BiP inactivation due to loss of the deAMPylation function of FICD causes a motor neuron disease
Adriana RebeloAriel RuizMaike F. DohrnMelanie WayandAmjad FarooqMatt C. DanziDanique BeijerBrooke AaronJana VandrovcováHenry HouldenLeslie MatalongaLisa AbreuGuy A. RouleauMehrdad A. EstiarLiedewei Van de VondelZiv Gan‐OrJonathan BaetsRebecca SchüleStephan Züchner
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Chaperone (clinical)
Foldase
Adenylylation
Endoplasmic-reticulum-associated protein degradation
Summary Some proteins are so much resistant to proteolysis and unfolding that they violate folding rules shared by the vast majority of proteins. These unusual proteins manage to fold into an active native conformation that is thermodynamically at best marginally, but often even less stable than the unfolded state. A huge energetic barrier traps these proteins kinetically in the folded state. The drawback of this situation is the need for a specialized chaperone that adds steric information to the proteins to cross this barrier on the folding pathway. Until now, our knowledge of these intriguing chaperones was restricted to the prodomains of secreted proteases, which function intramolecularly. Recent research has added more examples, which now include the membrane‐anchored lipase‐specific foldase and the pilus subunit specific chaperone, both acting intermolecularly. The case of the pilin chaperone is somewhat deviant in that steric information is definitely provided, but the pilus subunit adopts a thermodynamically favourable stable conformation.
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Proteolysis
Chaperonin
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Hitherto, membralin has been a protein of unknown function. Here, we show that membralin mutant mice manifest a severe and early-onset motor neuron disease in an autosomal recessive manner, dying by postnatal day 5–6. Selective death of lower motor neurons, including those innervating the limbs, intercostal muscles, and diaphragm, is predominantly responsible for this fatal phenotype. Neural expression of a membralin transgene completely rescues membralin mutant mice. Mechanistically, we show that membralin interacts with Erlin2, an endoplasmic reticulum (ER) membrane protein that is located in lipid rafts and known to be important in ER-associated protein degradation (ERAD). Accordingly, the degradation rate of ERAD substrates is attenuated in cells lacking membralin. Membralin mutations or deficiency in mouse models induces ER stress, rendering neurons more vulnerable to cell death. Our study reveals a critical role of membralin in motor neuron survival and suggests a novel mechanism for early-onset motor neuron disease.
Endoplasmic-reticulum-associated protein degradation
Mutant protein
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Abstract ATP-independent chaperones are usually considered to be holdases that rapidly bind to non-native states of substrate proteins and prevent their aggregation. These chaperones are thought to release their substrate proteins prior to their folding. Spy is an ATP-independent chaperone that acts as an aggregation inhibiting holdase but does so by allowing its substrate proteins to fold while they remain continuously chaperone bound, thus acting as a foldase as well. The attributes that allow such dual chaperoning behavior are unclear. Here, we used the topologically complex protein apoflavodoxin to show that the outcome of Spy’s action is substrate specific and depends on its relative affinity for different folding states. Tighter binding of Spy to partially unfolded states of apoflavodoxin limits the possibility of folding while bound, converting Spy to a holdase chaperone. Our results highlight the central role of the substrate in determining the mechanism of chaperone action.
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Foldase
Chemical chaperone
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Protein folding in crowding cellular environment often relies on the assistance of various chaperones. Hsp70 is one of the most ubiquitous chaperones in cells. Previous studies showed that the chaperone–client interactions at the open state tend to remodel the protein folding energy landscape and direct the protein folding as a foldase. In this work, we further investigate how the chaperone–client interaction strength modulates the foldase function of Hsp70 by using molecular simulations. The results showed that the time of substrate folding (including the whole folding step and substrate release step) has a non-monotonic dependence on the interaction strength. With the increasing of the chaperone–client interaction strength, the folding time decreases first, and then increases. More detailed analysis showed that when the chaperone–client interaction is too strong, even small number of chaperones–client contacts can maintain the substrate bound with the chaperone. The sampling of the transient chaperones–client complex with sparse inter-molecule contacts makes the client protein have chance to access the misfolded state even it is bound with chaperone. The current results suggest that the interaction strength is an important factor controlling the Hsp70 chaperoning function.
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Abstract Polypeptide chains experiences mechanical tension while translocating through cellular tunnel. In this scenario, interaction of tunnel-associated chaperones with the emerging polypeptide occurs under force; however, this force-regulated chaperone behaviour is not fully understood. We studied the mechanical chaperone activity of two tunnel-associated chaperones BiP and ERdj3 both in the absence and presence of force; and compared to their respective cytoplasmic homologs DnaK and DnaJ. We found that BiP/ERdj3 shows strong foldase activity under force; whereas their cytoplasmic homolog DnaK/DnaJ behave as holdase. Importantly, these tunnel-associated chaperones (BiP/ERdj3) revert to holdase in the absence of force, suggesting that mechanical chaperone activity differs depending on the presence or absence of force. This tunnel-associated chaperone-driven folding event generates additional mechanical energy of up to 54 zJ that could help protein translocation. The mechanical-chaperone behaviour can be explained by strain theory: chaperones with higher intrinsic deformability function as mechanical foldase (BiP, ERdj3), while chaperones with lower intrinsic deformability act as holdase (DnaK and DnaJ). Our study thus unveils the underlying mechanism of mechanically regulated chaperoning activity and provides a novel mechanism of co-translocational protein folding.
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Chemical chaperone
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In Escherichia coli, the binding of non-native protein substrates to the Hsp70 chaperone DnaK is mediated by the co-chaperone DnaJ. DnaJ accelerates ATP hydrolysis on DnaK, by closing the peptide-binding cleft of DnaK. GrpE catalysed nucleotide exchange and ATP re-binding then lead to substrate release from DnaK, allowing folding. Here we refold immunoglobulin 27 (I27) to better understand how DnaJ-DnaK-GrpE chaperones cooperate. When DnaJ is present, I27 is less likely to misfold and more likely to fold, whereas the unfolded state remains unaffected. Thus, the 'holdase' DnaJ shows foldase behaviour. Misfolding of I27 is fully abrogated when DnaJ cooperates with DnaK, which stabilizes the unfolded state and increases the probability of folding. Addition of GrpE shifts the unfolded fraction of I27 to pre-chaperone levels. These insights reveal synergistic mechanisms within the evolutionary highly conserved Hsp70 system that prevent substrates from misfolding and promote their productive transition to the native state. The bacterial Hsp70 chaperone system consists of DnaJ, DnaK and GrpE. To understand how these chaperones cooperate, Nunes et al. monitor refolding immunoglobulin domains using single-molecule force microscopy to demonstrate that the 'holdase' DnaJ can show foldase activity and suggest that GrpE can facilitate substrate release from DnaK.
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Foldase
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GroES
Chaperone (clinical)
Foldase
Chaperonin
Guanidinium chloride
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Single-molecule force spectroscopy provides access to the mechanics of biomolecules. Recently, magnetic and laser optical tweezers were applied in the studies of chaperones and their interaction with protein clients. Various aspects of the chaperone–client interactions can be revealed based on the mechanical probing strategies. First, when a chaperone is probed under load, one can examine the inner workings of the chaperone while it interacts with and works on the client protein. Second, when protein clients are probed under load, the action of chaperones on folding clients can be studied in great detail. Such client folding studies have given direct access to observing actions of chaperones in real-time, like foldase, unfoldase, and holdase activity. In this review, we introduce the various single molecule mechanical techniques and summarize recent single molecule mechanical studies on heat shock proteins, chaperone-mediated folding on the ribosome, SNARE folding, and studies of chaperones involved in the folding of membrane proteins. An outlook on significant future developments is given.
Foldase
Chaperone (clinical)
Force Spectroscopy
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Folding (DSP implementation)
Co-chaperone
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Foldase
Chaperone (clinical)
Chemical chaperone
Thermostability
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A bstract : Protein disulfide isomerase (PDI) is the physiological catalyst of native disulfide bond formation of nascent peptides in the cells. As a foldase, PDI has both isomerase and chaperone activities. The chaperone activity is intrinsic and independent of its isomerase activity. Both chaperone and isomerase activities are required for PDI to assist folding of denatured and reduced disulfide‐containing proteins. PDI may have great applications in protein production by bioengineering for its function as a foldase.
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Thermostability
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