Damaged mitochondria are flagged by ubiquitin (Ub) chains that are conjugated to proteins at the mitochondrial outer membrane (MOM). This process is regulated by the PINK1-mediated phosphorylation of Parkin (PRKN), an E3 ubiquitin protein ligase that, when activated, ubiquitinates a broad range of MOM proteins (Gladkova C et al., 2018; Sauve V et al., 2022). The conjugated Ub chains are recognized by cargo receptors such as sequestosome 1 (p62 or SQSTM1), nuclear dot protein 52 (NDP52) and optineurin (OPTN, also called FIP2 and NRP), which tether damaged mitochondria to components of the autophagy machinery promoting selective autophagy of mitochondria (mitophagy) (Onishi M et al., 2021; Goodall EA et al., 2022; Adriaenssens E et al., 2022).
OPTN is an Ub-binding adaptor protein, which is recruited to damaged mitochondria in a PINK1:PRKN-dependent manner (Wong YC & Holzbaur EL 2014; Heo JM et al., 2015; Lazarou M et al., 2015; reviewed in Qiu Y et al., 2022). Structural and biochemical studies demonstrate that OPTN binds polyUb chains via its C-terminal Ub binding domain (UBD) with a preference for M1- and K63-linked polyUb chains (Gleason CE et al., 2011; Nakazawa S et al., 2016; Li F et al., 2018). A surface plasmon resonance (SPR) and ITC binding assays show that OPTN and M1-linked polyUb form a complex with an equilibrium dissociation constant (Kd) value in the micromolar range (Nakazawa S et al., 2016; Li F et al., 2018).
OPTN interacts with TANK-binding kinase 1 (TBK1) (Morton S et al., 2008; Mankouri J et al., 2010; Gleason CE et al., 2011; Heo JM et al., 2015; Li F et al., 2016; Yamano K et al., 2024). Specifically, the N-terminal coiled coil domain of OPTN interacts with the C-terminal domain of TBK1 in human embryonic kidney 293T (HEK293T) cells (Morton S et al., 2008; Meena NP et al., 2016; Li F et al., 2016) with the micromolar affinity (Li F et al., 2016). Both constitutive (Mankouri J et al., 2010; Gleason CE et al., 2011; Richter B et al., 2016) and stimuli-induced (Pourcelot M et al., 2016; Meena NP et al., 2016) interactions between OPTN and TBK1 are observed in mouse and human cells. TBK1 has been shown to phosphorylate OPTN at S177, S473 or S513 enhancing the polyUb-binding function of OPTN (Richter B et al., 2016; Li F et al., 2018). TBK1-mediated phosphorylation of OPTN at S473 has been shown to broaden the binding specificity of OPTN to other polyUb chains including K48-linked (Li F et al., 2018). OPTN and TBK1 function in selective autophagy are critical for maintaining cellular homeostasis, and dysregulation of OPTN:TBK1-mediated autophagy has been implicated in disease pathogenesis (Wong YC & Holzbaur EL 2014; Harding O et al., 2021).
Interactions between OPTN and other proteins such as autophagy-related protein 9A (ATG9A) (Yamano K et al., 2020), myosin VI (MYO6), microtubule-associated proteins 1A/1B light chain 3B (MAP1LC3B or LC3) are implicated in autophagy.
This Reactome event shows association between TBK1, OPTN, and polyUb moieties of MOM proteins.
