The carboxy-terminal tail of the homeo domain protein alpha 2 is required for function with a second homeo domain protein.

The homeo domain protein alpha 2 from Saccharomyces cerevisiae has two roles in the a/alpha cell: With MCM1, alpha 2 turns off transcription of a-specific genes; with a1 (a second homeo domain protein), alpha 2 represses transcription of haploid-specific genes. From the carboxy-terminal side of the alpha 2 homeo domain extends an unstructured 22-amino-acid residue tail. In this paper we show that the carboxy-terminal tail of alpha 2 is required for formation of a stable a1/alpha 2-operator complex and is thus required for a1/alpha 2-mediated repression of transcription. In contrast, the tail is dispensable for alpha 2/MCM1-mediated repression. These results indicate that a short, unstructured tail mediates the interaction between two homeo domain proteins.