Nuclear F-actin: a functional component of baculovirus-infected lepidopteran cells?

Cellular functions known to involve actin are thought to occur in the cytoplasm. Even though actin has frequently been found in the nucleus, systems well-suited for studying the function of such nuclear actin are rare. We observed filamentous (F) actin within nuclei of IPLB-Sf-21 cells infected with Autographa californica M nuclear polyhedrosis virus (AcMNPV) as detected by laser confocal microscopy using fluorescent phalloidin probes. The nuclear F-actin co-localized with the major capsid protein of the virus during normal infections. Cytochalasin D, known to interfere with nucleocapsid morphogenesis, uncoupled the apparent co-localization of F-actin and the capsid protein and inhibited infectious progeny production. Inhibition was reversible throughout infection (even in the presence of a protein synthesis inhibitor) and the nuclear co-localization of F-actin and the capsid protein was re-established upon removal of the drug. These observations suggest that nuclear Factin plays a role in virus replication, and that AcMNPV-infected cells may constitute a useful system in which to expand our understanding of nuclear actin transport and function.