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MATERIAL AND METHODS

E rns is a structural glycoprotein of pestiviruses found to be attached to the virion and to membranes within infected cells via its C-terminus although it lacks a hydrophobic anchor sequence. The C-terminal sequence was hypothesized to fold into an amphipathic αhelix. Alanine insertion scanning revealed that the ability of the E rns C-terminus to bind membranes is considerably reduced by the insertion of a single amino acid at a wide variety of positions. Mutations decreasing the hydrophobicity of the apolar face of the putative helix led to reduction of membrane association. Proteinase K protection assays showed that E rns translated in vitro in the presence of microsomal membranes was protected whereas a mutant with an artificial transmembrane region and a short cytosolic tag was shortened by the protease treatment. A tag fused to the C-terminus of wt E rns was not accessible for antibodies within digitonin permeabilized cells but the variant with the tag located downstream of the artificial transmembrane region was detected under the same conditions. These results are in accordance with the model that the C-terminal membrane anchor of E rns represents an amphipathic helix embedded in plane into the membrane. The integrity of the membrane anchor was found to be important for recovery of infectious virus.