Equilibrium denaturation of X11 PDZ1-PDZ2 and its constituent isolated domains.A, CD-monitored equilibrium denaturation of X11 PDZ1-PDZ2, black circles, PDZ1, red circles, and PDZ2, green circles. Lines are the best fit to a two-state transition. B, fluorescence-monitored equilibrium unfolding of X11 PDZ1-PDZ2, black circles, and PDZ1, red circles. As discussed in the text, there was an excellent agreement between the thermodynamic parameters calculated for both PDZ1 and X11 PDZ1-PDZ2 when comparing CD and fluorescence. As detailed in the text, in the case of PDZ2, we could not detect any reliable transition due to the absence of Trp residues. Lines are the best fit to a two-state unfolding transitions. It is evident that, both by fluorescence and CD, the observed cooperativity of X-11 PDZ1-PDZ2 is remarkably higher than that observed for its isolated constituent domains. CD, circular dichroism.