Interaction between optineurin and Rab1a regulates autophagosome formation in neuroblastoma cells

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Optineurin (OPTN) is an adaptor protein containing several functional domains including a microtubule‐associated protein light chain 3 (LC3)‐interacting region (LIR), a ubiquitin‐binding domain (UBD), an NF‐κB‐essential molecule‐like domain (NEMO‐like), and a carboxyl (C)‐terminal C2H2 type of zinc finger (Ying & Yue, 2016). OPTN is involved in several cellular functions including autophagy regulation, NF‐κB activation (Nagabhushana, Bansal, & Swarup, 2011; Zhu, Wu, Zhao, & Ashwell, 2007), and membrane trafficking (Au, Puri, Ihrke, Kendrick‐Jones, & Buss, 2007; Nagabhushana et al., 2010; Sahlender et al., 2005). Mutations of the OPTN gene are associated with normal‐tension glaucoma and amyotrophic lateral sclerosis (ALS) (Maruyama et al., 2010; Minegishi et al., 2013; Osawa et al., 2011; Weishaupt et al., 2013; Zhu, Li, Chen, Zhang, & Wu, 2016).
Autophagy is an intracellular catabolic process by which cytoplasmic material is delivered to the lysosomes for degradation, contributing to cellular quality control. In particular, autophagy plays an essential role in removing aggregates of misfolded proteins and inclusion bodies, which are a signature of neurodegenerative diseases such as ALS, Alzheimer disease, Parkinson disease, and Huntington disease (Ciechanover & Kwon, 2015; Kesidou, Lagoudaki, Touloumi, Poulatsidou, & Simeonidou, 2013). Therefore, altered autophagic activity may be implicated in the pathogenesis of several pathological conditions including neurodegenerative diseases, cancer, and pathogenic infection (Kesidou et al., 2013).
There are multiple connections between OPTN and autophagy. OPTN has been shown to promote, as an autophagy receptor, the clearance of ubiquitin‐coated cytosolic Salmonella enterica (Wild et al., 2011). The OPTN protein binds to ubiquitinated cargos by UBD domain and then binds to autophagosome‐associated protein LC3 via its LIR domain (Ying & Yue, 2016). The OPTN protein is a selective autophagy adaptor, and it is involved in xenophagy (Wild et al., 2011), mitophagy (Richter et al., 2016), aggrephagy, and tumor suppression (Ying & Yue, 2016). In addition, OPTN can induce autophagy upon overexpression or mutation. The E50K OPTN mutation induces autophagosome formation, while the E478G OPTN mutation, found in ALS patients, is autophagy resistant (Shen, Li, Chen, Chern, & Tu, 2015). The molecular mechanisms of OPTN‐mediated autophagy remain largely unrevealed.
The Ras‐related proteins in brain (Rab) GTPases are a family of proteins playing a crucial role in the regulation of vesicle trafficking (Lopez de Armentia, Amaya, & Colombo, 2016). Among the Rab proteins involved in various stages of autophagy, Rab1 and Rab5 have been shown to participate in autophagosome formation, while Rab7 and Rab8B play a key role in autophagosome maturation (Jain & Ganesh, 2016). Yeast Ypt1 and its human functional homolog Rab1 are required for vesicle trafficking from the endoplasmic reticulum to the Golgi apparatus (Stenmark, 2009; Wang et al., 2015; Zerial & McBride, 2001). Recent data showed that the activity of Rab1 protein is required at the initial stage of autophagosome formation (Zoppino, Militello, Slavin, Alvarez, & Colombo, 2010). However, the identity and the role of all the molecular entities, including Rab1, involved in OPTN‐mediated autophagy are unknown.
In this study, we identified the Rab1a protein as a novel binding partner for OPTN. We demonstrated that the interaction between OPTN and Rab1a depends on the GTP‐binding ability of Rab1a. Finally, we showed that functional interaction between the two proteins occurs during autophagosome formation in neuroblastoma cells.
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