Acts as a GTPase-activating protein specific for RAB33B. Involved in the regulation of autophagosome maturation, the process in which autophagosomes fuse with endosomes and lysosomes. Interacts (via N-terminus) with MAP1LC3B, GABARAP and GABARAPL2. It is dispersed in the cytoplasm under nutrient-rich conditions. Localizes at autophagosomes under cell starving conditions.