| Summary: | Pestiviruses express the unique essential envelope protein E<sup>rns</sup>, which exhibits RNase activity, is attached to membranes by a long amphipathic helix, and is partially secreted from infected cells. The RNase activity of E<sup>rns</sup> is directly connected with pestivirus virulence. Formation of homodimers and secretion of the protein are hypothesized to be important for its role as a virulence factor, which impairs the host’s innate immune response to pestivirus infection. The unusual membrane anchor of E<sup>rns</sup> raises questions with regard to proteolytic processing of the viral polyprotein at the E<sup>rns</sup> carboxy-terminus. Moreover, the membrane anchor is crucial for establishing the critical equilibrium between retention and secretion and ensures intracellular accumulation of the protein at the site of virus budding so that it is available to serve both as structural component of the virion and factor controlling host immune reactions. In the present manuscript, we summarize published as well as new data on the molecular features of E<sup>rns</sup> including aspects of its interplay with the other two envelope proteins with a special focus on the biochemistry of the E<sup>rns</sup> membrane anchor.
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