Meta-Biol

• Pathways

Once MHC class II-peptide complexes are formed, they must be transported back to the cell surface. It is unclear how this occurs. LEs/lysosomes with peptide loaded MHC II molecules may move in a bidirectional manner in a stop-and-go fashion along microtubules to the plasma membrane, driven by the activities of the oppositely-directed motor proteins dynein and kinesin (Wubbolts et al. 1996, 1999, Chow et al. 2002, Rocha & Neefjes 2007). Ultimately, LEs/lysosomes fuse to the plasma membrane delivering the MHC II-peptide complexes to the surface (Raposo et al. 1996, Rocha & Neefjes 2007). RAB7-GTP present on LEs/lysosome membrane interacts with Rab7-interacting lysosomal protein (RILP) and oxysterol-binding protein-related protein 1L (ORP1L) to form a tripartite RILP-Rab7-ORP1L complex. RILP binds to the p150 dynactin subunit to recruit the dynein or kinesin motor proteins. ORP1L recruits this complex to betaIII spectrin domains, which appears to be critical for dynein motor activation and transport of LEs/lysosome vesicles to the cell periphery (Johansson et al. 2007, Rocha & Neefjes 2008).