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

¹ Department of Biological Sciences and the Institute for Biomolecular Structure and Function, Hunter College of the City University of New York, New York 10021,USA
² Department of Biochemistry and Microbiology, Cook College, Rutgers University, New Brunswick, New Jersey 08901, USA

Reprint requests to: Dr. Peter N. Lipke, Department of Biological Sciences, Hunter College, 695 Park Avenue, New York, NY 10021, USA; e-mail: lipke{at}genectr.hunter.cuny.edu; fax: 212-772-5227.

The yeast cell adhesion protein -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 -agglutinin has a ß-sheet-rich structure, with only 2% -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 -helical and low ß-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 -agglutinin showed reversible conversion from ß-rich to mixed /ß 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-ß and the /ß 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 ß-rich state by covalent and local charge interactions under native conditions, but form helices under non-native conditions.

Keywords: Cell adhesion protein; circular dichroism; conformational shift; glycoprotein; peptide conformation; secondary structure prediction

Abbreviations: Ig, immunoglobulin • CD, circular dichroism • CAPS, 3-(cyclohexylamino)-1-propanesulfonic acid • DTT, dithiothreitol • SDS, sodium dodecyl sulfate • EDTA, ethylenediaminetetraacetic acid • TFA, trifluoroacetic acid • HPLC, high performance liquid chromatography • TFE, trifluoroethanol

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