Yeast tbp forms tetramers and octamers in solution

The exact stoichiometry of yeast TATA-binding protein (yTBP) selfassociation has generated much controversy in the past few years. yTBP has boon proposed to exist in solution as dimers, tetramers, octamers or higher order oligomers, or combinations thereof. We used equilibrium analytical ultracentrifugation to investigate the stiochiometry and energetics of the self-assembly of yTBP over the salt concentration range of 60 mM to 1 mM KC1. The data are most consistent with an association model in which monomers (M) are in equilibrium with tetramers (T) and octamers (0). Furthermore, monomers, tetramers and octamers exist in the estimated physiological concentration range of TBP in the nucleus. Thus. yTBP self-association dictates the availability of monomeric (i.e., active) TBP available for transcription complex formation. Association is highly cooperative with octamer formation being favored by 7 kcal/mol over tetramers. At high [salt], the concentration of tetramers becomes negligible and the data are consistent with a monorner-octamer reaction mechanism. For each association step (M-T and T-0), the data are highly bipha.sk a a function of [salt], showing a decrease in Kassoc from 60 mM to 300 mM KG and an increase in Kassoc from 300 mM to l M KC1. Analysis of these [salt] dependences reveal that 3 moles of ions are absorbed at each association slop at low [salt] and 3 moles of ions are released at high [salt]. These results suggest that there is a [salt] dependent change in the mechanism of association of yTBP. which we propose arises from a change in ihe conformation of the N'-terminal domain.