Vesicular stomatitis virus (VSV), a prototype of non-segmented negative strand RNA viruses, packages an
RNA-dependent RNA polymerase (L) which, together with an associated
phosphoprotein (P), transcribes the genome
RNA, in vitro and in vivo, into mRNAs that are capped at the 5'-ends. However, unlike cellular guanlylyltransferase (GT), the
RNA polymerase incorporates
GDP in the capped structure, as Gp(alpha)p(beta)-p(alpha)A. In an effort to characterize the capping activity of the
RNA polymerase, we have purified recombinant L (rL)
protein expressed in insect cells. The rL, like the virion L polymerase, also caps transcribed mRNAs with identical unique cap structure. Interestingly, the purified rL is found to be tightly bound to the GT of the insect cell during all stages of purification. VSV grown in baby hamster kidney cells also packages cellular GT of the murine cell, suggesting that VSV L
protein or its associated
proteins may have a strong affinity for the cellular GT. The GT bound to rL, however, formed E-GMP complex, whereas no such complex was detected with the rL
protein. It appears that the L
protein may contain the putative active site for the unique capping reaction or the tightly bound cellular GT may by some unknown mechanism participate in the unique capping reaction.