Charged Residues in the Membrane Anchor of the Pestiviral E Protein Are Important for Processing and Secretion of E and Recovery of Infectious Viruses.

The pestivirus envelope protein E is anchored in membranes via a long amphipathic helix. Despite the unusual membrane topology of the E membrane anchor, it is cleaved from the following glycoprotein E1 by cellular signal peptidase. This was proposed to be enabled by a salt bridge-stabilized hairpin structure (so-called charge zipper) formed by conserved charged residues in the membrane anchor. We show here that the exchange of one or several of these charged residues reduces processing at the E carboxy-terminus to a variable extend, but reciprocal mutations restoring the possibility to form salt bridges did not necessarily restore processing efficiency. When introduced into an E-only expression construct, these mutations enhanced the naturally occurring E secretion significantly, but again to varying extents that did not correlate with the number of possible salt bridges. Equivalent effects on both processing and secretion were also observed when the proteins were expressed in avian cells, which points at phylogenetic conservation of the underlying principles. In the viral genome, some of the mutations prevented recovery of infectious viruses or immediately (pseudo)reverted, while others were stable and neutral with regard to virus growth.