Trs20 is required for TRAPP III complex assembly at the PAS and its function in autophagy.

The modular TRAPP complex acts as a guanine-nucleotide exchange factor (GEF) for Ypt/Rab GTPases. Whereas TRAPP I and TRAPP II regulate the exocytic pathway, TRAPP III functions in autophagy. The TRAPP subunit Trs20 is not required for assembly of core TRAPP or its Ypt1 GEF activity. Interestingly, mutations in the human functional ortholog of Trs20, Sedlin, cause spondyloepiphyseal dysplasia tarda (SEDT), a cartilage-specific disorder. We have shown that Trs20 is required for TRAPP II assembly and identified a SEDT-linked mutation, Trs20-D46Y, which causes a defect in this process. Here we show that Trs20 is also required for assembly of TRAPP III at the pre-autophagosomal structure (PAS). First, recombinant Trs85, a TRAPP III-specific subunit, associates with TRAPP only in the presence of Trs20, but not Trs20-D46Y mutant protein. Second, a TRAPP complex with Ypt1 GEF activity co-precipitates with Trs85 from wild type, but not trs20ts mutant, cell lysates. Third, live-cell colocalization analysis indicates that Trs85 recruits core TRAPP to the PAS via the linker protein Trs20. Finally, trs20ts mutant cells are defective in selective and non-selective autophagy. Together, our results show that Trs20 plays a role as an adaptor in the assembly of TRAPP II and TRAPP III complexes, and the SEDT-linked mutation causes a defect in both processes.