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      Structural and functional similarities between the capsid proteins of bacteriophages T4 and HK97 point to a common ancestry.

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          Abstract

          Gene product (gp) 24 of bacteriophage T4 forms the pentameric vertices of the capsid. Using x-ray crystallography, we found the principal domain of gp24 to have a polypeptide fold similar to that of the HK97 phage capsid protein plus an additional insertion domain. Fitting gp24 monomers into a cryo-EM density map of the mature T4 capsid suggests that the insertion domain interacts with a neighboring subunit, effecting a stabilization analogous to the covalent crosslinking in the HK97 capsid. Sequence alignment and genetic data show that the folds of gp24 and the hexamer-forming capsid protein, gp23*, are similar. Accordingly, models of gp24* pentamers, gp23* hexamers, and the whole capsid were built, based on a cryo-EM image reconstruction of the capsid. Mutations in gene 23 that affect capsid shape map to the capsomer's periphery, whereas mutations that allow gp23 to substitute for gp24 at the vertices modify the interactions between monomers within capsomers. Structural data show that capsid proteins of most tailed phages, and some eukaryotic viruses, may have evolved from a common ancestor.

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          Author and article information

          Journal
          Proc Natl Acad Sci U S A
          Proceedings of the National Academy of Sciences of the United States of America
          Proceedings of the National Academy of Sciences
          0027-8424
          0027-8424
          May 17 2005
          : 102
          : 20
          Affiliations
          [1 ] Department of Biological Sciences, Purdue University, 915 West State Street, West Lafayette, IN 47907, USA.
          Article
          0502164102
          10.1073/pnas.0502164102
          1129118
          15878991
          3874eaa6-effa-4407-92e1-9e2426f5a478
          History

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