FIG. 8.
Model of SOX-partner factor interactions essential for activation of a specific target gene by a SOX protein. (A) Functional domains of SOX proteins. Major transactivation potential is mapped to the distal portion of the C-terminal domain for both SOX2 (25) and SOX9 (34, 43). The interface region possibly interacting with the partner factors is indicated by broken lines. (B) Model for selection of a target site by SOX1/2/3 and SOX9. In the absence of the partner factor, binding of the SOX proteins to the SOX site occurs with low affinity (Kd of 3 × 10−9 M for SOX1 binding to DC5) and is not stable enough to exert a significant effect in an in vivo situation. Only when the partner factor lies next to the SOX site do SOX proteins more stably bind to their sites (e.g., Kd of 10−11 M, assuming affinity analogous to that for the GAL4DBD-GAL4 binding site) and elicit transactivation.