The Arabidopsis thaliana TRAPPIII subunit AtTRAPPC8/AtTRS85 is involved in ER functioning and autophagy

TRAPP (transport protein particle) tethering complexes are known for their function as Rab-GTPase exchange factors (GEFs). Two versions of the complex are considered functionally separate: TRAPPII, an activator of GTPases of the Rab11 family (RabA in plants) which functions in post-Golgi sorting, and TRAPPIII, activating the Rab1 family (RabD in plants) which regulates ER-to-Golgi trafficking and autophagy. In Arabidopsis thaliana, the TRAPPIII complex has been identified and its subunit composition established, but little is known about its functions. Here, we found that binary subunit interactions of the plant TRAPPIII complex are analogous to those of metazoan TRAPPIII, with the two large subunits TRAPPC8 and -C11 linking the TRAPP core and the small C12-C13 dimer. To gain insight into the functions of TRAPPIII in plants, we characterized two A. thaliana trappc8 mutants. The mutants display abnormalities in plant morphology, in particular in flower and seed development. They also have autophagic defects, constitutive ER stress response, and elevated levels of the ER lipid dolichol - an indispensable cofactor of protein glycosylation. These results show that plant TRAPPC8 is involved in multiple trafficking steps in the cells and they suggest a novel link between ER membrane turnover and dolichol levels. HIGHLIGHTArabidopsis thaliana TRAPPC8 is necessary for correct functioning of the ER, in particular for its lipid homeostasis. Dysruption of TRAPPC8 leads to defects in secretion, autophagosome formation, and plant development.