Cell, Vol. 98, 825-833, September 17, 1999. Viral Evolution Revealed by Bacteriophage PRD1 and Human Adenovirus Coat Protein Structures -- Summary The unusual bacteriophage PRD1 features a membrane beneath its icosahedral protein coat. The crystal structure of the major coat protein P3 reveals a molecule with three interlocking subunits. Surprisingly the P3 molecule closely resembles hexon, the equivalent protein in human adenovirus. Introduction The dsDNA bacteriophage PRD1 was discovered in the sewers of Kalamazoo, Michigan, in the 1970s. It is the prototypic example of Tectiviridae, with virions characterized by their unusual possession of an internal membrane that lies beneath the icosahedral protein capsid. PRD1 is also an unusual bacteriophage because it does not have a tail, as do the other dsDNA bacteriophages, but uses its membrane to "inject" the genome into the bacterium. The viral membrane is aquired during assembly from the host. An area of the bacterial plasma membrane, which contains virus-specific membrane proteins, buds into the cytosol and acts as a scaffold for the assembly of the coat proteins to form the procapsid. Image reconstructions from cryoelectron micrographs of complete virions and procapsids from a mutant strain which is unable to package the genome, show that both have the same diameters. Thus there is no capsid expansion nor are there changes in the capsomer proteins upon maturation when the genome is packaged into the empty PRD1 capsids, but structural rearrangements do occur in the membrane. This maturation machanism is quite unlike that for most other dsDNA bacteriophages, which exhibit considerable changes in their capsid size and organization on genome packaging. --

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