Pathway of assembly of herpesvirus capsids: An analysis using DNA+ temperature-sensitive mutants of pseudorabies virus

It was reported previously that nine different DNA⁺ is mutants of Pr virus were all defective in the processes leading to the cleavage of concatemeric viral DNA and to the formation of nucleocapsids. Six of these mutants were found to assemble capsids at the nonpermissive temperature; three were capsid^? (Ladin et al., 1980). In the present communication we have characterized these mutants further. Each of the mutants was rescued by one specific restriction fragment of wild type virus DNA indicating that none of these mutants is altered in more than one locus. Many of the mutants complemented one another well at a frequency indicating that they are probably mutated in different genes. Cells infected at the nonpermissive temperature (41°) with the DNA⁺ mutants synthesized all the virus proteins synthesized by wild type-infected cells (as detected by PAGE) with two notable exceptions: (1) ts1-infected cells synthesized a thermolabile 142K capsid protein. (2) In ts1, ts109, and tsJ (all capsid^? mutants)-infected cells, detectable amounts of 35K capsid protein did not accumulate. The 35 K capsid protein is normally processed from a precursor protein, the processed protein being detectable in the nucleus of the infected cells only. In capsid^?-infected cells processing of the 35K protein precursor did not occur. Furthermore, accumulation in the nucleus of all capsid proteins was impaired in all the capsid^?-infected cells. Because at least two of the three capsid^? mutants are mutated in different genes, we conclude that the processing of the 35K protein, capsid assembly, and accumulation of capsid proteins in the nuclei are interdependent events which occur only if all the proteins necessary for capsid assembly are functional. On the basis of these results, we propose that the 35K protein is processed during capsid assembly and that assembly of the proteins into capsids is required for the continued movement of capsid proteins to, and their accumulation in, the nuclei of infected cells.