Analysis of the proteolytic processing and activation of the rice tungro bacilliform virus reverse transcriptase.

Rice tungro bacilliform virus (RTBV) is a plant pararetrovirus and member of the badnavirus subgroup. Open reading frame (ORF) 3 encodes the viral capsid protein, protease (PR), and reverse transcriptase (RT). A DNA fragment of ORF 3 that contains PR and RT sequences was previously expressed in insect cells to produce the PR/RT polyprotein that was processed to yield p62 and p55. p62 and p55 share common N-terminal amino acid sequences and exhibit reverse transcriptase activity. Mass spectrometry was employed to determine the precise molecular weight of the p62 and p55 proteins and enabled determination of the C-termini for both proteins. ORFs encoding either p62 or p55 were constructed and expressed in insect cells using the baculoviruses 62R-BBac and 55R-BBac, respectively. The recombinant p62R and p55R proteins were purified separately and shown to have the same enzymatic activities as previously reported for the processed p62 and p55. The putative active site of the PR was mutated (mpr), and the resulting mpr/RT ORF was expressed in insect cells using the baculovirus mpr/RT-BBac. The mpr/RT polyprotein was not processed in insect cells, resulting in the accumulation of the approximately 87-kDa mpr/RT polyprotein. This study further extends the understanding of p62 and p55 and clarifies the role of the RTBV PR in processing of the RT.