Vaccinia virus protein A3 is required for the production of normal immature virions and for the encapsidation of the nucleocapsid protein L4

- We characterized an inducible mutant in the A3L gene.
- The phenotype of the inducible mutant is distinct from the thermosensitive mutant.
- Studying two classes of mutants leads to a better understanding of protein role.
- We suggest that A3 interacts with L4 to form the vaccinia virus core.

Maturation of the vaccinia virion is an intricate process that results in the organization of the viroplasm contained in immature virions into the lateral bodies, core wall and nucleocapsid observed in the mature particles. It is unclear how this organization takes place and studies with mutants are indispensable in understanding this process. By characterizing an inducible mutant in the A3L gene, we revealed that A3, an inner core wall protein, is important for formation of normal immature viruses and also for the correct localization of L4, a nucleocapsid protein. L4 did not accumulate in the viral factories in the absence of A3 and was not encapsidated in the particles that do not contain A3. These data strengthen our previously suggested hypothesis that A3 and L4 interact and that this interaction is critical for proper formation of the core wall and nucleocapsid.