November 03, 2005
Mapping the Physical Interactions of Malaria Proteins
Stylized protein interaction modules shown as white circles
(proteins) connected by lines (interactions) within malaria parasites (in green) in an infected human red blood cell.
Researchers have produced a detailed map that outlines thousands of
physical interactions that take place between proteins found in the
deadly malaria parasite, Plasmodium falciparum. This is the most
extensive description to date of how the parasite's proteins
interact.
The map provides new intelligence that will help in making decisions
about the best proteins to target with drugs and vaccines in the effort
to control this killer of as many as 2.7 million people a year
worldwide. Human malaria is transmitted by female Anopheles
mosquitoes, which serve as vectors for the parasites that cause the
illness. P. falciparum causes about 80 percent of all
human malaria infections and about 90 percent of the deaths.
“This organism is very difficult to study. And the protein interaction data can be important in yielding new hypotheses and new insights about how the organism works.”
Stanley Fields
The research team, which included scientists from Prolexys
Pharmaceuticals and the Howard Hughes Medical Institute (HHMI),
published their findings in the November 3, 2005 issue of the journal
Nature. Robert Hughes of Prolexys and HHMI investigator Stanley
Fields at the University of Washington were senior authors of the
article.
Until now, “there were only a handful of protein interactions
known from directed searches in Plasmodium falciparum,”
said Fields. “There had been no survey at the genome-wide
level.” Understanding which proteins interact is crucial because
interconnecting networks of proteins work together to guide the
parasite's metabolism and pathogenesis. Thus, mapping the multitude of
interactions among proteins is likely to lead to new insights about
drug targets or vulnerabilities in the parasite's defenses.
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This map shows countries with endemic malaria... more
Illustration: Courtesy of CDC
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In searching for interactions between proteins, the scientists used
a technique known as the yeast two-hybrid assay, which Fields invented.
In the assay used in the Nature study, the researchers inserted
the genes for malaria protein fragments into yeast cells and induced
the yeast to produce those fragments. The inserted malaria protein
fragments were also engineered to be expressed together with either of
two components of a regulatory protein. When malaria fragments that
interact were brought together, a telltale marker gene was then
triggered by the regulatory protein - detected by growth of yeast cells
on a Petri plate.
Using advanced analytical systems developed by Prolexys, the
researchers performed more than 32,000 yeast two-hybrid screens to
identify interacting malaria proteins. The scientists also used a
method for ensuring specific production of the target malaria protein
fragment in the yeast, as well as highly automated machinery for
performing the large-scale assays.
From these assays, the scientists identified 2,846 protein-protein
interactions, most of which included a protein whose function had not
yet been identified, said Fields. Knowledge of protein interactions can
help reveal that function, he said.
“It's guilt by association,” he said. “When we see
a protein that is uncharacterized interacting with a protein whose
function is known, it's likely that the uncharacterized protein also
shares that function.” What's more, as clusters of interacting
proteins are mapped, multiple unknown proteins can be considered likely
candidates to take part in the same biological function, he said.
The researchers analyzed the functions of known proteins involved in
the interactions to reveal such clusters. That analysis revealed groups
of proteins implicated in processes such as chromosome modification and
gene activation, as well as those involved in invading host cells.
“The interactions involved in host invasion will give the
parasitology community another focus to study this process,” said
Fields. “And some of these proteins may turn out to be useful
vaccine targets.”
The researchers also identified a group of interacting proteins that
the malaria parasite exports into the host cell. These interactions
could provide parasitologists with insights into how the parasite
modifies the host cell during infection, he said.
In the next phase of studies, the researchers are developing similar
protein interaction data for the parasite Plasmodium vivax,
another parasite that causes malaria. P. vivax is less virulent
than P. falciparum and is seldom fatal. “Our aim in
carrying out this analysis in a related but different parasite was that
we might see interactions that would be in common between
falciparum and vivax,” said Fields. Such knowledge
would yield insight into the basic cell biology of the parasites, he
said.
The researchers also are analyzing interactions between the malaria
parasite proteins and human proteins, which could yield insights into
the process of host cell infectivity, invasion and pathogenesis, said
Fields.
“Our hope is that the parasitology community will find this
knowledge useful to further their understanding of the basic biology of
Plasmodium and this understanding will ultimately lead to new
drugs or vaccine candidates,” said Fields. “This organism
is very difficult to study. And the protein interaction data can be
important - especially when combined with mass spectrometry data on
which proteins are present, with protein sequence data and with genetic
transcriptional profiles — in yielding new hypotheses and new insights
about how the organism works.”
Image: Colored transmission electron micrograph from Photo Researchers Inc., design by Marissa Vignali
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Jennifer Michalowski
