The φX174 protein J mediates DNA packaging and viral attachment to host cells

Packaging of viral genomes into their respective capsids requires partial neutralization of the highly negatively charged RNA or DNA. Many viruses, including the Microviridae bacteriophages φX174, G4, and α3, have solved this problem by coding for a highly positively charged nucleic acid-binding protein that is packaged along with the genome. The φX174 DNA-binding protein, J, is 13 amino acid residues longer than the α3 and G4 J proteins by virtue of an additional nucleic acid-binding domain at the amino terminus. Chimeric φX174 particles containing the smaller DNA-binding protein cannot be generated due to procapsid instability during DNA packaging. However, chimeric α3 and G4 phages, containing the φX174 DNA-binding protein in place of the endogenous J protein, assemble and are infectious, but are less dense than the respective wild-type species. In addition, host cell attachment and native gel migration assays indicate surface variations of these viruses that are controlled by the nature of the J protein. The structure of α3 packaged with φX174 J protein was determined to 3.5Å resolution and compared with the previously determined structures of φX174 and α3. The structures of the capsid and spike proteins in the chimeric particle remain unchanged within experimental error when compared to the wild-type α3 virion proteins. The amino-terminal region of the φX174 J protein, which is missing from wild-type α3 virions, is mostly disordered in the α3 chimera. The differences observed between solution properties of wild-type φX174, wild-type α3, and α3 chimera, including their ability to attach to host cells, correlates with the degree of order in the amino-terminal domain of the J protein. When ordered, this domain binds to the interior of the viral capsid and, thus, might control the flexibility of the capsid. In addition, the properties of the φX174 J protein in the chimera and the results of mutational analyses suggest that an evolutionary correlation may exist between the size of the J protein and the stoichiometry of the DNA pilot protein H, required in the initial stages of infection. Hence, the function of the J protein is to facilitate DNA packaging, as well as to mediate surface properties such as cell attachment and infection.