The yeast global transcriptional repressor Tup1 contains 7 WD repeats in its C-terminus that form a beta-propeller-like structure, in which the first and last WD repeats interact to make a closed circle. The WD domains of all proteins tested, including Tup1, form a compact structure resistant to trypsin digestion (Garcia-Higuera et al., Biochemistry 35 (1996) 13985-13994). We found that the in vitro formation of the trypsin-resistant core of Tup1 requires just five WD repeats (WD2-6). Deletion of the ST region between WD1 and WD2 destabilizes the trypsin-resistant core, but maintains Tup1 repression function in vivo. Linker insertion and point mutations in the WD repeats that compromise Tup1 repression function in vivo still maintain the trypsin-resistant core in vitro These results indicate that structural perturbation of the WD domain structure cannot explain the effects of these mutations on Tup1 repression function.