Enzyme involved in mRNA capping (Potential). Binds to GTP and might have guanylyltransferase activity. Together with the RNA-directed RNA polymerase P1 and protein P7, forms an transcriptional complex positioned near the channels situated at each of the five-fold vertices of the core (Probable). a 5'-end diphospho-ribonucleoside in mRNA + GTP + H(+) = a 5'-end (5'-triphosphoguanosine)-(ribonucleoside) in mRNA + diphosphate mRNA processing; mRNA capping. Located inside the inner capsid (PubMed:20190042). Found in the interior of spherical cytoplasmic structures, called virus factories, that appear early after infection and are the site of viral replication and packaging. Belongs to the phytoreovirus protein P5 family.