Macroautophagy is a bulk degradation system conserved in all eukaryotic cells. A 
ubiquitin-like protein, Atg8, and its homologues are essential for autophagosome 
formation and act as a landmark for selective autophagy of aggregated proteins 
and damaged organelles. In this study, we report evidence demonstrating that 
OATL1, a putative Rab guanosine triphosphatase-activating protein (GAP), is a 
novel binding partner of Atg8 homologues in mammalian cells. OATL1 is recruited 
to isolation membranes and autophagosomes through direct interaction with Atg8 
homologues and is involved in the fusion between autophagosomes and lysosomes 
through its GAP activity. We further provide evidence that Rab33B, an 
Atg16L1-binding protein, is a target substrate of OATL1 and is involved in the 
fusion between autophagosomes and lysosomes, the same as OATL1. Because both its 
GAP activity and its Atg8 homologue-binding activity are required for OATL1 to 
function, we propose a model that OATL1 uses Atg8 homologues as a scaffold to 
exert its GAP activity and to regulate autophagosomal maturation.