Molecular characterisation of the chaperone properties of Plasmodium falciparum heat shock protein 70
Heat shock protein 70 (called DnaK in prokaryotes) is one of the most prominent groups of chaperones whose role is to prevent and reverse protein misfolding. PfHsp70 is a heatinducible cytoplasm/nuclear localised Plasmodium falciparum Hsp70. PfHsp70 is thought to confer chaperone cytoprotection to P. falciparum during the development of malaria fever. The objective of this study was to examine the chaperone properties of PfHsp70 using a bioinformatics approach, coupled to in vivo and in vitro analysis. Structural motifs that qualify PfHsp70 as a typical Hsp70 chaperone were identified. Although PfHsp70 has a higher similarity to human Hsc70 than E. coli DnaK, in vivocomplementation assays showed that PfHsp70 was able to reverse the thermosensitivity of E. coli dnaK756 (a temperature sensitive strain whose DnaK is functionally compromised). Two residues (V401 and Q402) in the linker region of PfHsp70 that are critical for its in vivo function were identified. Constructs were generated that encoded the ATPase domain of PfHsp70 and the peptide binding domain of E. coli DnaK (to generate PfK chimera); and the ATPase domain of E. coli DnaK fused to the peptide binding domain of PfHsp70 (KPf). The two chimeras were tested for their ability to reverse the thermosensitivity of E. coli dnaK756 cells. Whilst KPf was able to reverse the thermosensitivity of the E. coli dnaK756 cells, PfK could not. Previously, PfHsp70 purification involved urea denaturation. Using a detergent, polyethylenimine (PEI), PfHsp70 was natively purified. Natively purified PfHsp70 had a basal ATPase activity approximately two times higher than the previously reported activity for the protein purified through urea denaturation. PfJ4, a type II Hsp40, could not stimulate the ATPase activity of PfHsp70 in vitro. Arch and hydrophobic pocket substitutions (A419Y, Y444A and V451F) were introduced in the PfHsp70 peptide binding domain. Similar substitutions were also introduced in the KPf chimera. PfHsp70-V451F (hydrophobic pocket mutant) had marginally compromised in vivo function. However, a similar mutation (V436F), introduced in KPf abrogated the in vivo function of this chimera. The arch and hydrophobic pocket derivatives of PfHsp70 exhibited marginally compromised in vivo function, whilst equivalent mutations in KPf did not affect its in vivo function. The ability of PfHsp70 and its arch/hydrophobic pocket mutants to suppress the heatinduced aggregation of malate dehydrogenase (MDH) in vitro was investigated. Whilst PfHsp70 arch mutants displayed marginal functional loss in vivo, data from in vitro studies revealed that their functional deficiencies were more severe. This is the first study in which an Hsp70 from a parasitic eukaryote was able to suppress the thermosensitivity of an E. coli DnaK mutant strain. Findings from the in vivo and in vitro assays conducted on PfHsp70 suggest that this protein plays a key role in the life-cycle of P. falciparum. Furthermore, this study raised insights that are pertinent to the current dogma on the Hsp70 mechanism of action.
School Location:South Africa
Source Type:Master's Thesis
Keywords:biochemistry microbiology biotechnology
Date of Publication:01/01/2007