SG in the cytoplasm to vacuole targeting (Cvt) pathway and bulk autophagy. The results presented here and elsewhere indicate that Atg26 might increase the efficacy of all autophagy-related pathways in P. pastoris, but not in other yeasts. Recently, it was shown that P. pastoris Atg26 (PpAtg26) is required for elongation of the pre-autophagosomal structure (PAS) into the micropexophagic membrane apparatus (MIPA) during micropexophagy. Therefore, we speculate that SG might facilitate elongation of any double membrane from the PAS and this enhancer function of SG becomes essential when extremely large double membranes are formed.

Addendum to:

The Requirement of Sterol Glucoside for Pexophagy in Yeast Is Dependent on the Species and Nature of Peroxisome Inducers

T.Y. Nazarko, A.S. Polupanov, R.R. Manjithaya, S. Subramani and A.A. Sibirny

Mol Biol Cell 2007; 18:106-18     

Degradation of lipid vesicles in the yeast vacuole requires function of Cvt17, a putative lipase

The vacuole/lysosome serves an essential role in allowing cellular components to be degraded and recycled under starvation conditions. Vacuolar hydrolases are key proteins in this process. In Saccharyomces cerevisiae, some resident vacuolar hydrolases are delivered by the cytoplasm to vacuole targeting (Cvt) pathway, which shares mechanistic features with autophagy. Autophagy is a degradative pathway that is used to degrade and recycle cellular components under starvation conditions. Both the Cvt pathway and autophagy employ double-membrane cytosolic vesicles to deliver cargo to the vacuole. As a result, these pathways share a common terminal step, the degradation of subvacuolar vesicles. We have identified a protein, Cvt17, which is essential for this membrane lytic event. Cvt17 is a membrane glycoprotein that contains a motif conserved in esterases and lipases. The active-site serine of this motif is required for subvacuolar vesicle lysis. This is the first characterization of a putative lipase implicated in vacuolar function in yeast.     

Autophagy in yeast: a review of the molecular machinery

Autophagy is a membrane trafficking mechanism that delivers cytoplasmic cargo to the vacuole/lysosome for degradation and recycling. In addition to non-specific bulk cytosol, selective cargoes, such as peroxisomes, are sorted for autophagic transport under specific physiological conditions. In a nutrient-rich growth environment, many of the autophagic components are recruited for executing a biosynthetic trafficking process, the cytoplasm to vacuole targeting (Cvt) pathway, that transports the resident hydrolases aminopeptidase I and alpha-mannosidase to the vacuole in Saccharomyces cerevisiae. Recent studies have identified pathway-specific components that are necessary to divert a protein kinase and a lipid kinase complex to regulate the conversion between the Cvt pathway and autophagy. Downstream of these proteins, the general machinery for transport vesicle formation involves two novel conjugation systems and a putative membrane protein complex. Completed vesicles are targeted to, and fuse with, the vacuole under the control of machinery shared with other vacuolar trafficking pathways. Inside the vacuole, a potential lipase and several proteases are responsible for the final steps of vesicle breakdown, precursor enzyme processing and substrate turnover. In this review, we discuss the most recent developments in yeast autophagy and point out the challenges we face in the future.     

ATG Genes Involved in Non-Selective Autophagy are Conserved from Yeast to Man,but the Selective Cvt and Pexophagy Pathways also Require Organism-Specific Genes

ATG genes encode proteins that are required for macroautophagy, the Cvt pathway and/or pexophagy. Using the published Atg protein sequences, we have screened protein and DNA databases to identify putative functional homologs (orthologs) in 21 fungal species (yeast and filamentous fungi) of which the genome sequences were available. For comparison with Atg proteins in higher eukaryotes, also the genomes of Arabidopsis thaliana and Homo sapiens were included. This analysis demonstrated that Atg proteins required for non-selective macroautophagy are conserved from yeast to man, stressing the importance of this process in cell survival and viability. Remarkably, the A. thaliana and human genomes encode multiple proteins highly similar to specific Atg proteins (paralogs), the function of which is unknown. The Atg proteins specifically involved in the Cvt pathway and/or pexophagy showed poor conservation, and were generally not present in A. thaliana and man. Furthermore, the receptor of Cvt cargo, Atg19, was only detected in S. cerevisiae. Nevertheless, Atg11, a protein that links receptor-bound cargo (peroxisomes, Cvt bodies) to the autophagic machinery was identified in all yeast species and filamentous fungi under study. This suggests that in fungi an organism-specific form of selective autophagy may occur, for which specialized Atg proteins have evolved.     

The selectivity of autophagy and its role in cell death and survival

Autophagy is a cellular process whose primary function is to degrade long-lived proteins and recycle cellular components. Beside macroautophagy, there are several forms of selective autophagy, including chaperone-mediated autophagy (CMA), cytoplasm to vacuole targeting (Cvt), pexophagy and mitophagy. In this review, we summarize what is currently known about selective autophagy, and discuss its role in cell death and survival. We also discuss possible mechanisms underlying the selectivity of macroautophagy.     

The selectivity of autophagy and its role in cell death and survival

Autophagy is a cellular process whose primary function is to degrade long-lived proteins and recycle cellular components. Beside macroautophagy, there are several forms of selective autophagy, including chaperone-mediated autophagy (CMA), cytoplasm to vacuole targeting (Cvt), pexophagy, and mitophagy. In this review, we summarize what is currently known about selective autophagy, and discuss its role in cell death and survival. We also discuss possible mechanisms underlying the selectivity of macroautophagy.