Picornain 3C-like protease is a thiol protease that cleaves at Gln-|-Gly or Gln-|-Ser sites in the P1 and P2 polyproteins. The VPg-NTB polyprotein may act as a membrane-anchor for the replication complex. a ribonucleoside 5'-triphosphate + RNA(n) = diphosphate + RNA(n+1) The NTB-VPg polyprotein is associated with endoplasmic-derived membranes that are active in viral replication. Specific enzymatic cleavages by picornain 3C-like protease in vivo yield mature proteins. Picornain 3C-like protease is autocatalytically processed. NTB exists as NTB-VPg polyprotein as well as NTB mature protein. VPg is uridylylated by the polymerase and is covalently linked to the 5'-end of genomic RNA. This uridylylated form acts as a nucleotide-peptide primer for the polymerase (By similarity). Belongs to the nepoviruses RNA1 polyprotein family. It is uncertain whether Met-1 or Met-122 is the initiator.