The tryptophan cluster: a hypothetical structure of the DNA-binding domain of the myb protooncogene product.

In the DNA-binding domain of the c-myb protooncogene product (c-Myb) which consists of three repeats of 51-52 amino acids, there are 3 perfectly conserved tryptophans in each repeat. Site-directed mutagenesis of these tryptophans showed that any single or multiple mutations of tryptophan to hydrophilic residues or alanine abolished or greatly reduced the sequence-specific DNA-binding activity, but mutations to hydrophobic amino acids retained considerable activity. Raman spectroscopic study showed that these tryptophans were buried in the protein core. These 3 tryptophans are proposed to form a cluster in the hydrophobic core in each repeat. This hypothetical structure is referred to as the "tryptophan cluster," and it may represent a characteristic property of a group of DNA-binding proteins including the myb- and ets-related proteins.