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

Abstract

The reovirus attachment protein σ1 mediates cell attachment and receptor binding and is thought to undergo conformational changes during viral disassembly. σ1 is a trimeric filamentous protein with an α-helical coil-coiled Tail, a triple β-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 σ1 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 σ1 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 σ1 Head loses stability at low pH and is more prone to detrimerize. Overall, the presence of the Body stabilizes the σ1 Head.Our results confirm a role of the aspartic acid cluster as a pH-dependent molecular switch, and highlight its role in enhancing σ1 stability at low pH.