Uncovering the molecular mechanisms regulating protein sorting in the endosomal/lysosomal system: implications in microbial clearance

Endocytic membrane traffic in mammalian cells has an essential role in delivering membrane components, receptor associated ligands and solute molecules to various intracellular destinations. A large number of cellular processes such as nutrient uptake, proliferation, apoptosis and other responses to extracellular signals depend upon this basic process. Similarly, dysregulation of endocytic transport has many clinical implications. For example, many diseases including cancer, lysosomal storage disorders, neurodegeneration, myopathies, liver diseases and immune dysfunction are caused by intracellular protein transport defects. The process of endocytosis begins with internalization of cargo/protein into small vesicles that quickly fuse with early endosomes or sorting endosomes. The internalized cargo is then recycled via an endocytic recycling compartment (ERC) or targeted for degradation in the lysosome. The RAS superfamily of monomeric GTP-binding proteins, or small GTPases, comprises a large family of regulatory molecules that collectively regulate endocytic trafficking in the cell. Among these proteins, the RAB, ARF and ARL GTPases play key roles in the control of intracellular membrane trafficking. We and others have recently identified ARL8b, a member of the ARL family, as the first small GTPase that localize to the lysosomes. We found that Arl8b knockdown results in a delay in delivery of pinocytosed and endocytosed cargo to lysosomes. Furthermore, ARL8b was found to bind and recruit subunits of the HOPS (HOmotypic fusion and Protein Sorting) complex to lysosomes. Since in specialized cell types such as dendritic cells, lysosomes also serve as the antigen loading compartment for receptors like CD1 and MHC class II, we therefore analyzed the role of ARL8b in antigen presentation. ARL8b knockdown and loss of HOPS recruitment to lysosomes results in defective trafficking of CD1 molecules to lysosomes, delayed formation of CD1•lipid antigen complexes and corresponding defects in T-cell activation. Furthermore, phagosome to lysosome delivery and fusion were delayed by ARL8b knockdown resulting in a defect in microbial killing. Together these findings mark ARL8b as a novel regulator of lysosomal trafficking pathways of special significance for host defense.