TEX264 is a major receptor for mammalian reticulophagy

Abstract

The endoplasmic reticulum (ER) is the main site of cellular protein and calcium homeostasis, as well as lipid synthesis in eukaryotic cells. Reticulophagy is the selective clearance and degradation of ER components and membranes by the cellular autophagy machinery. Recently, 2 groups (the laboratories of Noboru Mizushima and Wade Harper) independently identified the previously uncharacterized protein TEX264 (testis expressed gene 264) as a major receptor for selective reticulophagy in mammalian cells. Here we highlight and integrate the major findings of their recent work. Abbreviations: AIM: Atg8-interacting motif; AP-MS: affinity purification-mass spectrometry; ATL3: atlastin GTPase 3; Baf A₁: bafilomycin A₁; CCPG1: cell cycle progression 1; CRISPR: clustered regularly interspaced short palindromic repeats; GABARAP: gamma-aminobutyric acid receptor associated protein; GFP: green fluorescent protein; GyrI: gyrase inhibitor; IDR: intrinsically disordered region; IP: immunoprecipitation; KO: knockout; LIR: LC3-interacting region; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; MEF: mouse embryonic fibroblast; MS: mass spectrometry; MTOR: mechanistic target of rapamycin kinase; RB1CC1/FIP200: RB1-inducible coiled-coil 1; RFP: red fluorescent protein; RNAi: RNA interference; RTN3: reticulon 3; RTN3L: long isoform of RTN3; siRNA: small interfering RNA; SARS: selective autophagy receptors; ss: signal sequence; TEM: transmission electron microscopy, TEX264: testis expressed gene 264; TMT: tandem mass tagging.

Keywords: Autophagy; lysosome; macroautophagy; selective autophagy; stress.

Figure 1.

A model of the reticulophagy receptor TEX264 (teal) attached to the ER membrane via the transmembrane helix (residues 5–27). The gyrase inhibitor (GyrI)-like domain (residues 41–185) of TEX264 is followed by the IDR (residues 186–313) that binds via the LIR motif (F₂₇₃EEL) to LC3 (gray), which is conjugated to PE at the inner phagophore membrane. The ribosome size of approximately 20 nm creates the spatial gap between the ER and inner phagophore membrane. The IDR of the small molecule TEX264 is essential for bridging this gap and for the function of TEX264 as a reticulophagy receptor. The length of the IDR linker (residues 206–261), rather than the specific amino acid sequence, is a critical factor, as shown by the finding that this segment of the TEX264 IDR is functionally replaceable by the ATG13 IDR (residues 191–248). To visualize the LIR-LC3 complex, the crystal structure of LC3 bound to the LIR of SQSTM1 was used (PDB ID: 2ZJD). The transmembrane helix and GyrI-like domain of TEX264 have been modeled using the Phyre2 server with 72% and 99.7% confidence by the single highest-scoring template c3a0hJ and d1jyha, respectively.