The RAB-5 and RAB-7 GTPases regulate endosome to lysosome trafficking. Here, we show that Caenorhabditis elegans TBC-2 functions as a RAB-5 GAP. TBC-2 colocalizes with RAB-7 on late endosomes, and requires RAB-7 for membrane localization where TBC-2 could function to antagonize RAB-5 activity during early to late endosome maturation.

The Gal4p family of yeast zinc cluster proteins comprises regulators of multidrug resistance genes. For example, Pdr1p and Pdr3p bind as homo-or heterodimers to pleiotropic drug response elements (PDREs) found in promoters of target genes. Other zinc cluster activators of multidrug resistance genes include Stb5p and Yrr1p. To better understand the interplay among these activators, we have performed native co-immunoprecipitation experiments using strains expressing tagged zinc cluster proteins from their natural chromosomal locations. Interestingly, Stb5p is found predominantly as a Pdr1p heterodimer and shows little homodimerization. No interactions of Stb5p with Pdr3p or Yrr1p could be detected in our assays. In contrast to Stb5p, Yrr1p is only detected as a homodimer. Similar results were obtained using glutathione S-transferase pull-down assays. Importantly, the purified DNA binding domains of Stb5p and Pdr1p bound to a PDRE as heterodimers in vitro. These results suggest that the DNA binding domains of Pdr1p and Stb5p are sufficient for heterodimerization. Our data demonstrate a complex interplay among these activators and suggest that Pdr1p is a master drug regulator involved in recruiting other zinc cluster proteins to fine tune the regulation of multidrug resistance genes.