The HMG box of SRY is a calmodulin binding domain

The HMG box domain of the testis determining factor, SRY, includes a basic amphiphilic sequence common to calmodulin (CaM) binding proteins. By affinity chromatography, native gel electrophoresis and fluorescence spectroscopy, we show the calcium-dependent binding of SRY to CaM. Binding occurs via the HMG box and an SRY peptide of residues 57-80 binds CaM like the intact domain. SRY/CaM complex formation is specifically inhibited by the SRY DNA binding site sequence, AACAAT, but not a mutated sequence. Fluorescence spectra of the SRY/CaM complex indicate 1:1 stoichiometry and that binding is accompanied by a conformational change in SRY. The A domain of HMG1 also binds CaM and we propose that CaM binding is a property of the wider HMG box family, including SOX and TCF/LEF proteins. These results suggest that CaM may regulate the DNA binding activity of HMG box transcription factors.