INVOLVEMENT OF ACTIN MICROFILAMENTS IN THE REPLICATION OF HUMAN PARAINFLUENZA VIRUS TYPE-3

Several studies indicate that paramyxoviruses require a specific cellu lar factor(s) for transcription of their genomic RNAs. We previously r eported that the cellular cytoskeletal protein actin, in its polymeric form, participates in the transcription of human parainfluenza virus type 3 (HPIV3) in vitro. In the present study, we investigated the rol e of the polymeric form of actin, i.e., the actin microfilaments of th e cytoskeletal framework, in the reproduction of HPIV3 in vivo. Pulse- chase labeling analyses indicate that the viral nucleocapsid-associate d proteins, NP and P,are present predominantly in the cytoskeletal fra mework during infection. By in situ hybridization, we found that viral mRNAs and genomic RNA were synthesized from the nucleocapsids that we re bound to the cytoskeletal framework. Double immunofluorescent label ing and confocal microscopy of the cytoarchitecture revealed that the viral nucleocapsids are specifically localized on the actin microfilam ents. Treatment of cells with the actin-depolymerizing agent, cytochal asin D, resulted in the inhibition of viral RNA synthesis and ribonucl eoprotein accumulation. These results strongly suggest that actin micr ofilaments play an important role in the replication of HPIV3.