Primary structure and post-translational processing of the Berne virus peplomer protein.

The nucleotide sequence of the peplomer (P) protein gene of Berne virus (BEV), the torovirus prototype, was determined. The gene encodes an apoprotein of 1581 amino acids with an Mr of about 178K. The open reading frame was cloned behind the T7 RNA polymerase promoter and its translation product was identified as the BEV P protein precursor by in vivo expression and immunoprecipitation. The deduced amino acid sequence contains a number of domains which are typical for type I membrane glycoproteins: an N-terminal signal sequence, a putative C-terminal transmembrane anchor, and a cytoplasmic tail. Eighteen potential N-glycosylation sites, two heptad repeat domains, and a possible "trypsin-like" cleavage site were identified. The mature P protein consists of two subunits and their electrophoretic mobility upon endoglycosidase F treatment strongly suggests that the predicted cleavage site is functional in vivo. The heptad repeat domains are probably involved in the generation of an intra-chain coiled-coil secondary structure; similar inter-chain interactions can play a role in P protein oligomerization. Using a sucrose gradient assay the P protein was indeed shown to form dimers. The intra- and inter-chain coiled-coil interactions may stabilize the elongated BEV peplomers.