p62 itself is also a selective autophagy substrate ( Bjørkøy et al., 2005 Pankiv et al., 2007). p62 contains a self-polymerization PB1 domain, a conserved LC3-interacting region (LIR), and a ubiquitin-associating (UBA) domain. Among them, p62/sequestosome 1 (SQSTM1) acts as a cargo receptor for accumulation and autophagic degradation of ubiquitinated protein aggregates ( Bjørkøy et al., 2005 Komatsu et al., 2007 Pankiv et al., 2007). A family of Atg8/LC3 (mammalian Atg8 homologue)-interacting proteins act as receptors that mediate delivery of specific cargoes to the autophagic machinery via Atg8/LC3 binding ( Noda et al., 2010 Johansen and Lamark, 2011). How Atg proteins act coordinately in autophagosome formation remains largely unknown.Īutophagy acts as a quality control system by selectively removing protein aggregates (a process known as aggrephagy) and damaged organelles. Atg9-positive vesicles dynamically and transiently interact with DFCP1-positive structures ( Orsi et al., 2012). Among these membrane sources, PtdIns(3)P-enriched subdomains of the ER, called omegasomes, function as platforms for recruiting Atg proteins and act as cradles for generating autophagosomes ( Axe et al., 2008). The endoplasmic reticulum (ER), Golgi apparatus, endosomes, plasma membrane, and Atg9-positive vesicles have been shown to contribute to autophagosomal membranes in mammalian cells ( Young et al., 2006 Axe et al., 2008 Ravikumar et al., 2010 Orsi et al., 2012). The autophagy process in higher eukaryotes involves more complex membrane dynamics, and requires the concerted action of highly conserved Atg proteins and also metazoan-specific autophagy proteins ( Longatti and Tooze, 2009 Yang and Klionsky, 2010 Tian et al., 2010). The multi-membrane spanning protein Atg9 concentrates in vesicles and tubules that traffic to the PAS and trigger the hierarchical recruitment of other Atg proteins as well as supplying the membrane for autophagosome formation ( Suzuki et al., 2007 He et al., 2008 Mari et al., 2010). In yeast, all Atg proteins are recruited in a hierarchical order to the preautophagosomal structure (PAS), where autophagosomes are generated ( Nakatogawa et al., 2009). These Atg protein complexes include the Atg1 serine–threonine kinase complex, the Vps34 class III PtdIns(3)P kinase complex, the Atg2–Atg18 complex for Atg9-recycling, and the two ubiquitin-like conjugation systems (Atg8–phosphatidylethanolamine conjugates and Atg5–Atg12 conjugates Nakatogawa et al., 2009). A group of Atg proteins has been identified in yeast, members of which form distinct complexes that act at different steps of autophagosome formation. Macroautophagy (hereafter referred to as autophagy) is a lysosome-mediated degradation process that involves the formation of a closed double-membrane autophagosome and its subsequent fusion with lysosomes for degradation ( Xie and Klionsky, 2007 Nakatogawa et al., 2009). Our results indicate that under physiological conditions a scaffold protein endows cargo specificity and also elevates degradation efficiency by linking the cargo–receptor complex with the autophagic machinery. Unlike core autophagy genes, epg-7 is dispensable for starvation-induced autophagic degradation of substrate aggregates. EPG-7 interacts with multiple ATG proteins and colocalizes with ATG-9 puncta in various autophagy mutants. Mutations in epg-7 impair association of SQST-1 aggregates with LGG-1/Atg8 puncta. SQST-1 directly interacts with EPG-7 and colocalizes with EPG-7 aggregates in autophagy mutants. EPG-7 self-oligomerizes and is degraded by autophagy independently of SQST-1.
Here we demonstrate that during Caenorhabditis elegans embryogenesis, epg-7 functions as a scaffold protein mediating autophagic degradation of several protein aggregates, including aggregates of the p62 homologue SQST-1, but has little effect on other autophagy-regulated processes. Previous studies show that a family of Atg8-interacting proteins function as receptors linking specific cargoes to the autophagic machinery. The mechanism by which protein aggregates are selectively degraded by autophagy is poorly understood.
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