Profilin is required for optimal actin-dependent transcription of respiratory syncytial virus genome RNA

Transcription of human respiratory syncytial virus (RSV) genome RNA exhibit ed an obligatory need for the host cytoskeletal protein actin. Optimal tran scription, however, required the participation of another cellular protein that was characterized as profilin by a number of criteria. The amino acid sequence of the protein, purified on the basis of its transcription-optimiz ing activity in vitro, exactly matched that of profilin, RSV transcription was inhibited 60 to 80% by antiprofilin antibody or poly-L-proline, molecul es that specifically bind profilin, Native profilin, purified from extracts of lung epithelial cells by affinity binding to a poly-L-proline matrix, s timulated the actin-saturated RSV transcription by 2.5- to 3-fold. Recombin ant profilin, expressed in bacteria, stimulated viral transcription as effe ctively as the native protein and was also inhibited by poly-L-proline, Pro filin alone, in the absence of actin, did not activate viral transcription. It is estimated that at optimal levels of transcription, every molecule of viral genomic RNA associates with approximately the following number of pr otein molecules: 30 molecules of, 120 molecules of phosphoprotein P, and 60 molecules each of actin and profilin, Together, these results demonstrated for the first time a cardinal role for profilin, an actin-modulatory prote in, in the transcription of a paramyxovirus RNA genome.