The N-terminal amphipathic alpha-helix of viperin mediates localization to the cytosolic face of the endoplasmic reticulum and inhibits protein secretion.

Viperin is an evolutionarily conserved interferon-inducible protein that localizes to the endoplasmic reticulum (ER) and inhibits a number of DNA and RNA viruses. In this study, we report that viperin specifically localizes to the cytoplasmic face of the ER and that an amphipathic alpha-helix at its N terminus is necessary for the ER localization of viperin and sufficient to promote ER localization of a reporter protein, dsRed. Overexpression of intact viperin but not the amphipathic alpha-helix fused to dsRed induced crystalloid ER. Consistent with other proteins that induce crystalloid ER, viperin self-associates, and it does so independently of the amphipathic alpha-helix. Viperin expression also affected the transport of soluble but not membrane-associated proteins. Expression of intact viperin or an N-terminal alpha-helix-dsRed fusion protein significantly reduced secretion of soluble alkaline phosphatase and reduced its rate of ER-to-Golgi trafficking. Similarly, viperin expression inhibited bulk protein secretion and secretion of endogenous alpha(1)-antitrypsin and serum albumin from HepG2 cells. Converting hydrophobic residues in the N-terminal alpha-helix to acidic residues partially or completely restored normal transport of soluble alkaline phosphatase, suggesting that the extended amphipathic nature of the N-terminal alpha-helical domain is essential for inhibiting protein secretion.