Completing Autophagy: Formation and Degradation of the Autophagic Body and Metabolite Salvage in Plants

Abstract

Autophagy is an evolutionarily conserved process that occurs in yeast, plants, and animals. Despite many years of research, some aspects of autophagy are still not fully explained. This mostly concerns the final stages of autophagy, which have not received as much interest from the scientific community as the initial stages of this process. The final stages of autophagy that we take into consideration in this review include the formation and degradation of the autophagic bodies as well as the efflux of metabolites from the vacuole to the cytoplasm. The autophagic bodies are formed through the fusion of an autophagosome and vacuole during macroautophagy and by vacuolar membrane invagination or protrusion during microautophagy. Then they are rapidly degraded by vacuolar lytic enzymes, and products of the degradation are reused. In this paper, we summarize the available information on the trafficking of the autophagosome towards the vacuole, the fusion of the autophagosome with the vacuole, the formation and decomposition of autophagic bodies inside the vacuole, and the efflux of metabolites to the cytoplasm. Special attention is given to the formation and degradation of autophagic bodies and metabolite salvage in plant cells.

Keywords: Atg proteins; SNARE proteins; autophagosome; tonoplast; vacuole.

Figure 1

Schematic diagram of macroautophagy in cells of yeast and plants (upper part of the drawing) and in cells of animals (bottom part of the drawing). In yeast and plant cells, the autophagosome fuses to the tonoplast, creating the autophagic body inside the vacuole. In animal cells, the autophagosome fuses with the lysosome, giving the autolysosome. The autophagic body inside the vacuole and the content of autolysosome are rapidly degraded, allowing reuse of metabolites.

Figure 2

Schematic diagram depicting trafficking and fusion of the autophagosome to the vacuole and degradation of the autophagic body inside the vacuole in yeast. PI3P and Atg8 anchored in the outer membrane of the autophagosome are involved in autophagosome trafficking and bind autophagosome with FYVE and coiled-coil domain-containing protein 1 (FYCO1). The complex autophagosome-FYCO1-Ypt7 moves along microtubules in the direction of the plus end by the binding of Ypt7 to kinesin motor proteins. Proteins Vam3, Vam7, Vti1, Ykt6, Ypt7, and complexes Ccz1-Mon1 and HOPS are involved in the fusion of the autophagosome and vacuole. The newly formed autophagic body inside the vacuole is rapidly degraded by lytic enzymes. Proteins involved in the degradation of the autophagic body are proteinase A (Pep4), proteinase B (Prb1), and putative lipase Atg15. Other proteins that are probably involved in the degradation of the autophagic body are Atg22, Atg42, Prc, and Ybr139. It is suggested that Atg22, Avt3, and Avt4 are involved in metabolite efflux from the vacuole to the cytoplasm. Question marks indicate the hypothetical involvement of proteins in the degradation of the autophagic body and metabolite efflux from the vacuole to the cytoplasm.

Figure 3

Schematic diagram depicting trafficking and fusion of the autophagosome to the vacuole and the degradation of the autophagic body inside the vacuole in plants. PI3P and ATG8 anchored in the outer membrane of the autophagosome are involved in autophagosome trafficking and bind autophagosome with FYCO1 protein. The complex autophagosome-FYCO1-RAB7 moves along microtubules in the direction of the plus end by the binding of RAB7 to kinesin motor proteins. Protein VTI12 is involved in the fusion of the autophagosome and vacuole. RABG3B is located on the surface of the autophagosome but the involvement of this protein in the fusion of the autophagosome and vacuole in plants remains unclear. It is suggested that the homologous yeast proteins Ykt6, Vam3, Ypt7, and complex HOPS are involved in the fusion of autophagosome and vacuole in plants. Additionally, it is suggested that plant proteins CHMP1, FREE1, VPS2.1, CFS1, and the complex EXO70B1 are involved in the autophagosome trafficking, autophagosome-vacuole fusion, and the release of the autophagic body into the vacuole lumen. The newly formed autophagic body inside the vacuole is rapidly degraded by lytic enzymes. One of them can be the vacuolar processing enzyme γ (VPEγ). Proteins involved in metabolite efflux from the vacuole to the cytoplasm during autophagy in plants have not been described so far. Only permease AVT3 was confirmed in Arabidopsis thaliana, but the involvement of this permease in the transport of metabolites coming from the degradation of the autophagic body is not confirmed. Question marks indicate the hypothetical involvement of plant proteins and complexes, or plant homologs of yeast proteins, during autophagy.

Figure 4

Schematic diagram of microautophagy in yeast and plants. Microautophagy may occur by vacuolar membrane invagination, which increases, creating the autophagic tube in yeast, and the cargo is enclosed inside the vacuole, forming a vesicle surrounded by a single two-layer membrane—an autophagic body. The occurrence of the autophagic tube in plants has not been confirmed. The autophagic body may also be formed by an arm-shaped protrusion of the tonoplast capturing a cargo into the vacuole.