The transition state for coupled folding and binding of a disordered DNA binding domain resembles the unbound state

The basic zippers (bZIPs) are one of two large eukaryotic families of transcription factors whose DNA binding domains are disordered in isolation but fold into stable -helices upon target DNA binding. Here we systematically disrupt pre-existing helical propensity within the DNA binding region of the homodimeric bZIP domain of cAMP-response element binding protein (CREB) using Ala-Gly scanning and examine the impact on target binding kinetics. We find that the secondary structure of the transition state strongly resembles that of the unbound state. The closest residue to the dimerisation domain that has been examined is largely folded within both unbound and transition states; dimerisation apparently propagates additional helical propensity into the basic region. The results are consistent with electrostatically-enhanced DNA binding, followed by rapid folding from the folded zipper outwards. Interestingly, despite taking the exact experimental approach suggested for testing it, we find no evidence for disorder-mediated rate enhancement predicted by fly-casting theory.