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prevent clearance of the infected erythrocyte by the spleen. Other remodeling
processes increase the uptake of nutrients by the cell. 68,69 With the complete
sequence of the genome of the parasite in hand, it was found that proteins that
are exported from the parasite to the erythrocyte contain a unique sequence,
KxLxE/Q/D, where 'x' denotes any amino acid. This sequence is termed the
'Plasmodium EXport ELement' or 'PEXEL' sequence. Furthermore, it was
found that the sequence is cleaved between the leucine and the 'x' that follows
to give a new amino terminus that is acylated to yield the protein that is
exported. These modifications occur during the process of export, and all
proteins found in the cytosol of the red blood cells contain the Ac-Leu-x-
sequence at the amino terminus. Thus, this process is a critical function of the
interaction between the parasite and the erythrocyte.
In 2010, two papers appeared back to back in Nature 70,71 to announce that
an aspartic proteinase, plasmepsin V, is the enzyme responsible for the cleavage
of the PEXEL sequence in the ER of the parasite of protein exported from the
parasite into the erythrocyte cytosol. The cleavage and acylation reactions must
occur consecutively, as an expressed protein with an x-Q amino terminal
sequence was not acylated. 70 Both papers also showed a reduction in cleavage
when inhibitors of the retroviral enzyme, HIV protease, were added, allowing
the conclusion that an enzyme of the aspartic proteinase class was involved in
the cleavage reaction. Further discussion of the effect of HIV protease inhibi-
tors will appear in the last section of this chapter.
The observation that plasmepsin V is involved in the critical function of
protein export has raised the possibility that this enzyme may be the optimal
target for drug discovery in an effort to obtain new therapeutics to treat
malaria. Success in this venture will require the production of larger amounts of
protein for structural studies, inhibitor screening, and optimization.
A recent study 72 has modeled the structure of plasmepsin V based on the
known structure of members of the aspartic proteinase family. In this report,
the authors did not consider the effect of the three sequence inserts that make
plasmepsin V unique among the family. While it is unlikely that those inserts
into the sequence have any effect on the active-site binding properties, it
remains to be seen what the real structure is based on crystallography and
structure determination by X-ray methods (Figure 11.5).
11.4.2 Plasmepsin IX and Plasmepsin X
At this point, only a single meeting abstract each has appeared describing
studies on plasmepsin IX 73 and plasmepsin X 74 . The latter citation indicates
that PM X is involved in an important process in the mosquito vector. A
reference to plasmepsin X appears in a paper fromMoura, Dame, and Fidock 61
where it was observed that a parasite line overexpressing PM X was four times
less sensitive to the effects of a potent diphenylurea compound. At this time,
efforts are under way in several laboratories to determine the biological role of
PM IX and X, and to prepare sucient amounts of pure protein for studies of
their structure and function.
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