The nucleus is a quality control center for non-imported mitochondrial proteins

Abstract

Mitochondrial import deficiency causes cellular stress due to the accumulation of non-imported mitochondrial precursor proteins. Despite the burden mis-localized mitochondrial precursors place on cells, our understanding of the systems that dispose of these proteins is incomplete. Here, we catalog the location and steady-state abundance of mitochondrial precursor proteins during mitochondrial impairment in S. cerevisiae. We find that a number of non-imported mitochondrial proteins localize to the nucleus, where they are eliminated by proteasome-based nuclear protein quality control. Recognition of mitochondrial precursors by the nuclear quality control machinery requires the presence of an N-terminal mitochondrial targeting sequence (MTS), and impaired breakdown of precursors leads to their buildup in nuclear-associated foci. These results identify the nucleus as a key destination for the disposal of non-imported mitochondrial precursors.

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

“…1C, S1C). Consistent with previous reports, our data indicate that under normal conditions Oxa1 precursors associate with the ER surface only very transiently (Hansen et al, 2018;Shakya et al, 2020;, and that Ema19 has a role in regulating the amount or extent of this interaction.…”

“…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.…”

“…Precursor proteins that accumulate in the cytosol can be highly toxic (Wang and Chen, 2015;Wrobel et al, 2015) and elicit response programs to counteract these deleterious consequences (Nargund et al, 2012;Weidberg and Amon, 2018;Boos et al, 2019;Song et al, 2020;Zöller et al, 2020). 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).…”

Increased levels of the mitochondrial import factor Mia40 prevent the aggregation of polyQ proteins in the cytosol

“…Thereby, mitochondrial protein biogenesis directly depends on the cytosolic chaperone capacity. Presumably as a consequence of their strong tendency to sequester chaperones, the cytosolic accumulation of precursor proteins induces a sudden growth arrest and triggers the increased expression of components of the chaperone and proteasome system (Boos et al, 2019, Boos, Labbadia et al, 2020, Mårtensson, Priesnitz et al, 2019, Shakya, Barbeau et al, 2020, Wang & Chen, 2015, Weidberg & Amon, 2018, Wrobel et al, 2015). Obviously, the post-translational import mode of mitochondrial proteins poses a threat for cellular proteostasis which is met by an adaptive network of cytosolic factors that can deal with unfolded precursors.…”

The mitochondrial surface receptor Tom70 protects the cytosol against mitoprotein-induced stress

“…Mitochondrial protein biogenesis strictly depends on the cytosolic chaperone capacity (Becker et al, 1996;Ben-Menachem et al, 2018;Deshaies et al, 1988;Doring et al, 2017;Hoseini et al, 2016;Stein et al, 2019;Terada et al, 1996). Presumably as a consequence of their strong tendency to sequester chaperones, precursor proteins accumulating in the cytosol induce a sudden growth arrest, trigger the heat shock response to increase components of the chaperone and proteasome system, and activate specific factors on the mitochondrial surface that clean off translocation intermediates (Boos et al, 2019;Boos et al, 2020;Martensson et al, 2019;Shakya et al, 2020;Wang and Chen, 2015;Weidberg and Amon, 2018;Wrobel et al, 2015).…”