illustration by Mark Allen Miller

Malaria's Weakness

With different approaches, two HHMI researchers land on an enzyme critical to the malaria parasite's destructive ways.

In a hallway during an October 2008 science meeting in Bangkok, the chitchat was guarded. The two malaria researchers held their cards close to their chests. Dan Goldberg and Alan Cowman realized, however, that they were working on the same research question, approaching it from opposite directions, and quickly closing in on an answer.

They agreed to share information and keep each other informed of their progress. In the end, their papers were published back-to-back in the February 4, 2010, issue of Nature. Both reveal the central role of one enzyme, plasmepsin V, in the malaria parasite's survival.

“The parasite does lots of things that dramatically affect the physical properties of the red blood cell. It takes over the cell for its own purpose,” says Goldberg, an HHMI investigator and infectious disease physician at Washington University in St. Louis.

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Life Cycle of Malaria

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When an infected mosquito injects the malaria parasite, Plasmodium falciparum, into a human's bloodstream, the parasite travels to the liver where it multiplies for 1-2 weeks. Parasites then break out of the liver cells and invade red blood cells. During this stage of its life cycle, the amoeba-like organism cloaks itself within a red blood cell, wrapped in a bit of cell membrane, and converts the cell into a hiding place from the immune system while it reproduces. This coup relies on the parasite shipping 200-300 of its proteins into the cell, which camouflage the cell and turn it into a sticky glob that clings to blood vessel walls. The proteins also help gather nutrients for the hungry parasite so it can continue to multiply.

Cowman, an HHMI international research scholar at the Walter and Eliza Hall Institute of Medical Research in Melbourne, Australia, and his team were searching for the enzyme that chops proteins down for export in a form the parasite can recognize. In 2004, Cowman's lab had identified a specific sequence of amino acids called a PEXEL motif that appeared in all exported proteins. He had a hunch that a protease might recognize the PEXEL sequence and cut at that site. Meanwhile, Goldberg and his group had been characterizing a protease enzyme, called plasmepsin V (PMV), which they had found in the endoplasmic reticulum of the parasite but didn't know what it did.

The two pieces of the puzzle snapped into place for Goldberg and Cowman when a third group published its results in 2008. Michael Marletta's laboratory at the University of California, Berkeley, showed that nearly all exported malaria proteins were cleaved at the PEXEL motif and the process took place in the endoplasmic reticulum.

“We said, ‘Aha! We've got a protease of unknown function in the right place. We bet it's the processing enzyme,’” says Goldberg. They set about to test that hypothesis.

Both groups isolated PMV from parasites and added it to PEXEL-containing proteins in a test tube. “It cleaved 100 percent and we knew straight away—we've got it!” says Cowman, a molecular parasitologist.

Goldberg wanted to test whether the chopping activity of PMV was critical for protein export and, ultimately, the survival of the parasite. His postdoc Ilaria Russo made a catalytically dead version of PMV—it could bind PEXEL proteins but did not cut them. She flooded live parasites with the mutant enzyme to compete with normal PMV. Through the microscope, she saw that in cells containing only parasites with the dead enzyme, the parasites appeared sick. In cells that carried a parasite with dead enzyme and another parasite with normal PMV, both parasites were healthy. The normal PMV exported enough proteins to maintain parasite health. Clearly, PMV's protein-chopping action was essential for the parasite's growth.

Cowman's group also did a clever experiment to test whether PMV's cleavage of PEXEL was required for proper export of a protein. They created proteins that “looked” like they had been cleaved properly by PMV but were actually cleaved by another enzyme, called signal peptidase. These proteins built up inside the parasite.

“It has to be PMV handing off the cleaved protein to something else for export,” says Cowman. “It was one of the most clear-cut experiments I've ever seen.”

An inhibitor of PMV, which would shut down protein export and kill the parasite, has potential to be a powerful drug against malaria. “PMV is similar to the HIV protease for which very good drugs already exist,” says Cowman. “We should be able to develop very good drugs against this as well, which would inhibit the parasite at a final point in its life cycle.”

About six months after the Bangkok meeting, Goldberg got in touch with Cowman to let him know his group was wrapping things up. The two decided to submit their manuscripts simultaneously—even getting on the phone together to hit the “send” button. “It was a pleasure to do this alongside Alan Cowman's group,” says Goldberg. “This is the way I think science should work. We both had discovered something and we wanted both groups to get credit.”

Scientist Profile

Investigator
Washington University in St. Louis
Microbiology, Parasitology
Senior International Research Scholar
The Walter and Eliza Hall Institute of Medical Research
Cell Biology, Parasitology