The GET pathway safeguards against non-imported mitochondrial protein stress

Abstract

SUMMARYDeficiencies in mitochondrial import cause the toxic accumulation of non-imported mitochondrial precursor proteins. Numerous fates for non-imported mitochondrial precursors have been identified, including proteasomal destruction, deposition into protein aggregates, and mis-targeting to other organelles. Amongst organelles, the endoplasmic reticulum (ER) has emerged as a key destination for non-imported mitochondrial proteins, but how ER-targeting of these proteins is achieved remains unclear. Here, we show that the guided entry of tail-anchored proteins (GET) complex is required for ER-targeting of endogenous mitochondrial multi-transmembrane proteins. Without a functional GET pathway, non-imported mitochondrial proteins destined for the ER are alternatively sequestered into Hsp42-dependent protein foci. The ER targeting of non-imported mitochondrial proteins by the GET complex prevents cellular toxicity and facilitates re-import of mitochondrial proteins from the ER via the recently identified ER-SURF pathway. Overall, this study outlines an important and unconventional role for the GET complex in mitigating stress associated with non-imported mitochondrial proteins.

The ER protein Ema19 facilitates the degradation of nonimported mitochondrial precursor proteins

“…A number of recent studies reported alternative approaches to follow the import reaction which resulted in surprising observations: (1) Ribosome profiling revealed that cytosolic chaperones and the signal recognition particle play crucial roles in distinguishing mitochondrial and secretory proteins already at very early steps in their synthesis (Schibich et al, 2016;Doring et al, 2017;Costa et al, 2018); (2) proximity labeling suggested that some mitochondrial proteins, in particular hydrophobic inner membrane proteins, explore the mitochondrial surface already during their synthesis (Jan et al, 2014;Williams et al, 2014; Vardi-Oknin and Arava, 2019; Wang et al, 2019) and that, in vivo, many (if not most) mitochondrial surface proteins are in direct proximity to the ER (Hung et al, 2017;Cho et al, 2020); (3) systematic screens of GFP-tagged protein libraries showed that many mitochondrial proteins are prone to accumulate in non-mitochondrial locations under certain growth conditions, in particular on the ER and within the nucleus (Vitali et al, 2018;Backes et al, 2020;Saladi et al, 2020;Shakya et al, 2020;Xiao et al, 2020) and, maybe even more surprising, observed non-mitochondrial residents in mitochondria (Ruan et al, 2017;Bader et al, 2020); and (4) genetic screens reported a very close cooperation of the mitochondrial and ER surface in protein biogenesis (Kornmann et al, 2009;Papic et al, 2013;Okreglak and Walter, 2014;Gamerdinger et al, 2015;Wohlever et al, 2017;Hansen et al, 2018;Vitali et al, 2018;Dederer et al, 2019;Matsumoto et al, 2019). Thus, in vivo, the surfaces of the ER and of mitochondria apparently vividly cooperate to sort proteins to the correct intracellular location.…”

“…The stability of many precursor proteins in the cytosol is low thanks to surveillance of the ubiquitin/proteasome system for uninserted precursors (Bragoszewski et al, 2017;Kowalski et al, 2018;Paasch et al, 2018;Saladi et al, 2020;Shakya et al, 2020). The association of mitochondrial precursors to the ER surface can retard their degradation and facilitate their productive targeting to the TOM complex by a process referred to as ER-SURF (Hansen et al, 2018;Xiao et al, 2020).…”

ER-SURF: Riding the Endoplasmic Reticulum Surface to Mitochondria

“…Get3, a cytosolic chaperone and targeting factor, was found to directly interact with some mitochondrial precursors, in addition to its ER-destined clients [50]. Moreover, if mitochondrial precursor proteins accumulate in the cytosol, they can be "rescued" by the GET pathway, which directs them onto the ER surface from where they finally reach the mitochondrial import machinery; this GET-mediated detour might be particularly relevant for carrier proteins and prevents their incorporation into non-productive protein aggregates [51].…”

“…When mitochondrial import sites are limiting so that precursor proteins accumulate outside of mitochondria, a large number of precursors of mitochondrial membrane proteins were found to associate with the ER surface [56] and to induce the unfolded protein response pathway of the ER [57]. Since these precursor proteins, in particular those of the carriers, have a highly toxic potential [24,58,59], ER binding might serve as a safeguard mechanism [51]. This is because the ER surface is coated by a number of cellular chaperones.…”

The intermembrane space protein Mix23 is a novel stress-induced mitochondrial import factor

“…The protein import into mitochondria is under surveillance of the proteasome. Arrested mitochondrial import intermediates induce severe stress, presumably because of the accumulation of non-imported mitochondrial precursor proteins in the cytosol (32)(33)(34)(35)(36)(37). Conditions that perturb mitochondrial import trigger a characteristic stress response that leads to an induction in expression of many chaperones and the constituents of the proteasome (Fig.…”