Transactivation domain

The transactivation domain or trans-activating domain (TAD) is a transcription factor scaffold domain which contains binding sites for other proteins such as transcription co-regulators. These binding sites are frequently referred to as activation functions (AFs).

TADs are named after their amino acid composition. These amino acids are either essential for the activity or simply the most abundant in the TAD. Transactivation by the Gal4 transcription factor is mediated by acidic amino acids, whereas hydrophobic residues in Gcn4 play a similar role. Hence, the TADs in Gal4 and Gcn4 are referred to as acidic or hydrophobic, respectively.

In general we can distinguish four classes of TADs:

• acidic domains (called also “acid blobs” or “negative noodles", rich in D and E amino acids, present in Gal4, Gcn4 and VP16).
• glutamine-rich domains (contains multiple repetitions like "QQQXXXQQQ", present in SP1)
• proline-rich domains (contains repetitions like "PPPXXXPPP" present in c-jun, AP2 and Oct-2)
• isoleucine-rich domains (repetitions "IIXXII", present in NTF-1)

Alternatively, since similar amino acid compositions does not necessary mean similar activation pathways, TADs can be grouped by the process they stimulate, either initiation or elongation.

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