EXPRESSION OF L-PROTEIN OF VESICULAR STOMATITIS-VIRUS INDIANA SEROTYPE FROM RECOMBINANT BACULOVIRUS IN INSECT CELLS - REQUIREMENT OF A HOSTFACTOR(S) FOR ITS BIOLOGICAL-ACTIVITY IN-VITRO

The 241-kDa large (L) protein of vesicular stomatitis virus (VSV) Indi ana serotype, a multifunctional catalytic subunit of the viral RNA pol ymerase, has been expressed in Spodoptera frugiperda cells infected wi th recombinant baculovirus BacPAK6-L containing the L gene under the c ontrol of a polyhedrin promoter. The recombinant L protein was biologi cally active and supported viral MRNA synthesis in vitro. When the exp ressed L protein was purified by phosphocellulose column chromatograph y, it eluted in two peaks, one at 0.4 M NaCl (peak I) and the second a t 0.75 M NaCL (peak II). The L protein in peak I showed significant tr anscriptional activity in an in vitro transcription reconstitution exp eriment, whereas the L protein in peak II was inactive. Interestingly, the addition of cytoplasmic extract from uninfected Sf21 cells to pea k II completely restored transcription in vitro, indicating the requir ement of a host factor(s) for the activity of the L protein. This fact or is relatively heat stable and is dissociable from the recombinant L protein. It is also present in BHK, COS, and HeLa cells in detectable levels. The role of the putative host protein(s) in the activation of the L protein is discussed.