DNA-bound Bas1 recruits Pho2 to activate ADE genes in Saccharomyces cerevisiae.

Expression of the genes in the ADE regulon of Saccharomyces cerevisiae is repressed by the presence of purine bases in the extracellular medium and derepressed when cells are grown in the absence of purines. Derepression requires the transcriptional activators Bas1 and Pho2, as well as the biosynthetic intermediates 5'-phosphoribosyl-4-succinocarboxamide-5-aminoimidazole (SAICAR) and 5'-phosphoribosyl-4-carboxamide- 5-aminoimidazole (AICAR). In this study, we investigated if nuclear localization and binding to promoter DNA by the activators are regulated by purines.

Functional mapping of Bas2. Identification of activation and Bas1-interaction domains.

The transcriptional activator protein Bas2 is required to express more than 20 genes in pathways for purine nucleotide and histidine biosynthesis, phosphate utilization, and the HO endonuclease by acting with co-regulator proteins Bas1, Pho4, and Swi5. The role that Bas2 plays in transcriptional activation may be to unmask latent activation domains in the co-regulator and to promote ternary complex formation between Bas2, the co-regulator, and DNA. We show that Bas2 also contributes to transcriptional activation by providing an activation domain.

The Vps27p Hse1p complex binds ubiquitin and mediates endosomal protein sorting.

Membrane proteins that are degraded in the vacuole of Saccharomyces cerevisiae are sorted into discrete intralumenal vesicles, analogous to the internal membranes of multi-vesiculated bodies (MVBs). Recently, it has shown that the attachment of ubiquitin (Ub) mediates sorting into lumenal membranes. We describe a complex of Vps27p and Hse1p that localizes to endosomal compartments and is required for the recycling of Golgi proteins, formation of lumenal membranes and sorting of ubiquitinated proteins into those membranes.