Free peptide is a co-factor in DM-mediated peptide release.

<p>(a) Competitive binding analysis of P1, P3 and P7 MTSL-Cys substituted HA peptide variants to DR1. Data represent the mean and SD of three independent experiments. Lines indicate the fit of the data to a logistic equation. <i>K</i>_D values for the peptides as listed in the legend are respectively 83.3 nM, 28.8 μM, 237.9 nM and 404.7 nM. (b) Top view of the HA peptide P7 Leu→Cys substituted, labeled at this position with MTSL (HAsp7) and complexed with HLA-DR. The peptide is in orange, the α-chain is in green and the β-chain in yellow. The most energetically favored orientation of the probe (white) is shown. The model was generated using PyMol <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0003722#pone.0003722-DeLano1" target="_blank">[45]</a>. Coordinates taken from ref. 41. (c) Spectra of HAsp7 peptide as recorded when free in solution (top), completely bound to HLA-DR (center) or as a composite of the two states (bottom). The broadening of the spectral lines and a slight shift in their positions in the spectrum appear evident. (d) Dissociation of HAsp7 peptide from DR1 was measured as described in Results. Data is plotted as the % of free (unbound) peptide detected at each time point as quantitated by spectral subtraction methods. Data points represent one of three independent experiments. Data points referring to the C sample at <i>t</i> = 0 and <i>t</i> = 24 are hidden below other data points.