U Researchers Determine Structure, Produce 1st Images of Protein that Aids Virus Needs to Replicate and Spread

U Researchers Determine Structure, Produce 1st Images of Protein that Aids Virus Needs to Replicate and Spread

Oct 3, 2005 6:00 PM

Researchers at the University of Utah School of Medicine have produced the first high-resolution images of a protein that the AIDS virus must use to replicate and spread through the human body-a possible step toward new drugs against the disease.

The first structures of the VPS4 protein have shed light on its role as HIV-1 buds or escapes from cells, according to the U of U researchers, who published their findings in the Oct. 4 edition of EMBO Journal (European Molecular Biology Organization) and the Sept. 20 online edition of The Proceedings of the National Academy of Sciences.

By understanding how the virus buds from a cell, we might be able to develop drugs to inhibit the virus from spreading, said Wesley I. Sundquist, Ph.D., professor of biochemistry at the U of U School of Medicine and head of the research team. Although VPS4 itself may not be an ideal drug target, because it is so important for the cell, the proteins structure does tell us more about the budding process.

To exit cells, HIV uses the same cellular machinery used to make tiny compartments inside cells, called vesicles, which help the cell to store, transport or digest cellular products. Vesicle formation is a complex process involving the assembly and disassembly of a large number of proteins. When HIV leaves an infected cell, it uses those same proteins to form viral particles that break through the cellular membrane and spread the virus throughout the body. VSP4 directs the disassembly of these proteins, and therefore is essential for virus replication.

These proteins are like little machines and have to do complex things, Sundquist said. We are now learning what a lot of them look like, although we still dont understand how many of them work.

Scientists already knew that VPS4 is needed to form vesicles. Sundquist and Christopher P. Hill, Ph.D., professor of biochemistry and a corresponding author of the EMBO Journal study, showed several years ago that the AIDS virus needs VPS4 to bud, but the proteins precise role in vesicle formation and budding has been unclear. Led by doctoral candidate Anna Scott, first author on the two studies, the U researchers produced images of two regions on VPS4-the MIT Domain and the ATPase region.

The structure of these two regions suggests how VPS4 binds to another protein complex and then unfolds those proteins in a process that must take place before vesicle formation, according to Sundquist.

The MIT (Microtubule Interacting and Transport) Domain forms a bundle of three helixes that bind to the ESCRT-III (Endosomal Sorting Complexes Required for Transport) protein complex. The ESCRT (pronounced escort) complexes sort other proteins as part of the process of vesicle formation. These proteins must be assembled and then disassembled for multiple rounds of vesicle formation to take place, according to Sundquist.

An ATPase is a protein complex that converts chemical energy into mechanical energy. Although the region of ATPase on the VPS4 protein has not been studied before, the structure showed that this region is similar to ATPases known to unfold other proteins. The U researchers therefore suspect that the ATPase region of VPS4 unfolds the ESCRT complexes to recycle them. (In 1998, Markus Babst, Ph.D., assistant professor of biology at the U, was the first researcher to show that VPS4 APTase is necessary for vesicle formation.)

When VPS4 unfolds the ESCRT protein, the ATPase region forms a ring through which single strands of ESCRT protein are pulled before a vesicle is formed, the U researchers believe.

Its like taking a single strand from a plate of spaghetti and by sucking it through your pursed lips, Sundquist said. He, Hill, and the other Utah researchers now are trying to confirm that VPS4 does actually work by unfolding the ESCRT complex.

Researchers still have many more questions than answers about what happens in the process of vesicle formation, according to Sundquist. For example, they do not understand whether disassembly of the protein complex is mechanistically required for vesicle formation or HIV budding.

This is a pretty complex process and we dont understand how most of it works, he said.

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