Functional Role Of Heat Shock Protein 90 From Plasmodium Falciparum

Abstract

Molecular chaperones have emerged in recent years as major players in many aspects of cell biology. Molecular chaperones are also known as heat shock proteins (HSPs) since many were originally discovered due to their increased synthesis in response to heat shock. They were initially identified when Drosophila salivary gland cells were exposed to a heat shock at 37°C for 30 min and then returned to their normal temperature of 25°C for recovery. A “puffing” of genes was found to have occurred in the chromosome of recovering cells, which was later shown to be accompanied by an increase in the synthesis of proteins with molecular masses of 70 and 26 kDa. These proteins were hence named “heat shock proteins”. The first identification of a function for HSPs was the discovery in Escherichia coli that five proteins synthesized in response to heat shock were involved in λ phage growth. The products of the groEL and groES genes were found to be essential for phage head assembly while the dnaK, dnaJ and grpE gene products were essential for λ phage replication. It was later shown that GroEL and GroES are part of a chaperonin system for protein folding in the prokaryotic cytosol while DnaK is a member of the Hsp70 family that works in conjunction with the DnaJ (Hsp40) co-chaperone and the nucleotide exchange factor GrpE to promote phage replication by dissociating the DnaB helicase from the phage-encoded P protein. Since then, a large number of other proteins collectively referred to as HSPs have been discovered. However, heat shock is not the only signal that induces synthesis of heat shock proteins. Stress of any kind, such as nutrient deprivation, chemical treatment and oxidative stress among others causes increased production of HSPs and therefore, they are also known as stress proteins. The term “molecular chaperone” was originally used to describe the function of nucleoplasmin, a Xenopus oocyte protein that promotes nucleosome assembly by binding tightly to histones and donating the bound histone to chromatin. However, since then, chaperones have been defined as “a family of unrelated classes of proteins that mediate the correct assembly of other proteins, but are not themselves components of the final functional structure”. This view of molecular chaperones, though undoubtedly correct, doesn’t capture the multifaceted roles they have since been discovered to play in cellular processes. In recent years, molecular chaperones have been shown to perform other functions in addition to the maintenance of protein homeostasis: translocation of proteins across organelle membranes, quality control in the endoplasmic reticulum, turnover of misfolded proteins as well as signal transduction. As a result, many chaperones are also essential under non-stress conditions and play crucial roles in cell growth and development, cell-cell communication and regulation of gene expression. Heat shock protein 90 (Hsp90) is one of the most abundant and highly conserved molecular chaperones in organisms ranging from bacteria to all branches of eukarya. It has been shown to be essential for cell viability in Saccharomyces cerevisiae, Schizosaccharomyces pombe and Drosophila melanogaster. Although the bacterial homolog HtpG is dispensable under normal conditions, it is important for cell survival during heat shock. In addition to its role as general chaperone in protein folding following stress, Hsp90 has a more specialized role as a chaperone for several protein kinases and transcription factors. Many Hsp90 client proteins are signaling proteins involved in regulation of cell growth and survival. These proteins are critically dependent on Hsp90 for their maturation and conformational maintenance resulting in a key role for Hsp90 in these processes. Recent reports have also highlighted a role for Hsp90 in linking the expression of genetic and epigenetic variation in response to environmental stress with morphological development in Drosophila melanogaster and Arabidopsis thaliana. In Candida albicans, Hsp90 augments the development of drug resistance, implicating a role for Hsp90 in the evolution of infectious diseases. The malarial parasite, Plasmodium falciparum, is the causative agent of the most lethal form of human malaria. The parasite life cycle involves two hosts: an invertebrate mosquito vector and a vertebrate human host. As the parasite moves from the mosquito to the human body, it experiences an increase in temperature resulting in a severe heat shock. The mechanisms by which the parasite adapts to changes in temperature have not been deciphered. Our laboratory has been interested in investigating the role of heat shock proteins during acclimatization of the parasite to such temperature fluctuations. Heat shock proteins of the Hsp40, Hsp60, Hsp70 and Hsp90 families have been characterized in the parasite and are being examined in our laboratory. This thesis pertains to understanding the functional role of Plasmodium falciparum Hsp90 (PfHsp90) during adaptation of the parasite to fluctuations in environmental temperature. The parasite expresses a single gene for cytosolic Hsp90 on chromosome 7 (PlasmoDB accession no.: PF07_0029) coding for a protein of 745 amino acids with a pI of 4.94 and Mw of 86 kDa. Eukaryotic Hsp90 regulates several protein kinases and transcription factors involved in cell growth and differentiation pathways resulting in a crucial role for Hsp90 in developmental processes. A role for PfHsp90 in parasite development, therefore, seems likely. Indeed, PfHsp90 has previously been implicated in parasite development from the ring stage to the trophozoite stage during the intra-erythrocytic cycle. Pharmacological inhibition of PfHsp90 function using geldanamycin (GA), a specific inhibitor of Hsp90 activity, abrogates stage progression. These experiments suggest that PfHsp90 may play a critical role in parasite development. This is further substantiated by the fact that several pathogenic protozoan parasites such as Leishmania donovani, Trypanosoma cruzi, Toxoplasma gondii and Eimeria tenella depend on Hsp90 function during different stages of their life cycles. It appears, therefore, that a principal role of Hsp90 in protozoan parasites may be the regulation of their developmental cycles. However, the precise functions of PfHsp90 during the intra-erythrocytic cycle of the malarial parasite are not clear. In this study we have carried out a functional analysis of PfHsp90 in the malarial parasite. We have examined the role of PfHsp90 in parasite development during repeated exposure to febrile temperatures. We have investigated its involvement in parasite development during a commonly used synchronization protocol involving cyclical changes in temperature. We have examined the interaction of GA with the Hsp90 multi-chaperone complex from P. falciparum as well as the human host. Finally, we have carried out a systems level analysis of chaperone networks in the malarial parasite as well as its human host using an in silico approach. We have analyzed the protein-protein interactions of PfHsp90 in the chaperone network and predicted putative cellular processes likely to be regulated by parasite chaperones, particularly PfHsp90.