A Baculovirus Mutant Defective in PKIP, a Protein Which Interacts with a Virus-Encoded Protein Kinase

We have found that a temperature-sensitive mutant of the baculovirus Ac M NPV, tsB97, is defective in PKIP, the product of ORF24 which was previously found to interact with and stimulate the activity of a virus-encoded protein kinase, PK-1. The mutant lacks the ability to form plaques and occlusion bodies at the nonpermissive temperature. The mutant displays several properties which suggest a defect in the latter half of the late phase of infection; these properties include a delay in the shutoff of host protein synthesis, the presence of aberrant electron-dense bodies associated with the virogenic stroma, and the production of few, if any, progeny budded virus. A study of the expression of selected late genes showed no difference in the timing or level of transcription or translation of most late genes. However, elevated levels of the late 6.9K protein, a protamine-like protein, were observed in mutant-infected cells at 24 h postinfection, suggesting a defect in the regulation of this protein. Two polypeptides, 40 and 6 kDa, exhibited considerably higher levels of steady-state phosphorylation in wt-infected cells versus tsB97-infected cells at 24 h p.i. and could be candidates for PK-1/PKIP-mediated phosphorylation. The tsB97 mutant also displayed a severe defect in very late gene transcription which accounts for its inability to form occlusion bodies. The effect of PKIP on very late gene transcription may be a secondary effect of the block in the late phase of infection. PKIP showed no ability to transactivate expression from a very late promoter in transient expression assays.