Post-translational processing of the tobacco etch virus 49-kDa small nuclear inclusion polyprotein: identification of an internal cleavage site and delimitation of VPg and proteinase domains.

The 49,000-dalton (49-kDa) small nuclear inclusion (NI) protein of tobacco etch virus (TEV) has two distinct functions associated with it. An N-terminal segment is covalently attached to the genomic length RNA and likely involved in RNA replication, while the C-terminal half is associated with a proteolytic activity critical for genome expression. The junction delineating these two proteins has not been identified. We have analyzed naturally occurring cleavage products of TEV NI proteins and have identified a possible internal cleavage site between Glu and Gly residues at TEV 49-kDa NI protein amino acids 189-190. Similar 49-kDa-derived products are formed in cell-free translation studies in minor amounts upon the addition of excess amounts of NI protein. Cleavage of the 49-kDa (430 amino acids) protein is predicted to result in the formation of two products, 21-kDa (189 amino acids) and 27 kDa (241 amino acids) in size. Complementary DNA encoding the 27-kDa C-terminal portion of the 49-kDa protein gene was cloned into various DNA sequences. This allowed us to express the 27-kDa protein alone or as part of higher molecular weight polyproteins containing flanking TEV or foreign protein sequences. This 27-kDa amino acid sequence had a proteolytic activity similar to the 49-kDa-associated activity.