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Filling in the Gaps of HIV

Seeing the whole picture can mean a lot when it comes to figuring out HIV. Researchers at the University of Missouri Bond Life Sciences Center are gaining a clearer idea of what a key protein in HIV looks like, which will help explain the flexible protein’s vital role in the virus life cycle.

The protein the researchers imaged is a building block that forms the virus’ capsid, a protective shell surrounding the virus’ genes. The journal Science published their findings online June 4.

“The capsid acts as an invisibility cloak that hides the virus’ genetic information, the genome, while it is being copied in a hostile environment for the virus,” said Stefan Sarafianos, a virologist at Bond LSC and lead author of the study. “Fine-tuned capsid stability is critical for successful infection: too stable a capsid shell and the cargo is never delivered properly; not stable enough and the contents are detected by our immune defenses, triggering an antiviral response. Capsid stability is a key to the puzzle, and to solve it you have to understand its structure.”

This is the most complete model yet of an HIV-1 capsid protein. In a virus, the protein combines in groups of five or six — called pentamers and hexamers, respectively — that assemble into a mosaic that forms the capsid shell. Roughly 1,500 copies of the protein, grouped into about 250 hexamers and 12 pentamers, comprise the capsid.

HIV, or human immunodeficiency virus, is the retrovirus that leads to AIDS — acquired immunodeficiency syndrome. Roughly 1.2 million people live with HIV in the United States, according to the Centers for Disease Control and Prevention. Globally, about 35 million people were living with HIV in 2013.

Over the years, scientists have employed various techniques and tricks to figure out the structure of the capsid protein. But until now, the clearest image had been made of a mutated version of the protein. It was a compromise: the mutation made the protein stable enough that the scientists could get a good snapshot, but they couldn’t see the detailed interactions between hexamers.

Sarafianos’ lab figured out how to get the full picture: a detailed image of the unmodified proteins that filled all the gaps between hexamers.The team used a technique called X-ray crystallography to unravel the protein’s secrets. Basically, they took many copies of the protein and coaxed them into forming a patterned, crystalline lattice. Next they shot high-powered X-ray beams at the crystal. By interpreting how the X-rays scattered when they ricocheted off the proteins, the researchers made a 3-D map of the protein.

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