Trs85 (Gsg1), a Component of the TRAPP Complexes, Is Required for the Organization of the Preautophagosomal Structure during Selective Autophagy via the Cvt Pathway

Khuyen Meiling-Wesse,Ulrike D Epple,Roswitha Krick,Henning Barth,Anika Appelles,Christiane Voss,Eeva-Liisa Eskelinen,Michael Thumm

doi:10.1074/jbc.m501701200

Khuyen Meiling-Wesse, Ulrike D Epple + Show 6 more

Open Access

https://doi.org/10.1074/jbc.m501701200

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Abstract

Autophagosomes and Cvt vesicles are limited by two membrane layers. The biogenesis of these unconventional vesicles and the origin of their membranes are hardly understood. Here we identify in Saccharomyces cerevisiae Trs85, a nonessential component of the TRAPP complexes, to be required for the biogenesis of Cvt vesicles. The TRAPP complexes function in endoplasmic reticulum-to-Golgi and Golgi trafficking. Growing trs85delta cells show a defect in the organization of the preautophagosomal structure. Although proaminopeptidase I is normally recruited to the preautophagosomal structure, the recruitment of green fluorescent protein-Atg8 depends on Trs85. Autophagy proceeds in the absence of Trs85, albeit at a reduced rate. Our electron microscopic analysis demonstrated that the reduced autophagic rate of trs85delta cells does not result from a reduced size of the autophagosomes. Growing and starved cells lacking Trs85 did not show defects in vacuolar biogenesis; mature vacuolar proteinase B and carboxypeptidase Y were present. Also vacuolar acidification was normal in these cells. It is known that mutations impairing the integrity of the ER or Golgi block both autophagy and the Cvt pathway. But the phenotypes of trs85delta cells show striking differences to those seen in mutants with defects in the early secretory pathway. This suggests that Trs85 might play a direct role in the Cvt pathway and autophagy.

Highlights

Starvation-induced autophagy is an unselective, degradative pathway that delivers cytosolic material to the lysosome [1,2,3]
The formation of double membrane-layered Cvt vesicles and autophagosomes out of the preautophagosomal structure (PAS) seems to require novel mechanistic features, since none of the yeast t-SNAREs has been located to the PAS [17]
The yeast NSF Sec18 is not involved in the biogenesis of these vesicles [18]

Summary

Introduction

Starvation-induced autophagy is an unselective, degradative pathway that delivers cytosolic material to the lysosome (vacuole) [1,2,3]. It is well conserved between eukaryotes such as fungi, plants, and mammals. Within the vacuole the autophagic bodies are lysed dependent on the putative lipase Atg15 [16], and the cytosolic material is degraded by the various vacuolar hydrolases. In S. cerevisiae the Cvt (cytoplasm to vacuole targeting) pathway was discovered as a variant of the starvation-induced unselective autophagy. The selective autophagic degradation of dispensable peroxisomes, is another variant of autophagy It takes place when yeast cells are shifted from a medium inducing the proliferation of peroxisomes to a medium containing glucose [19]

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