CD and Fluorescence Measurements.

CD and fluorescence spectra were recorded using an Aviv 425 circular dichroism spectrometer (Aviv Biomedical, Inc.) equipped with a fluorescence emission-scanning monochromator. The QTY protein sample was buffer exchanged by dialysis into CD buffer [10 mM sodium phosphate (pH 7.4), 150 mM NaF, 1 mM Tris(2-carboxyethyl)phosphine]. The sample was filtered through a 0.2-µm filter before measurement. For far UV CD, spectra between 183 nm and 260 nm were collected with a 1-nm step size, 1-nm bandwidth, and 15-s averaging time in 0.1-cm path length cuvettes. The protein concentration was ∼1.2 µM.