Unfolding of bZIP dimers formed by the ATF-2 and c-Jun transcription factors is not a simple two-state transition

The varied selectivity of bZIP transcription factors stems from the fact that they are dimers consisting of two not necessarily identical subunits held together by a leucine zipper dimerization domain. Determining their stability is therefore important for understanding the mechanism of formation of these transcription factors. The most widely used approach for this problem consists of observing temperature-induced dissociation of the bZIPs by any means sensitive to their structural changes, particularly optical methods. In calculating thermodynamic characteristics of this process from such data it is usually assumed that it represents a two-state transition. However, scanning micro-calorimetric study of the temperature-induced unfolding/dissociation of the three bZIPs formed by the ATF-2 and c-Jun proteins, i.e. the two homodimers (ATF-2/ATF-2) and (c-Jun/c-Jun) and the heterodimer (ATF-2/c-Jun), showed that this process does not represent a two-state transition, as found previously with the GCN4 homodimeric bZIP protein. This raises doubt about all indirect estimates of bZIP thermodynamic characteristics based on analysis of their optically-observed temperature-induced changes.