  Plasmodium falciparum 70 kDa heat shock proteins (PfHsp70s) are expressed at
all stages of the pathogenic erythrocytic phase of the malaria parasite
lifecycle. There are six PfHsp70s, all of which have orthologues in other
plasmodial species, except for PfHsp70-x which is unique to P. falciparum. This
paper highlights a number of original results obtained by a detailed
bioinformatics analysis of the protein. Large scale sequence analysis indicated
the presence of an extended transit peptide sequence of PfHsp70-x which
potentially directs it to the endoplasmic reticulum (ER). Further analysis
showed that PfHsp70-x does not have an ER-retention sequence, suggesting that
the protein transits through the ER and is secreted into the parasitophorous
vacuole (PV) or beyond into the erythrocyte cytosol. These results are
consistent with experimental findings. Next, possible interactions between
PfHsp70-x and exported P. falciparum Hsp40s or host erythrocyte DnaJs were
interrogated by modeling and docking. Docking results indicated that
interaction between PfHsp70-x and each of the Hsp40s, regardless of biological
feasibility, seems equally likely. This suggests that J domain might not
provide the specificity in the formation of unique Hsp70-Hsp40 complexes, but
that the specificity might be provided by other domains of Hsp40s. By studying
different structural conformations of PfHsp70-x, it was shown that Hsp40s can
only bind when PfHsp70-x is in a certain conformation. Additionally, this work
highlighted the possible dependence of the substrate binding domain residues on
the orientation of the {\alpha}-helical lid for formation of the substrate
binding pocket.
