Molecule of the Month: Malaria Parasite PTEX

The parasites that cause malaria have an unusual multi-step life cycle. They are carried by infected mosquitoes and are injected into the bloodstream when they bite. The parasite then infects liver cells and multiplies. Finally, the parasite escapes into the bloodstream, invading red blood cells and reproducing inside them. Most of the symptoms of malaria is caused at this final step, as the parasite bursts red blood cells and moves on to attack more cells.

When the malaria parasite is inside a red blood cell, it secretes hundreds of different proteins that remodel the cell. These effector proteins do many things. They help the parasite absorb nutrients and get rid of wastes. They also make the cell act in ways that are beneficial to the parasite, but not necessarily beneficial to the infected person, such as evading the immune system and gluing the infected red blood cell to the wall of the blood vessel. The PTEX complex (Plasmodium translocon of exported proteins) pumps these effector proteins out of the parasite and into the red blood cell.

The core of PTEX is composed of three parts, as seen in PDB ID 6e10. HSP101 is an AAA+ motor that uses ATP to unfold effector proteins and thread them through the center of PTEX. EXP2 forms a transmembrane pore, providing a path for the unfolded protein through the parasite membrane. PTEX150 holds the motor and the pore together. This structure also includes a short piece of the cargo protein inside HSP101, which is better seen in the interactive JSmol below.

PTEX exports a family of proteins, termed erythrocyte membrane protein 1, that are displayed on the surface of red blood cells, radically changing their properties. Instead of circulating normally in the bloodstream, these proteins adhere infected red blood cells to the wall of the circulatory system, fixing them in one place. This is a great advantage to the parasite, since it allows them to evade mechanisms used by the spleen for finding and removing infected cells. VAR2CSA, shown here from PDB ID 7jgh and 7nnh, is one of this family of parasite proteins. It binds to molecules on cells in the placenta, causing severe problems both in the mother and the growing fetus. Researchers are currently exploring VAR2CSA as a target for vaccines to fight malaria in pregnant women.