Two Nonoverlapping Domains on the Norwalk Virus Open Reading Frame 3 (ORF3) Protein Are Involved in the Formation of the Phosphorylated 35K Protein and in ORF3-Capsid Protein Interactions

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Abstract
Expression of the Norwalk virus open reading frame 3 (ORF3) in Spodoptera frugiperda (Sf9) cells yields two major forms, the predicted 23,000-molecular-weight (23K) form and a larger 35K form. The 23K form is able to interact with the ORF2 capsid protein and be incorporated into virus-like particles. In this paper, we provide mass spectrometry evidence that both the 23K and 35K forms are composed only of the ORF3 protein. Two-dimensional gel electrophoresis and phosphatase treatment showed that the 35K form results solely from phosphorylation and that the 35K band is composed of several different phosphorylated forms with distinct isoelectric points. Furthermore, we analyzed deletion and point mutants of the ORF3 protein. Mutants that lacked the C-terminal 33 amino acids (ORF31-179, ORF31-152, and ORF31-107) no longer produced the 35K form. An N-terminal truncation mutant (ORF351-212) and a site-directed mutant (ORF3T201V) were capable of producing the larger form, which was converted to the smaller form by treatment with protein phosphatase. These data suggest that the region between amino acids 180 and 212 is phosphorylated, and mass spectrometry showed that amino acids Arg196 to Arg211 are not phosphorylated; thus, phosphorylation of the serine-threonine-rich region from Thr181 to Ser193 must be involved in the generation of the 35K form. Studies of the interaction between the ORF2 protein and full-length and mutated ORF3 proteins showed that the full-length ORF3 protein (ORF3FL), ORF31-179, ORF31-152, and ORF351-212 interacted with the ORF2 protein, while an ORF31-107 protein did not. These results indicate that the region of the ORF3 protein between amino acids 108 and 152 is responsible for interaction with the ORF2 protein.