Four domains of Ada1 form a heterochromatin boundary through different mechanisms

In eukaryotic cells, there are two chromatin states, silenced and active, and the formation of a so-called boundary plays a critical role in demarcating these regions; however, the mechanisms underlying boundary formation are not well understood. In this study, we focused on S. cerevisiae ADA1, a gene previously shown to encode a protein with a robust boundary function. Ada1 is a component of the histone modification complex Spt-Ada-Gcn5-acetyltransferase (SAGA) and the SAGA-like (SLIK) complex, and it helps to maintain the integrity of these complexes. Domain analysis showed that four relatively small regions of Ada1 (Region I; 66-75 aa, II; 232-282 aa, III; 416-436 aa and IV; 476-488 aa) have a boundary function. Among these, Region II could form an intact SAGA complex, whereas the other regions could not. Investigation of cellular factors that interact with these small regions identified a number of proteasome-associated proteins. Interestingly, the boundary functions of Region II and Region III were affected by depletion of Ump1, a maturation and assembly factor of the 20S proteasome. These results suggest that the boundary function of Ada1 is functionally linked to proteasome processes and that the four relatively small regions in ADA1 form a boundary via different mechanisms.