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Crystal structure solves virus puzzle.

A new computer model of the crystal structure of a monkey virus reveals how nature builds a viral coat using only five-sided building blocks.

Some viruses, such as those that cause polio and the common cold, consist of genetic material encased in shells made from five-sided and six-sided bundles of protein. The coexistence of pentagonal and hexagonal bundles results in a tightly knit shell in which all bundle edges are bound together. But some larger viruses, including those associated with warts and cervical cancer, lack six-sided bundles. Without at least some hexagons, not all of the pentagons can touch on all sides, because pentagons fit together in a way that produces six-sided holes.

To learn how viruses solve this structural problem, Harvard crustallographer Robert C. Liddington and his colleagues used synchrotron radiation to probe the crystal structure of simian virus 40, a cousin of the papillomavirus that causes warts in humans. In the Nov. 28 NATURE, they describe how the five-sided bundles link up.

The coat on simian virus 40 contains a total of 360 proteins. Each proten consists of 361 amino acids strung together and folded so that one 60-amino-acid arm reaches out. The 360 proteins form 72 pentamers -- five-sided building blocks containing five proteins. The arms emerging from the various proteins dangle in slightly different directions.

Twelve pentamers (blue) fit tightly with five neighbors. But the other 60 pentamers (multicolored) must each connect with six neighbors. "You've got a mismatch of symmetry," says Liddington. What allows them to fit in are the flexible arms, which allow distinct kinds of interactions.

The pentamers link themselves into a tightly bound surface. A multicolored pentamer extends one arm to an arm of the blue and three other arms to the outstretched arms of three multi-colored neighbors. These fours pairs of arms twist aroun each other, linking the pentamers, explains Liddington.

The pentamer's remaining arm then latches onto the multicolored neighbor to the left of the blue one; that neighbor, however, does not extend an arm back. Likewise, the multicolored pentamer fails to extend an arm to the outstreched arm of the multicolored neighbor just to the right of the blue one. Thus, these pentamers connect via linkages with the blue one, creating three-way ties.
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Title Annotation:simian virus 40
Author:Pennisi, Elizabeth
Publication:Science News
Date:Dec 7, 1991
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