Semliki Forest virus capsid protein expressed by a baculovirus recombinant.

We have constructed a recombinant baculovirus which expressed the Semliki Forest (SFV) capsid (C) gene as a fusion protein under the control of the polyhedrin gene promoter. The sequence coding for C and part of the envelope E3 region were expressed as a polyprotein precursor. Spodoptera frugiperda (Sf9) insect cells infected with the recombinant virus produced a protein reacting with polyclonal rabbit antiserum that had been raised against the 33 kDa authentic C protein purified from SFV. Primer extension analyses showed that transcription from the polyhedrin promoter started in the late/very late consensus initiation motif. Due to its autoprotease activity, the recombinant C protein was contranslationally cleaved within its C-terminus and the mature protein remained undegraded for long periods of time. Spodoptera frugiperda cells infected with recombinant virus synthesized large amounts of C protein. The recombinant protein was karyophilic as is authentic SFV C protein and it was translocated into the nucleus; there, it was associated with nucleolus-like structures.