Autophagy is a catabolic process used by eukaryotic cells for the degradation and recycling of cytosolic proteins and excess or defective organelles. In yeast, autophagy is primarily a response to nutrient limitation, whereas in higher eukaryotes it also plays a role in developmental processes. Due to its essentially unlimited degradative capacity, it is critical that regulatory mechanisms are in place to modulate the timing and magnitude of the autophagic response. One set of proteins that seems to function in this regard includes a complex that contains the Atg1 kinase. Aside from Atg1, the proteins in this complex participate primarily in either nonspecific autophagy or specific types of autophagy, including the cytoplasm to vacuole targeting pathway, which operates under vegetative growth conditions, and peroxisome degradation. Accordingly, these proteins are prime candidates for factors that regulate the conversion between these pathways, including the change in size of the sequestering vesicle, the most obvious morphological difference. The atg17⌬ mutant forms a reduced number of small autophagosomes. As a result, it is defective in peroxisome degradation and is partially defective for autophagy. Atg17 interacts with both Atg1 and Atg13, via two coiled-coil domains, and these interactions facilitate its inclusion in the Atg1 complex. INTRODUCTIONCells must be able to respond to changes in their environment. These changes may be dictated in a random manner as the result of varying nutrient conditions or they may be part of a precise program that is part of a developmental plan, and the response may be either biosynthetic or catabolic. In recent years, increased emphasis has been placed upon the degradative pathways that allow cells to respond to stress or to specific hormonal cues. One of the primary pathways that play a role in cellular remodeling through degradation is autophagy (Klionsky, 2004;Levine and Klionsky, 2004). Autophagy presumably occurs in all eukaryotic cells and has been documented from yeast to humans (Reggiori and Klionsky, 2002;Wang and Klionsky, 2003). It involves the sequestration of cytoplasm within double-membrane cytosolic vesicles (Klionsky and Ohsumi, 1999;Kim and Klionsky, 2000;Klionsky and Emr, 2000). The completed vesicles subsequently fuse with endosomes and/or lysosomes, allowing the breakdown and recycling of the cargo.The protein components that form the autophagic machinery were first identified through molecular genetic studies in various yeasts, including Saccharomyces cerevisiae, Pichia pastoris, and Hansenula polymorpha (Tsukada and Ohsumi, 1993;Thumm et al., 1994;Harding et al., 1995;Titorenko et al., 1995;Tuttle and Dunn, 1995). Homologues of most of the corresponding genes, termed ATG , have been identified in higher eukaryotes, and recent studies have demonstrated the importance of autophagy in human health and disease (Cuervo, 2004;Shintani and Klionsky, 2004a). Autophagy is generally thought to occur in a nonspecific manner; however, studies in yeast have provide...

Autophagy is a conserved degradative pathway that is induced in response to various stress and developmental conditions in eukaryotic cells. It allows the elimination of cytosolic proteins and organelles in the lysosome/vacuole. In the yeast Saccharomyces cerevisiae, the integral membrane protein Atg9 (autophagy-related protein 9) cycles between mitochondria and the preautophagosomal structure (PAS), the nucleating site for formation of the sequestering vesicle, suggesting a role in supplying membrane for vesicle formation and/or expansion during autophagy. To better understand the mechanisms involved in Atg9 cycling, we performed a yeast two-hybrid–based screen and identified a peripheral membrane protein, Atg11, that interacts with Atg9. We show that Atg11 governs Atg9 cycling through the PAS during specific autophagy. We also demonstrate that the integrity of the actin cytoskeleton is essential for correct targeting of Atg11 to the PAS. We propose that a pool of Atg11 mediates the anterograde transport of Atg9 to the PAS that is dependent on the actin cytoskeleton during yeast vegetative growth.

The molecular mechanisms of peroxisome biogenesis have begun to emerge; in contrast, relatively little is known about how the organelle functions as cells age. In this report, we characterize age-related changes in peroxisomes of human cells. We show that aging compromises peroxisomal targeting signal 1 (PTS1) protein import, affecting in particular the critical antioxidant enzyme catalase. The number and appearance of peroxisomes are altered in these cells, and the organelles accumulate the PTS1-import receptor, Pex5p, on their membranes. Concomitantly, cells produce increasing amounts of the toxic metabolite hydrogen peroxide, and we present evidence that this increased load of reactive oxygen species may further reduce peroxisomal protein import and exacerbate the effects of aging

Autophagy is a catabolic pathway for the degradation of cytosolic proteins or organelles and is conserved among all eukaryotic cells. The hallmark of autophagy is the formation of double-membrane cytosolic vesicles, termed autophagosomes, which sequester cytoplasm; however, the mechanism of vesicle formation and the membrane source remain unclear. In the yeast Saccharomyces cerevisiae, selective autophagy mediates the delivery of specific cargos to the vacuole, the analog of the mammalian lysosome. The transmembrane protein Atg9 cycles between the mitochondria and the pre-autophagosomal structure, which is the site of autophagosome biogenesis. Atg9 is thought to mediate the delivery of membrane to the forming autophagosome. Here, we characterize a second transmembrane protein Atg27 that is required for specific autophagy in yeast. Atg27 is required for Atg9 cycling and shuttles between the pre-autophagosomal structure, mitochondria, and the Golgi complex. These data support a hypothesis that multiple membrane sources supply the lipids needed for autophagosome formation.