**Mechanism of Cargo Recognition During Selective Autophagy**

Yasunori Watanabe1,2 and Nobuo N. Noda2 *1Graduate School of Life Science, Hokkaido University 2Institute of Microbial Chemistry, Tokyo Japan* 

## **1. Introduction**

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> Autophagy is an intracellular bulk degradation system conserved among eukaryotes from yeast to mammals. It is responsible for the degradation of cytosolic components and organelles in response to nutrient deprivation. There are three main types of autophagy: macroautophagy, microautophagy and chaperone-mediated autophagy (CMA). Microautophagy sequesters cytoplasmic components and delivers them for degradation by direct invagination or protrusion/septation of the lysosomal or vacuolar membrane (Mijaljica et al., 2011; Uttenweiler and Mayer, 2008). CMA targets specific cytosolic proteins that are trapped by the heat shock cognate protein of 70 kDa (hsc70) and, through interaction with lysosome-associated membrane protein type 2A (LAMP-2A), they are then translocated into the lysosomal lumen for rapid degradation (Orenstein and Cuervo, 2010). Macroautophagy, hereafter referred to as autophagy, is the most well characterized process of the three. During autophagy, double membrane structures called autophagosomes sequester a portion of the cytoplasm and fuse with the lysosome (or vacuole in the case of yeast and plants) to deliver their inner contents into the organelle lumen (Mizushima, 2007; Mizushima et al., 2010). Analyses of autophagy-related (Atg) proteins have unveiled dynamic and diverse aspects of mechanisms that underlie membrane formation during autophagy (Mizushima et al., 2010; Nakatogawa et al., 2009). As the contents of autophagosomes are indistinguishable from their surrounding cytoplasm (Baba et al., 1994), autophagy has long been considered a nonselective catabolic pathway. Recent studies, however, have provided evidence for the selective degradation of various targets by autophagy. In autophagy-deficient neuronal cells, intracellular protein aggregates accumulate and eventually lead to neurodegeneration, suggesting that autophagy selectively degrades harmful protein aggregates (Hara et al., 2006; Komatsu et al., 2006). Damaged or superfluous organelles, such as mitochondria and peroxisomes, and even intracellular infectious pathogens are also selectively degraded by autophagy (Goldman et al., 2010; Gutierrez et al., 2004; Manjithaya et al., 2010; Nakagawa et al., 2004; Noda and Yoshimori, 2009). In the budding yeast *Saccharomyces cerevisiae*, αmannosidase and aminopeptidase I are selectively transported to the vacuole through autophagic pathways (Baba et al., 1997; Hutchins and Klionsky, 2001).

> Although the precise molecular mechanisms of cargo selection by autophagy are yet to be established, an increasing number of autophagic receptors that are responsible for recognition of specific cargoes have been identified. These include Atg19 and Atg34 in the

selective transport of vacuolar enzymes to the vacuole through autophagy (Leber et al., 2001; Scott et al., 2001; Suzuki et al., 2010), p62 and neighbor of BRCA1 gene 1 (NBR1) in the autophagic degradation of ubiquitinated protein aggregates (Bjorkoy et al., 2005; Kirkin et al., 2009), PpAtg30 in pexophagy (autophagic degradation of peroxisome) (Farre et al., 2008), and Atg32 and Nix1 in mitophagy (autophagic degradation of mitochondria) (Kanki et al., 2009; Novak et al., 2010; Okamoto et al., 2009). Most of these receptors interact directly with Atg8-family proteins, which are crucial factors in autophagosome biogenesis.

We have been studying the mechanisms of specific cargo recognition during autophagy, especially those of the selective delivery of vacuolar enzymes into the vacuole in yeast. We summarize here the current knowledge of such mechanisms as revealed by biochemical and structural studies.
