Current Protein and Peptide Science - Volume 15, Issue 3, 2014

During the last decade, network approaches became a powerful tool to describe protein structure and dynamics. Here, we describe first the protein structure networks of molecular chaperones, then characterize chaperone containing sub-networks of interactomes called as chaperone-networks or chaperomes. We review the role of molecular chaperones in short-term adaptation of cellular networks in response to stress, and in long-term adaptation discussing their putative functions in the regulation of evolvability. We provide a general overview of possible network mechanisms of adaptation, learning and memory formation. We propose that changes of network rigidity play a key role in learning and memory formation processes. Flexible network topology provides ‘ learning-competent’ state. Here, networks may have much less modular boundaries than locally rigid, highly modular networks, where the learnt information has already been consolidated in a memory formation process. Since modular boundaries are efficient filters of information, in the ‘learning-competent’ state information filtering may be much smaller, than after memory formation. This mechanism restricts high information transfer to the ‘learning competent’ state. After memory formation, modular boundary-induced segregation and information filtering protect the stored information. The flexible networks of young organisms are generally in a ‘learning competent’ state. On the contrary, locally rigid networks of old organisms have lost their ‘learning competent’ state, but store and protect their learnt information efficiently. We anticipate that the above mechanism may operate at the level of both protein-protein interaction and neuronal networks.

Cardiomyocytes are best known for their spontaneous beating activity, large cell size, and low regenerative capacity during adulthood. The mechanical activity of cardiomyocytes depends on a sophisticated contractile apparatus comprised of sarcomeres whose rhythmic contraction relies on Ca2+ transients with a high level of energy consumption. Hence the proper folding and assembly of the sarcomeric and other accessory proteins involved in those diverse functions (i.e., structural, mechanical, energy exchange and production) is critical for muscle mechanics. Chaperone proteins assist other polypeptides to reach their proper conformation, activity and/or location. Consequently, chaperone-like functions are important for the healthy heart but assume greater relevance during cardiac diseases when such chaperone proteins are recruited: 1) to protect cardiac cells against adverse effects during the pathological transition, and 2) to mitigate certain pathogenic mechanisms per se. Protein misfolding is observed as a consequence of inappropriate intracellular environment with acquired conditions (e.g., ischemia/reperfusion and redox imbalance) or because of mutations, which can modify primary to quaternary protein structures. In this review, we discuss the importance of cardiac chaperones while emphasizing the genetic origin (modification of gene/protein sequence) of cardiac protein misfolding and their consequences on the cardiomyocytes leading to organ dysfunction and failure.

Immunophilins comprise a family of intracellular proteins with peptidyl-prolyl-(cis/trans)-isomerase activity. These foldases are abundant, ubiquitous, and able to bind immunosuppressant drugs, from which the term immunophilin derives. Family members are found in abundance in virtually all organisms and subcellular compartments, and their amino acid sequences are conserved phylogenetically. Immunophilins possess the ability to function as molecular chaperones favoring the proper folding and biological regulation of their biological actions. Their ability to interact via their TPR domains with the 90-kDa heat-shock protein, and through this chaperone, with several signalling cascade factors is of particular importance. Among the family members, the highly homologous proteins FKBP51 and FKBP52 were first characterized due to their ability to interact with steroid hormone receptors. Since then, much progress has been made in understanding the mechanisms by which they regulate receptor signaling and the resulting roles they play not only in endocrine processes, but also in cell architecture, neurodifferentiation, and tumor progression. In this article we review the most relevant features of these two immunophilins and their potential as pharmacologic targets.

Homeostasis requires effective action of numerous biological pathways including those working along a genome. The variety of processes functioning in the nucleus is considerable, yet the number of employed factors eclipses this total. Ideally, individual components assemble into distinct complexes and serially operate along a pathway to perform work. Adding to the complexity is a multitude of fluctuating internal and external signals that must be monitored to initiate, continue or halt individual activities. While cooperative interactions between proteins of the same process provide a mechanism for rapid and precise assembly, the inherent stability of such organized structures interferes with the proper timing of biological events. Further prolonging the longevity of biological complexes are crowding effects resulting from the high concentration of intracellular macromolecules. Hence, accessory proteins are required to destabilize the various assemblies to efficiently transition between structures, avoid off-pathway competitive interactions, and to terminate pathway activity. We suggest that molecular chaperones have evolved, in part, to manage these challenges by fostering a general and continuous dynamic protein environment within the nucleus.

The most conserved cellular response to stress is the expression of heat shock proteins (hsp). These proteins participate in the repair of cellular damage after the stress, which is necessary for a positive recovery and confers further protection from subsequent insults. Since these proteins are expressed in subcellular compartments, it was thought that their function during stress conditions was circumscribed to the intracellular environment. However, it is now well established that hsp can also be present outside cells, where they appear to display a function different than the well understood chaperone role. Extracellular hsp act as alert stress signals priming other cells, particularly of the immune system, to avoid the propagation of the insult and favoring resolution. A very pertinent question to ask is what is the mechanism for the export of these proteins into the extracellular environment, since they do not possess a secretory leading signal? Different mechanisms have been proposed, including translocation across the plasma membrane and release associated with lipid vesicles, an endolysosomal pathway, and the passive release after cell death by necrosis. Extracellular hsp appears to be in membrane-bound and membrane-free forms. They could be associated with substrate or free of client proteins. All of these variants of extracellular hsp suggest that their interactions with cells may be quite diverse, both in target cell-types and the activating signal pathways. This review addresses some of our current knowledge about the function and release of extracellular hsp, in particular the major inducible form, Hsp70.

Molecular chaperones, central to cellular protein homeostasis, are conserved within species. Hsp90 and its cochaperones participate in major cellular functions such as cell growth, response to biotic and abiotic stresses and differentiation, and are critical to the regulation of these functions. Regulation is done through their interacting with client proteins in various cellular compartments under specific conditions. Plant Hsp90 and its co-chaperones resemble their mammalian counterparts in their structure. They were shown to participate in diverse and unique pathways such as defense mechanism against pathogens, regulation of gene expression by regulation of the silencing of RNAs, transport of pre-proteins into chloroplasts and response to heat stress. In many cases, the Hsp90 interaction with the co-chaperone is a prerequisite to interaction with client proteins and regulation of their function. While our understanding of the interaction of plant Hsp90 and its co-chaperones has been greatly enhanced, the large number of isoforms in plants and the diverse molecular pathways specific to plants still leave many open questions about the regulation, specificity, and biophysical characteristics of the complexes formed and their contribution to the cellular homeostasis.

The HSP90 chaperone is a highly conserved protein from bacteria to higher eukaryotes. In eukaryotes, this chaperone participates in different large complexes, such as the HSP90 heterocomplex, which has important biological roles in cell homeostasis and differentiation. The HSP90-heterocomplex is also named the HSP90/HSP70 cycle because different co-chaperones (HIP, HSP40, HOP, p23, AHA1, immunophilins, PP5) participate in this complex by assembling sequentially, from the early to the mature complex. In this review, we analyze the conservation and relevance of HSP90 and the HSP90-heterocomplex in several protozoan parasites, with emphasis in Plasmodium spp., Toxoplasma spp., Leishmania spp. and Trypanosoma spp. In the last years, there has been an outburst of studies based on yeast two-hybrid methodology, co-immunoprecipitation-mass spectrometry and bioinformatics, which have generated a most comprehensive protein-protein interaction (PPI) network of HSP90 and its co-chaperones. This review analyzes the existing PPI networks of HSP90 and its co-chaperones of some protozoan parasites and discusses the usefulness of these powerful tools to analyze the biological role of the HSP90-heterocomplex in these parasites. The generation of a T. gondii HSP90 heterocomplex PPI network based on experimental data and a recent Plasmodium HSP90 heterocomplex PPI network are also included and discussed. As an example, the putative implication of nuclear transport and chromatin (histones and Sir2) as HSP90-heterocomplex interactors is here discussed.

This manuscript reviews published Hsp90 inhibitors and Hsp90-binding compounds. The main goal of the article is to overview the structures of existing Hsp90 inhibitors and to draw correlations between compound structure and binding affinity. Furthermore, it is important to emphasize all thermodynamic binding data especially the data that includes the enthalpies, entropies, heat capacities, and the volumes of binding. This information is important for the goal of drawing the principles of a more rational drug design. The distinction between the observed and the intrinsic binding parameters should also be emphasized. Out of nearly 280 compounds surveyed, most could be classified to three chemical groups (resorcinol-bearing, geldanamycin derivatives, and purine-pyrimidine derivatives) and only few of them have determined intrinsic thermodynamic parameters other than the dissociation/inhibition constant. Since most compounds are being developed as anticancer agents, the available cell growth inhibition data is also included. Possible limitations of the use of enzymatic inhibition and the cancer cell growth inhibition data is discussed.