Characterization of a herpes simplex virus regulatory protein: aggregation and phosphorylation of a temperature-sensitive variant of ICP 4

The viral polypeptide ICP 4 (or Vmw 175) is synthesized during the immediate early phase of infection by herpes simplex virus (HSV) and is required during the viral reproductive cycle for efficient transcription of delayed early viral genes. Replication of mutant strains of HSV-1 such as tsLB 2 that encode a temperature-sensitive variant of ICP 4 does not proceed beyond the immediate early phase in cells that are infected and maintained at the nonpermissive temperature (NPT). Under these conditions, the immediate early viral polypeptides accumulate to levels that are 10 to 100 fold greater than normal. We have investigated the use of tsLB 2-infected cells maintained at the NPT as a source for substantial amounts of ICP 4 for further characterization. Extraction of ICP 4 from tsLB 2-infected cells requires 0.5 M NaCl and yields aggregates that contain ICP 4, ICP 6, ICP 27, and lesser amounts of other proteins. These large aggregates cannot be disrupted under nondenaturing conditions and thus are not a suitable source for native ICP 4. We have used this overproduced ICP 4 as an antigen to generate ICP 4-specific antibody and for characterization of the primary structure of ICP 4. Analysis of acid-hydrolysed 32P-labeled ICP 4 revealed that the major phosphorylated residues in ICP 4 are phosphoserine and phosphothreonine.