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. 2009 Jul 17;5(7):e1000567. doi: 10.1371/journal.pgen.1000567

Figure 5. Model for the initial steps of MMTV promoter activation.

Figure 5

Before hormone addition the MMTV promoter is silent and associated with a repressive complex that includes HP1γ (step 1). After hormone addition the activated complex of pPR-pErk-pMsk, as well as PCAF and BAF, are recruited to the MMTV promoter. For simplicity PR is shown as a monomer, though the active form is a homodimer. Msk and PCAF phosphoacetylates H3 leading to H3S10phK14ac (step 2). This modification displaces the repressive complex and anchors the BAF complex, enabling ATP-dependent H2A/H2B displacement (step 3). The nucleosome opening facilitates NF1 binding generating a stable platform that exposes previously hidden HREs for the recruitment of additional PR and BAF complexes, coactivator and eventually promoter activation (step 4). Depletion of BAF prevents progression of the activation process to step 2: no histone displacement is observed, NF1 cannot bind and consequently less PR/BAF complexes are bound to the promoter. Depletion of PCAF has a similar effect, most likely by labilization of BAF binding, blocking the activation process at step 2.