TÄMÄ ARTIKKELI ON OIKEIN HYVÄ. Pitää kerrata usein. Kertoo S26proteosomin synteesistä ja hajoamisesta.5.11. 2019).
As will be described below, proteasomes are also rapidly cleared by autophagy using at least two proteaphagic routes (Marshall and Vierstra, 2015; Marshall et al., 2015, 2016; Cohen-Kaplan et al., 2016; Waite et al., 2016; Nemec et al., 2017)."
Dynamic Regulation of the 26S Proteasome: From Synthesis to Degradation
Richard S. Marshall
Richard D. Vierstra- Department of Biology, Washington University in St. Louis, St. Louis, MO, United States
Most of this turnover is catalyzed by the 26S proteasome, an intricate, multi-subunit proteolytic machine.
Proteasomes recognize and degrade proteins first marked with one or more chains of poly-ubiquitin, the addition of which is actuated by hundreds of ligases that individually identify appropriate substrates for ubiquitylation. Subsequent proteasomal digestion is essential and influences a myriad of cellular processes in species as diverse as plants, fungi and humans. Importantly, dysfunction of 26S proteasomes is associated with numerous human pathologies and profoundly impacts crop performance, thus making an understanding of proteasome dynamics critically relevant to almost all facets of human health and nutrition. Given this widespread significance, it is not surprising that sophisticated mechanisms have evolved to tightly regulate 26S proteasome assembly, abundance and activity in response to demand, organismal development and stress.
These include
- controls on transcription and chaperone-mediated assembly,
- influences on proteasome localization and activity by an assortment of binding proteins and post-translational modifications, and
- ultimately the removal of excess or damaged particles via autophagy.
This turnover is also influenced by two distinct biomolecular condensates that coalesce in the cytoplasm,
- one attracting damaged proteasomes for autophagy, and
- the other reversibly storing proteasomes during carbon starvation to protect them from autophagic clearance.
...
Autophagy-Mediated Control of 26S Proteasome Abundance
While the synthesis and assembly of proteasomes has been
studied for over a decade, their turnover had remained obscure until
recently. Proteasomes are stable complexes (Pack et al., 2014), with a half-life of 16 h in mouse embryonic fibroblasts (Tomita et al., 2019) and over 2 weeks when measured in rat liver cells (Tanaka and Ichihara, 1989),
but under specific conditions their degradation can be rapid and
extensive. One turnover mechanism involves caspase-mediated cleavage.
Following induction of apoptosis in human cells, the RP subunits RPT5,
RPN2 and RPN10 are cleaved by caspase-3, resulting in impaired
proteasome activity and the accumulation of ubiquitylated substrates (Sun et al., 2004). Similarly, caspase-3 activation in D. melanogaster cells leads to cleavage of the α2, α4 and β4 subunits of the CP, and the RPT1 subunit of the RP (Adrain et al., 2004).
Presumably these impaired proteasomes are then removed, possibly by
autophagy (see below). A second pathway is the removal of non-functional
proteasome subunits by the UPS itself prior to their integration into
the holo-proteasome. Hsp42 was shown to be important in yeast by
coalescing these polypeptides into cytoplasmic condensates from which
they are cleared by active 26S proteasomes (Peters et al., 2015; Nahar et al., 2019).
A third pathway for degrading 26S proteasomes that has
recently gained in appreciation is autophagy, via a route termed
proteaphagy (Figure 5; Marshall and Vierstra, 2015; Marshall et al., 2015, 2016).
Autophagy involves the delivery of cytoplasmic material to the vacuole
(in plants and yeast) or lysosome (in mammals) for breakdown by resident
hydrolases (Reggiori and Klionsky, 2013; Gatica et al., 2018; Marshall and Vierstra, 2018a; Levine and Kroemer, 2019).
It is the preferred catabolic route for large, heterogeneous
cytoplasmic material, such as protein aggregates, organelles, lipid
droplets, or even invading pathogens whose sizes exceed the spatial
capacity of proteasomes. The defining feature of the most common
autophagic route, macroautophagy (referred to here as autophagy), is the
de novo formation of a cup-shaped membrane called the phagophore
(or isolation membrane) that encircles portions of cytoplasm. The
phagophore ultimately seals to generate a double membrane-bound
autophagosome, the outer membrane of which then fuses with the vacuole
or lysosome to release the internal vesicle as an autophagic body (see Figure 6).
The contents of the autophagic body and its limiting membrane are
rapidly consumed by a collection of vacuolar hydrolases with acidic pH
optima (Parzych and Klionsky, 2018),
with the constituent amino acids, fatty acids, carbohydrates and
nucleotides ultimately re-used for survival or to power new growth.
... Autophagic degradation of proteasomes in response to amino acid starvation has also been reported in mammals (Figure 5B; Cohen-Kaplan et al., 2016). Surprisingly, and in contrast to the situation in plants and yeast (Marshall et al., 2015, 2016), starvation-induced proteaphagy in HeLa cells is accompanied by increased subunit ubiquitylation on RPN1, RPN10 and RPN13 (Cohen-Kaplan et al., 2016). The attached poly-ubiquitin chains appear essential for proteaphagy, as siRNA-mediated silencing of the E1 or over-expression of a ubiquitin variant lacking the internal lysine residues necessary for chain concatenation reduced rates of proteasome degradation. This turnover requires the autophagy receptor p62/SQSTM1, which recognizes ubiquitylated cargo via its UBA domain and ATG8/LC3 via a canonical AIM (Noda et al., 2008, 2010; Cohen-Kaplan et al., 2016). It thus appears that, at least in the HeLa cell system, starvation induces significant ubiquitylation of proteasomes to promote recognition by the autophagy machinery (Figure 5B; Cohen-Kaplan et al.,
2016).
In addition to starvation-induced proteaphagy, a second pathway has been described in plants and yeast that enables clearance of non-functional 26S proteasomes (Marshall et al., 2015, 2016; Nemec et al., 2017). This proteaphagic route occurs independently of the Atg1 kinase..
..an elegant example of convergent evolution, where different interacting motifs are exploited to generate the same outcome, namely tethering of ubiquitylated proteasomes to autophagic membranes..
.. Cue5 and its human counterpart TOLLIP have been implicated in the autophagic clearance of various aggregation-prone proteins (Lu et al., 2014) and, intriguingly, inhibitor-induced proteaphagy in yeast is likewise preceded by aggregation of 26S proteasomes into peri-vacuolar insoluble protein deposit (IPOD)-type structures (Kaganovich et al., 2008; Marshall et al., 2016), suggesting some degree of overlap between the proteaphagy and aggrephagy machineries..
.. The IPODs seen upon proteasome inhibition are distinct from PSGs (Marshall and Vierstra, 2018b), although there might be some overlap between the two types of puncta during early stages of carbon starvation (Peters et al., 2016). The two condensates can be easily distinguished based on their co-localization with either Blm10 (in PSGs) or the aggregation-prone prion protein Rnq1 (in IPODs; Figure 4B)..
..IPOD formation is dependent on the oligomeric chaperone Hsp42, which helps coalesce aggregated proteins (Specht et al., 2011; Malinovska et al., 2012; Miller et al., 2015). The accumulation of yeast 26S proteasomes into IPODs upon inhibition, and their subsequent autophagic breakdown, were also found to require this aggregase (Marshall et al., 2016). Where inactive 26S proteasomes become ubiquitylated is currently unclear; one possibility is that dysfunctional proteasomes are first ubiquitylated and then delivered to IPODs with the help of Hsp42, while the other is that Hsp42 first delivers dysfunctional proteasomes into IPODs, which are then ubiquitylated through one or more IPOD-resident E3s..
.. observations imply that proteaphagy can be initiated for both the whole 26S particle, and for the individual CP and RP sub-complexes separately..
Clearly, an important feature of inhibitor-induced proteaphagy is its ability to discriminate between functional and dysfunctional particles. One possibility is that stalled or compromised 26S proteasomes acquire a distinct conformation that is recognized by Hsp42 and/or the ubiquitylation machinery, which directs their accumulation into IPODs. An intriguing factor in this scenario was Ecm29, as it binds specifically to mutant forms of 26S proteasomes, and thus could detect inappropriate conformations induced by inactivation (Lehmann et al., 2010; Lee S. Y. et al., 2011; Panasenko and Collart, 2011; Park et al., 2011). However, analysis of yeast Δecm29 mutants suggested this it is not involved in proteaphagy (Marshall and Vierstra, 2018b). Several E3s have been detected in association with 26S proteasomes that could instead provide this quality control (Xie and Varshavsky, 2000; Crosas et al., 2006; Panasenko and Collart, 2011), some of which ubiquitylate specific subunits (Besche et al., 2014; Fu et al., 2018), but their function(s) in relation to proteaphagy, if any, remain to be determined. Further work is certainly required to fully unravel the mysteries surrounding this last chapter in the life of a proteasome.
2019
... Autophagic degradation of proteasomes in response to amino acid starvation has also been reported in mammals (Figure 5B; Cohen-Kaplan et al., 2016). Surprisingly, and in contrast to the situation in plants and yeast (Marshall et al., 2015, 2016), starvation-induced proteaphagy in HeLa cells is accompanied by increased subunit ubiquitylation on RPN1, RPN10 and RPN13 (Cohen-Kaplan et al., 2016). The attached poly-ubiquitin chains appear essential for proteaphagy, as siRNA-mediated silencing of the E1 or over-expression of a ubiquitin variant lacking the internal lysine residues necessary for chain concatenation reduced rates of proteasome degradation. This turnover requires the autophagy receptor p62/SQSTM1, which recognizes ubiquitylated cargo via its UBA domain and ATG8/LC3 via a canonical AIM (Noda et al., 2008, 2010; Cohen-Kaplan et al., 2016). It thus appears that, at least in the HeLa cell system, starvation induces significant ubiquitylation of proteasomes to promote recognition by the autophagy machinery (Figure 5B; Cohen-Kaplan et al.,
2016).
In addition to starvation-induced proteaphagy, a second pathway has been described in plants and yeast that enables clearance of non-functional 26S proteasomes (Marshall et al., 2015, 2016; Nemec et al., 2017). This proteaphagic route occurs independently of the Atg1 kinase..
..an elegant example of convergent evolution, where different interacting motifs are exploited to generate the same outcome, namely tethering of ubiquitylated proteasomes to autophagic membranes..
.. Cue5 and its human counterpart TOLLIP have been implicated in the autophagic clearance of various aggregation-prone proteins (Lu et al., 2014) and, intriguingly, inhibitor-induced proteaphagy in yeast is likewise preceded by aggregation of 26S proteasomes into peri-vacuolar insoluble protein deposit (IPOD)-type structures (Kaganovich et al., 2008; Marshall et al., 2016), suggesting some degree of overlap between the proteaphagy and aggrephagy machineries..
.. The IPODs seen upon proteasome inhibition are distinct from PSGs (Marshall and Vierstra, 2018b), although there might be some overlap between the two types of puncta during early stages of carbon starvation (Peters et al., 2016). The two condensates can be easily distinguished based on their co-localization with either Blm10 (in PSGs) or the aggregation-prone prion protein Rnq1 (in IPODs; Figure 4B)..
..IPOD formation is dependent on the oligomeric chaperone Hsp42, which helps coalesce aggregated proteins (Specht et al., 2011; Malinovska et al., 2012; Miller et al., 2015). The accumulation of yeast 26S proteasomes into IPODs upon inhibition, and their subsequent autophagic breakdown, were also found to require this aggregase (Marshall et al., 2016). Where inactive 26S proteasomes become ubiquitylated is currently unclear; one possibility is that dysfunctional proteasomes are first ubiquitylated and then delivered to IPODs with the help of Hsp42, while the other is that Hsp42 first delivers dysfunctional proteasomes into IPODs, which are then ubiquitylated through one or more IPOD-resident E3s..
.. observations imply that proteaphagy can be initiated for both the whole 26S particle, and for the individual CP and RP sub-complexes separately..
Clearly, an important feature of inhibitor-induced proteaphagy is its ability to discriminate between functional and dysfunctional particles. One possibility is that stalled or compromised 26S proteasomes acquire a distinct conformation that is recognized by Hsp42 and/or the ubiquitylation machinery, which directs their accumulation into IPODs. An intriguing factor in this scenario was Ecm29, as it binds specifically to mutant forms of 26S proteasomes, and thus could detect inappropriate conformations induced by inactivation (Lehmann et al., 2010; Lee S. Y. et al., 2011; Panasenko and Collart, 2011; Park et al., 2011). However, analysis of yeast Δecm29 mutants suggested this it is not involved in proteaphagy (Marshall and Vierstra, 2018b). Several E3s have been detected in association with 26S proteasomes that could instead provide this quality control (Xie and Varshavsky, 2000; Crosas et al., 2006; Panasenko and Collart, 2011), some of which ubiquitylate specific subunits (Besche et al., 2014; Fu et al., 2018), but their function(s) in relation to proteaphagy, if any, remain to be determined. Further work is certainly required to fully unravel the mysteries surrounding this last chapter in the life of a proteasome.
2019
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