An unusual aspartic acid cluster in the reovirus attachment fiber σ1 mediates stability at low pH

The reovirus attachment protein {sigma}1 mediates cell attachment and receptor binding and is thought to undergo conformational changes during viral disassembly. {sigma}1 is a trimeric filamentous protein with an -helical coil-coiled Tail, a triple {beta}-spiral Body, and a globular Head. The Head domain features an unusual and conserved aspartic acid cluster at the trimer interface, which forms the only significant intra-trimer interactions in the Head, and must be protonated to allow trimer formation. Here we show that all domains of {sigma}1 are remarkably thermostable across a wide range of pH, even at the low pH of the stomach. Interestingly, we determine the optimal pH for stability to be between pH 5-6, a value close to the pH of the endosome and of the jejunum. The {sigma}1 Head is stable at acidic and neutral pH, but detrimerizes at basic pH. When Asp345 in the aspartic acid cluster is mutated to asparagine, the {sigma}1 Head loses stability at low pH and is more prone to detrimerize. Overall, the presence of the Body stabilizes the {sigma}1 Head. Our results confirm a role of the aspartic acid cluster as a pH-dependent molecular switch, and highlight its role in enhancing {sigma}1 stability at low pH.