Characterization of poliovirus 3A protein: Membrane topology, epitope shielding, and inhibition of secretion

Because poliovirus replication occurs in the cytoplasm, the 3A-containing proteins were assumed to be cytosolic. However, their orientation with respect to the ER membrane was not fully examined. Previous studies have shown evidence to support a luminal, as well as a cytosolic, orientation for 3A. Therefore, to accurately interpret experimental results, it was important to investigate the topology of all 3A-containing proteins that accumulate during infection. Using two approaches, immunofluorescence analyses following differential permeabilization and proteinase susceptibility assays, the N-termini of 3A and 3AB, in addition to 3A-2, a mutant 3A protein containing an amino-terminal serine insertion, were shown to be cytosolic.; During poliovirus infection, the immunofluorescence signal of an amino-terminal epitope of 3A-containing proteins was found to be markedly shielded compared to 3A protein expressed in isolation. Shielding was more pronounced in cells infected with wild-type poliovirus than in cells with temperature-sensitive mutant virus that contains a mutation in the 3D polymerase coding region adjacent to the 3AB binding site. These results suggest that direct binding of the poliovirus RNA-dependent RNA polymerase may occlude the amino terminus of 3A-containing polypeptides in the RNA replication complex.; Poliovirus protein 3A, an 87-amino acid protein, is a potent inhibitor of protein secretion in mammalian cells, blocking protein traffic from the endoplasmic reticulum (ER) to the Golgi complex through an unknown mechanism. The function of viral protein 3A in blocking protein secretion is abrogated by mutations near the amino terminus of the protein. To identify specific residues in 3A that were crucial for secretion inhibition, site-directed mutagenesis was performed, and mutant 3A proteins were tested for their ability to inhibit secretion using a secreted alkaline phosphatase (SEAP) assay. The results indicated that there are at least two residues that are critical for poliovirus 3A-mediated inhibition of ER-to-Golgi protein traffic, Lys9 and Pro18. In addition, co-expression of an inhibition-deficient 3A protein with wild-type 3A protein did not result in loss of inhibition. This supports the hypothesis that mutations that abrogate secretion inhibition are loss of function mutations, perhaps resulting from the inability of mutant 3A to interact with its normal binding partner.