All proteins are composed of linear chains of amino acids - with one end called the N-terminus and the opposite end the C-terminus - that are folded in three-dimensional shapes. In the CA protein, amino acid chains are twisted into several rods, called a-helices, with extensions - called side chains - that protrude from the main chain to interact with other folded regions of the protein.

The two-?…ngstrom structure showed the positioning of these a-helices and, for the first time, the location of the atoms in the side chains. "We could precisely delineate all the chemical interactions that stabilize the hexamer," says Yeager.

The center of the CA hexamer is formed by 6 N-terminal ends of the CA protein subunits. The C-terminal domains form a "floppy" belt around this central core, connecting adjacent hexamers. The fact that the belt is not held rigidly in place, helps explain how the honeycomb shape of the capsid forms. "The curvature of the capsid is not constant," says Pornillos. "Now we can see in atomic detail how flexibility in CA makes this happen."

The group discovered another set of interactions critical to stabilizing the capsid - connections between the N-terminal and C-terminal ends of adjacent CA protein molecules in one hexamer. "Think of the fingers of one hand as the N-terminal domain and the palm as the C-terminal," says Yeager. "Imagine the fingers of one hand being cradled in the palm of the other, and so on as if you had six hands in a ring."

Knowing precisely how and where CA proteins interact gives researchers clues on how to interfere with these connections. One approach is to design small molecules that can insert themselves at strategic positions, impeding capsid assembly or making the capsid less stable.

While finding HIV therapies is a main driver for Yeager's work, he points out that it also provides fundamental insights into biology. "Determining the assembly of a relatively simple structure like the capsid of a virus can help us understand how more complex biological structures inside the cell are organized," he explains.In addition to Yeager, Pornillos, and Ganser-Pornillos of Scripps Research and the University of Virginia, co-authors of the paper "X-Ray Structures of the Hexameric Building Block of the HIV Capsid," include Yuanzi Hua and C. David Stout at Scripps Research, and Brian N. Kelly, Frank G. Whitby, Wesley I. Sundquist, and Christopher P. Hill at the University of Utah School of Medicine. For more information, see cell/abstract/S0092-8674(09)00580-7.

This study was funded by the National Institutes of Health and the George E. Hewitt Foundation for Medical Research. Facilities supported by the National Institutes of Health and the U.S. Department of Energy were also used to collect data.

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