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Kalde, Monika; Elliott, Liam; Ravikumar, Raksha; Rybak, Katarzyna; Altmann, Melina; Klaeger, Susan; Wiese, Christian; Abele, Miriam; Al, Benjamin; Kalbfuss, Nils; Qi, Xingyun; Steiner, Alexander; Meng, Chen; Zheng, Huanquan; Kuster, Bernhard; Falter-Braun, Pascal; Ludwig, Christina; Moore, Ian and Assaad, Farhah F. (2019): Interactions between Transport Protein Particle (TRAPP) complexes and Rab GTPases in Arabidopsis. In: Plant Journal, Vol. 100, No. 2: pp. 279-297

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Transport Protein Particle II (TRAPPII) is essential for exocytosis, endocytosis, protein sorting and cytokinesis. In spite of a considerable understanding of its biological role, little information is known about Arabidopsis TRAPPII complex topology and molecular function. In this study, independent proteomic approaches initiated with TRAPP components or Rab-A GTPase variants converge on the TRAPPII complex. We show that the Arabidopsis genome encodes the full complement of 13 TRAPPC subunits, including four previously unidentified components. A dimerization model is proposed to account for binary interactions between TRAPPII subunits. Preferential binding to dominant negative (GDP-bound) versus wild-type or constitutively active (GTP-bound) RAB-A2a variants discriminates between TRAPPII and TRAPPIII subunits and shows that Arabidopsis complexes differ from yeast but resemble metazoan TRAPP complexes. Analyzes of Rab-A mutant variants in trappii backgrounds provide genetic evidence that TRAPPII functions upstream of RAB-A2a, allowing us to propose that TRAPPII is likely to behave as a guanine nucleotide exchange factor (GEF) for the RAB-A2a GTPase. GEFs catalyze exchange of GDP for GTP;the GTP-bound, activated, Rab then recruits a diverse local network of Rab effectors to specify membrane identity in subsequent vesicle fusion events. Understanding GEF-Rab interactions will be crucial to unravel the co-ordination of plant membrane traffic.

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