Selective autophagy of non-ubiquitylated targets in plants: looking for cognate receptor/adaptor proteins

Abstract

Cellular homeostasis is essential for the physiology of eukaryotic cells. Eukaryotic cells, including plant cells, utilize two main pathways to adjust the level of cytoplasmic components, namely the proteasomal and the lysosomal/vacuolar pathways. Macroautophagy is a lysosomal/vacuolar pathway which, until recently, was thought to be non-specific and a bulk degradation process. However, selective autophagy which can be activated in the cell under various physiological conditions, involves the specific degradation of defined macromolecules or organelles by a conserved molecular mechanism. For this process to be efficient, the mechanisms underlying the recognition and selection of the cargo to be engulfed by the double membrane autophagosome are critical, and not yet well understood. Ubiquitin (poly-ubiquitin) conjugation to the target appears to be a conserved ligand mechanism in many types of selective autophagy, and defined receptors/adaptors recognizing and regulating the autophagosomal capture of the ubiquitylated target have been characterized. However, non-proteinaceous and non-ubiquitylated cargoes are also selectively degraded by this pathway. This ubiquitin-independent selective autophagic pathway also involves receptor and/or adaptor proteins linking the cargo to the autophagic machinery. Some of these receptor/adaptor proteins including accessory autophagy-related (Atg) and non-Atg proteins have been described in yeast and animal cells but not yet in plants. In this review we discuss the ubiquitin-independent cargo selection mechanisms in selective autophagy degradation of organelles and macromolecules and speculate on potential plant receptor/adaptor proteins.

The Arabidopsis Abiotic Stress-Induced TSPO-Related Protein Reduces Cell-Surface Expression of the Aquaporin PIP2;7 through Protein-Protein Interactions and Autophagic Degradation

“…It would be interesting to investigate whether the TSPO/PIP2;7 interaction is affected by heme binding to TSPO. TSPO is actively degraded in the plant cell by a selective autophagic pathway (Vanhee et al, 2011b;Veljanovski and Batoko, 2014). We confirm here that TSPO physically interacts with ATG8, but it is not yet clear whether the interacting ATG8 molecules are membrane-bound or soluble.…”

“…We confirm here that TSPO physically interacts with ATG8, but it is not yet clear whether the interacting ATG8 molecules are membrane-bound or soluble. It is possible that TSPO acts as a selective autophagy receptor (Veljanovski and Batoko, 2014) and could initiate the recruitment of the autophagosome formation machinery through ATG8 binding or by interacting with phagophore-bound ATG8. Where and how the TSPO/PIP2;7 complex is selectively engulfed in autophagosomes remains to be determined.…”

Mechanisms and roles of mitophagy in neurodegenerative diseases

“…Typically, selective macroautophagy can be classified into mitophagy, pexophagy, reticulophagy, ribophagy, etc. In higher eukaryotes, selective autophagy also includes chaperone‐mediated autophagy (CMA), as well as two similar processes, namely endosomal microautophagy (e‐MI) and chaperone‐assisted selective autophagy (CASA) . Nonetheless, how a substrate is targeted for sequestration and segregated from other cellular parts remains one of the major issues in this research field.…”

Origin of the Autophagosomal Membrane in Plants

“…A role for selective autophagy in plants has been established in recent years, especially in the degradation of the ER, mitochondria, chloroplasts, peroxisomes, and exocyst-positive organelle as well as TSPO-binding proteins for cellular homeostasis (Wang et al, 2010; Floyd et al, 2012; Li and Vierstra, 2012; Michaeli and Galili, 2014; Veljanovski and Batoko, 2014; Lin et al, 2015; Xie et al, 2015). However, the mechanism of autophagosome initiation for selective autophagy has not been well characterized.…”