The SOX9 Dimerization Domain And Its Role in Cooperative DNA Binding

Nucleic acid binding proteins are key regulators in developmental processes and controlling the onset of diseases. Human SOX9 is a group E member of the SOX transcription family of proteins characterized by a conserved amino acid region first identified as the high mobility group (HMG) with high affinity for DNA. In parallel, a 38 amino acid region preceding the HMG domain described as the dimerization domain supports the formation of cooperative dimers on promoters with inverted sites, by binding to the HMG domain. Cooperative binding occurs when the binding at a first site increases the rate at which binding occurs at a second site between identical or non-identical proteins. Using mutagenesis and electrophoretic mobility shift assays (EMSA) the transdimerization model of dimeric SOX9 binding to DNA was studied. The molecular model of the SOX9 dimeric complex is characterized as a dimer by way of the amphipathic helix of the dimerization domain of one SOX9 protein docks on the HMG domain of an identical SOX9 protein in the DNA-bound state. Cooperative binding of one protein to the consensus-binding site may facilitate binding to the non-consensus site. This model provides insight to the characterized mutations associated with Campomelic dysplasia, an embryonic malformation of the skeletal and reproductive system.