Studies on the conditions required for structural and functional maturation of rabies virus glycoprotein (G) in G cDNA-transfected cells

When the rabies virus G cDNA was expressed with the help of T7 RNA polymerase provided by a recombinant vaccinia virus (RVV-T7), functional G proteins were produced in terms of their ability to induce low pH-dependent syncytium formation and the formation of conformational epitopes, including the acid-sensitive epitope recognized by mAb #1-30-44. Such an ability and the 1-30-44 epitope formation, however, were not associated with the G gene products when G cDNA was expressed without the help of RVV-T7 using a tetracycline-regulated expression vector (pTet-G), although they were normally transported to the surface of established G protein-producing BHK-21 (G-BHK) cells. But, when the G-BHK cells were treated with 2.5 mm sodium butyrate (NaB) after the removal of tetracycline, we could observe not only a much increased frequency of G protein-producing cells, but also the greatly enhanced maturation of the protein. Another short acylate, sodium propionate (NaP), similarly induced increased G protein synthesis at a concentration of 2.5 mm as NaB; however, such proteins were mostly not endowed with the fusion activity nor the 1-30-44 epitope, while NaP at a higher concentration as 5.0 mm did induce similarly the increased production and enhanced maturation of G protein, including the 1-30-44 epitope formation. From these results, we conclude that functional maturation of G protein to acquire fusogenic activity is correlated with 1-30-44 epitope formation, and 2.5 mm NaB not only stimulates G protein production, but also provides such cellular conditions as are required for the structural and functional maturation of the protein.