Single molecule imaging reveals the collective and independent search mechanisms of cFos and cJun on DNA

Abstract

AP-1 proteins are members of the basic leucine zipper (bZIP) protein family of dimeric transcription factors, responsible for controlling many integral cellular processes. These proteins form dimers with each other, and their aberrant expression can lead to a number of cancer types. The oncogenic transcription factor AP-1 binds its target TRE site (5’TCA[G/C]TGA), however the physical mechanism of how this is achieved is not understood. Such an understanding is essential to know how these proteins function, and could offer the potential to uncover new drug targets. The archetypal AP-1 complex is formed by cFos and cJun, which heterodimerise via their bZIP domains. Here, we set out to investigate how these proteins interact with DNA using a real-time single molecule fluorescence imaging approach. Using DNA tightropes as a substrate, we determine that the AP-1 bZIP dimers cJun:cFos and cJun:cJun rapidly scan DNA using a 1D diffusional search with an average diffusion constant of 0.14 µm2s-1 and 0.26 µm2s-1 respectively. Remarkably, we also found that cFos was able to bind to and diffuse on DNA (0.29 µm2s-1) both as a monomer and homodimer. Periods of diffusion were punctuated by pauses, suggesting a mechanism for how AP-1 may rapidly find its target sites on DNA. Taken together the results we have obtained indicate a considerably more complex and graded interaction between cFos, cJun and DNA than has been reported previously.