Environmentally induced reversible conformational switching in the yeast cell adhesion protein α‐agglutinin

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Abstract
The yeast cell adhesion protein α-agglutinin is expressed on the surface of a free-living organism and is subjected to a variety of environmental conditions. Circular dichroism (CD) spectroscopy shows that the binding region of α-agglutinin has a β-sheet-rich structure, with only ∼2% α-helix under native conditions (15–40°C at pH 5.5). This region is predicted to fold into three immunoglobulin-like domains, and models are consistent with the CD spectra as well as with peptide mapping and site-specific mutagenesis. However, secondary structure prediction algorithms show that segments comprising ∼17% of the residues have high α-helical and low β-sheet potential. Two model peptides of such segments had helical tendencies, and one of these peptides showed pH-dependent conformational switching. Similarly, CD spectroscopy of the binding region of α-agglutinin showed reversible conversion from β-rich to mixed α/β structure at elevated temperatures or when the pH was changed. The reversibility of these changes implied that there is a small energy difference between the all-β and the α/β states. Similar changes followed cleavage of peptide or disulfide bonds. Together, these observations imply that short sequences of high helical propensity are constrained to a β-rich state by covalent and local charge interactions under native conditions, but form helices under non-native conditions.