Norwalk Virus Nonstructural Protein p48 Forms a Complex with the SNARE Regulator VAP-A and Prevents Cell Surface Expression of Vesicular Stomatitis Virus G Protein

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Abstract
Norwalk virus (NV), a reference strain of human calicivirus in the Norovirus genus of the family Caliciviridae, contains a positive-strand RNA genome with three open reading frames. ORF1 encodes a 1,789-amino-acid polyprotein that is processed into nonstructural proteins that include an NTPase, VPg, protease, and RNA-dependent RNA polymerase. The N-terminal protein p48 of ORF1 shows no significant sequence similarity to viral or cellular proteins, and its function in the human calicivirus replication cycle is not known. The lack of sequence similarity to any protein in the public databases suggested that p48 may have a unique function in the NV replication cycle or, alternatively, may perform a characterized function in replication by a unique mechanism. In this report, it is shown that p48 displays a vesicular localization pattern in transfected cells when fused to the fluorescent reporter EYFP. A predicted transmembrane domain at the C terminus of p48 was not necessary for the observed localization pattern, but this domain was sufficient to redirect localization of EYFP to a fluorescent pattern consistent with the Golgi apparatus. A yeast two-hybrid screen identified the SNARE regulator vesicle-associated membrane protein-associated protein A (VAP-A) as a binding partner of p48. Biochemical assays confirmed that p48 and VAP-A interact and form a stable complex in mammalian cells. Furthermore, expression of the vesicular stomatitis virus G glcyoprotein on the cell surface was inhibited when cells coexpressed p48, suggesting that p48 disrupts intracellular protein trafficking.