Current Proteomics - Volume 16, Issue 1, 2019

Proposed models of the function of Hsp90 are characterised by high flexibility of the dimeric state and conformational changes regulated by both nucleotide binding and hydrolysis, and by co-chaperone interactions. In addition to its dimeric state, Hsp90 self-associates upon particular stimuli. The Hsp90 dimer is the building block up to the hexamer that we named “cosy nest”, and the dodecamer results from the association of two hexamers. Oligomers exhibit chaperone activity, but their exact mechanism of action has not yet been determined. One of the best ways to elucidate how oligomers might operate is to study their interactions with co-chaperone proteins known to regulate the Hsp90 chaperone cycle, such as p23 and Aha1. In this review, we summarise recent results and conclude that Hsp90 oligomers are key players in the chaperone cycle. Crucible-shaped quaternary structures likely provide an ideal environment for client protein accommodation and folding, as is the case for other Hsp families. Confirmation of the involvement of Hsp90 oligomers in the chaperone cycle and a better understanding of their functionality will allow us to address some of the more enigmatic aspects of Hsp90 activity. Utilising this knowledge, future work will highlight how Hsp90 oligomers and co-chaperones cooperate to build the structures required to fold or refold numerous different client proteins.

Molecular chaperones have several critical functions in protein metabolism. Among them, some are involved in processes that culminate in the extraction of entangled polypeptides from protein aggregates, releasing unfolded structures prone to be refolded or directed to degradation. This action avoids the effect of toxic aggregates on cells and tissues. Molecular chaperones belonging to the Hsp100 family are widely distributed from unicellular and sessile organisms up to fungi and plants, exerting key functions related to the reduction of the effects caused by different forms of stress. The Hsp100 proteins belong to the AAA+ (ATPases Associated with diverse cellular Activities) family and form multichaperone systems with Hsp70 and small Hsp chaperones families. However, Hsp100 are absent in metazoan, where protein disaggregation action is performed by a system involving the Hsp70 family, including Hsp110 and J-protein co-chaperones. Here, the structural and functional aspects of these protein disaggregation systems will be reviewed and discussed in the perspective of the Hsp100 system absent in the metazoan kingdom. This feature focuses on Hsp100 as a hot spot for drug discovery against human infectious diseases such as leishmaniasis and malaria, as Hsp100 is critical for microorganisms. The current data available for Hsp100 in Leishmania spp. and Plasmodium spp. are also reviewed.

Protein homeostasis, or proteostasis, is required for proper cell function and thus must be under tight maintenance in all circumstances. In crowded cell conditions, protein folding is sometimes unfavorable, and this condition is worsened during stress situations. Cells cope with such stress through the use of a Protein Quality Control system, which uses molecular chaperones and heat shock proteins as its major players. This system aids with folding, avoiding misfolding and/or reversing aggregation. A pivotal regulator of the response to heat stress is Heat Shock Factor, which is recruited to the promoters of the chaperone genes, inducting their expression. This mini review aims to cover our general knowledge on the structure and function of this factor.

Telomere length maintenance is important for genome stability and cell division. In most eukaryotes, telomeres are maintained by the telomerase ribonucleoprotein (RNP) complex, minimally composed of the Telomerase Reverse Transcriptase (TERT) and the telomerase RNA (TER) components. In addition to TERT and TER, other protein subunits are part of the complex and are involved in telomerase regulation, assembly, disassembly, and degradation. Among them are some molecular chaperones such as Hsp90 and its co-chaperone p23 which are found associated with the telomerase RNP complex in humans, yeast and probably in protozoa. Hsp90 and p23 are necessary for the telomerase RNP assembly and enzyme activity. In budding yeast, the Hsp90 homolog (Hsp82) is also responsible for the association and dissociation of telomerase from the telomeric DNA by its direct interaction with a telomere end-binding protein (Cdc13), responsible for regulating telomerase access to telomeres. In addition, AAA+ ATPases, such as Pontin and Reptin, which are also considered chaperone- like proteins, associate with the human telomerase complex by the direct interaction of Pontin with TERT and dyskerin. They are probably responsible for telomerase RNP assembly since their depletion impairs the accumulation of the complex. Moreover, various RNA chaperones, are also pivotal in the assembly and migration of the mature telomerase complex and complex intermediates. In this review, we will focus on the importance of molecular chaperones for telomerase RNP biogenesis and how they impact telomere length maintenance and cellular homeostasis.

Different signaling cascades including the Cell Wall Integrity (CWI), the High Osmolarity Glycerol (HOG) and the Ca2+/calcineurin pathways control the cell wall biosynthesis and remodeling in fungi. Pathogenic fungi, such as Aspergillus fumigatus and Candida albicans, greatly rely on these signaling circuits to cope with different sources of stress, including the cell wall stress evoked by antifungal drugs and the host's response during infection. Hsp90 has been proposed as an important regulatory protein and an attractive target for antifungal therapy since it stabilizes major effector proteins that act in the CWI, HOG and Ca2+/calcineurin pathways. Data from the human pathogen C. albicans have provided solid evidence that loss-of-function of Hsp90 impairs the evolution of resistance to azoles and echinocandin drugs. In A. fumigatus, Hsp90 is also required for cell wall integrity maintenance, reinforcing a coordinated function of the CWI pathway and this essential molecular chaperone. In this review, we focus on the current information about how Hsp90 impacts the aforementioned signaling pathways and consequently the homeostasis and maintenance of the cell wall, highlighting this cellular event as a key mechanism underlying antifungal therapy based on Hsp90 inhibition.

Bacteria use an impressive arsenal of secretion systems (1-7) to infect their host cells by exporting proteins, DNA and DNA-protein complexes via cell membranes. They use chaperone-usher pathways for host colonization as well. To be targeted for transportation across one (Gram-positive) or two membranes (Gram-negative), clients must be selected, guided and unfolded to pass through type 3 (T3SS) or type 4 (T4SS) secretion systems. For these processes, bacteria count on secretory chaperones that guide macromolecular transport via membranes. Moreover, if we know how these processes occur, we might be able to stop them and avoid bacterial infections. Thus, structural and functional characterizations of secretory chaperones become interesting, as these proteins are the perfect targets for blocking bacteria action. Therefore, this review focuses on a story of known mechanisms of chaperone- secretion assisted transport with special attention on virulence proteins and DNA transport in bacteria.

Hsp70 members occupy a central role in proteostasis and are found in different eukaryotic cellular compartments. The mitochondrial Hsp70/J-protein machinery performs multiple functions vital for the proper functioning of the mitochondria, including forming part of the import motor that transports proteins from the cytosol into the matrix and inner membrane, and subsequently folds these proteins in the mitochondria. However, unlike other Hsp70s, mitochondrial Hsp70 (mtHsp70) has the propensity to self-aggregate, accumulating as insoluble aggregates. The self-aggregation of mtHsp70 is caused by both interdomain and intramolecular communication within the ATPase and linker domains. Since mtHsp70 is unable to fold itself into an active conformation, it requires an Hsp70 escort protein (Hep) to both inhibit self-aggregation and promote the correct folding. Hep1 orthologues are present in the mitochondria of many eukaryotic cells but are absent in prokaryotes. Hep1 proteins are relatively small and contain a highly conserved zinc-finger domain with one tetracysteine motif that is essential for binding zinc ions and maintaining the function and solubility of the protein. The zinc-finger domain lies towards the C-terminus of Hep1 proteins, with very little conservation outside of this domain. Other than maintaining mtHsp70 in a functional state, Hep1 proteins play a variety of other roles in the cell and have been proposed to function as both chaperones and co-chaperones. The cellular localisation and some of the functions are often speculative and are not common to all Hep1 proteins analysed to date.

Background: Liquid chromatography coupled with targeted mass spectrometry underwent rapid technical evolution during last years and has become widely used technology in clinical laboratories. It offers confident specificity and sensitivity superior to those of traditional immunoassays. However, due to controversial reports on reproducibility of SRM measurements, the prospects of clinical appliance of the method are worth discussing. Objective: The study was aimed at assessment of capabilities of SRM to achieve a thorough assembly of the human plasma proteome. Method: We examined set of 19 human blood plasma samples to measure 100 proteins, including FDA-approved biomarkers, via SRM-assay. Results: Out of 100 target proteins 43 proteins were confidently detected in at least two blood plasma sample runs, 36 and 21 proteins were either not detected in any run or inconsistently detected, respectively. Empiric dependences on protein detectability were derived to predict the number of biological samples required to detect with certainty a diagnostically relevant quantum of the human plasma proteome. Conclusion: The number of samples exponentially increases with an increase in the number of protein targets, while proportionally decreasing to the logarithm of the limit of detection. Analytical sensitivity and enormous proteome heterogeneity are major bottlenecks of the human proteome exploration.