Expression of the IE1 transactivator of Autographa californica nuclear polyhedrosis virus during viral infection

Abstract

The immediate-early IE1 protein of Autographa californica nuclear polyhedrosis virus (AcMNPV) is an important regulator of viral gene transcription. To provide a tool for further analysis of the expression and function of IE1, a polyclonal antiserum was raised against IE1 expressed in bacteria. Immunoblot analysis of infected cell lysates was used to monitor the accumulation of IE1 throughout the viral life cycle. When extracts were prepared in the presence of phosphatase inhibitors, only one protein band was detected on SDS-polyacrylamide gels. However, in the absence of phosphatase inhibitors, at least four distinct electrophoretic species were detected. Mobility shift assays were conducted using an enhancer DNA probe and whole cell extracts prepared at different times postinfection. Results indicated that the enhancer-binding activity of IE1 increased from 4 to 72 hr postinfection. DNA-protein complexes formed with infected cell extracts migrated more slowly than those formed with transfected cell extracts. This effect was more pronounced with extracts prepared in the presence of phosphatase inhibitors. Supershift experiments with IE1 antiserum confirmed that IE1 was a component of DNA-protein complexes in both transfected and infected cell extracts. A titration experiment was done to determine the minimal amounts of IE1 required for activation of the 39k promoter in the presence and absence of a cis-linked enhancer element. These analyses indicated that the intracellular levels of IE1 are not sufficient for enhancer-independent activation of the 39k promoter during the early phase of viral infection. Quantitative immunoblots revealed that the amount of IE1 in budded virus was less than 0.68 mole per mole of viral DNA, suggesting that IE1 is not a structural protein of AcNPV.