A new type of intracellular retention signal identified in a pestivirus structural glycoprotein

Sorting of membrane proteins into intracellular organelles is crucial for cell function. Viruses exploit intracellular transport and retention systems to concentrate envelope proteins at the site of virus budding. In pestiviruses, a group of important pathogens of pigs and ruminants closely related to human hepatitis C virus, the E-rns protein translated from the viral RNA is secreted from the infected cells and found in the serum of infected animals. Secretion of the protein is regarded as crucial for its function as a viral virulence factor associated with its RNase activity. However, similar to 95% of the E-rns molecules are retained within the infected cell. Fusion of different Erns fragments to the C terminus of CD72 allowed identification of a retention signal within the C-terminal 65 aa of the viral protein. This C-terminal sequence represents its membrane anchor and folds into an amphipathic helix binding in-plane to the membrane surface. Residues L183, I190, and L208 are important for intracellular location of E-rns. Presentation of the retention signal on the cytoplasmic instead of the luminal face of the ER membrane in CD8 alpha fusion proteins still led to retention. Thus, E-rns contains in its C-terminal amphipathic helix an intracellular retention signal that is active on both faces of the membrane.-Burrack, S., Aberle, D., Burck, J., Ulrich, A. S., Meyers, G. A new type of intracellular retention signal identified in a pestivirus structural glycoprotein. FASEB J. 26, 3292-3305 (2012). www.fasebj.org