Members of the large SOX family of transcription factors are widely conserved and at least 20 are found in mammals. Structurally SOX proteins exhibit a high mobility group (HMG) motif that binds the DNA minor groove. SOX family members are often found in multiprotein complexes that regulate their ability to affect transcription. SOX proteins play important roles in early development and are often used as markers to assess the differentiation of specific cell lineages.
SOX5 (SRY-related HMG box) is a 75 - 80 kDa Group D member of the SOX family of DNA-binding proteins. It is expressed in fetal chondrocytes and skeletal muscle, with a short form appearing in spermatids. SOX5 form homodimers and heterodimerizes with SOX6. Human SOX5 is 763 amino acids (aa) in length and contains two coiled-coil regions (aa 193 - 274 and 448 - 515), plus one Q/glutamine (aa 234 - 272) and one HMG box (aa 556 - 624). HMG domains participate in DNA-binding and protein-protein interaction. There are two alternate start sites at Met14 and Met417 (which creates a short/testis form), plus one variant with a three aa substitution for aa 389 - 763, another variant with a two aa substitution for aa 1 - 388, and a third variant with a one aa substitution for aa 534 - 763. Over aa 417 - 763, human SOX5 is 97% aa identical to mouse SOX5.