Identification of Viral Membrane Proteins Required for Cell Fusion and Viral Dissemination That Are Modified during Vaccinia Virus Persistence

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Wild-type vaccinia virus WR strain forms non-fusogenic (F-) large plaques and is hemagglutinin positive (HA+) under normal conditions of virus infection. We have analyzed a collection of spontaneous, highly attenuated mutants of vaccinia virus isolated from persistently infected Friend erythroleukemia cells (E. Paez, S. Dallo, and M. Esteban, J. Virol. 61, 2642-2647, 1987) for the ability to express HA during virus infection. After 14 cell passages, all the mutants isolated were hemadsorption negative (HAD-) and did not synthesize a HA that could be recognized by anti-HA monoclonal antibodies. All these HA- mutants induced extensive cell-cell fusion (F+), with the exception of two mutants (65-16 and 101-14) isolated from late cell passages. Nucleotide sequence analysis of the HA gene in these two mutants confirmed the HA- phenotype. A frameshift mutation very close to the initiation codon resulted in premature translational termination. The truncated gene now only encodes the first 25 amino acids. Analysis of progeny from "wild-type," like early serial passage virus (5-3) X mutant back crosses, shows that for one late passage non-fusogenic small-plaque mutant (101-14) among large plaque progeny there is good correspondence between the ability to fuse and the absence of a viral HA and that each large plaque mutant contains a normal 14 kDa membrane protein. However, with a second serial passage mutant 65-16, which, like 101-14, is a nonfusogenic small-plaque variant, there is again an excellent correlation between the inability to synthesize HA and the ability to fuse, but there is no correlation of plaque size with a normal 14 kDa viral membrane protein, as most large plaque mutants encode a larger, i.e., 17 kDa protein. Rescue experiments of 65-16 with bona fide cloned 14 kDa protein gene confirm that the ability to regulate plaque size and cell fusion in this mutant is due to a protein other than the 14 kDa protein. Marker rescue experiments indicated that the map position of the additional mutation coincided with a small deletion occurring in the HindIII F fragment. This deletion affected the 5′-end and promoter sequences of the 37 kDa envelope protein gene and produced a lack of expression of this protein, shown by others to be involved in the formation of extracellular enveloped virus in infected cells. These results shown that viruses with mutations of viral membrane proteins required for cell fusion and viral dissemination are selected during vaccinia virus persistence in cell culture.