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The transport protein particle (TRAPP) is a hetero-multimeric complex involved in the trafficking of COP II (coat protein complex II) vesicles. TRAPP is present in different eukaryotes from yeast to vertebrates and occurs in three distinct modifications with function in different intracellular transport steps. All forms contain a core of five essential subunits, and the different species of TRAPP are formed by the addition of various subunits. A recently identified TRAPP-associated protein, Tca17, is supposed to be involved in the regulation of the transport complex. We have determined the three-dimensional structure of yeast Tca17 by X-ray crystallography at a resolution of 1.8 Å. It adopts the longin fold characteristic for the Bet5 family of TRAPP subunits, and it also shares a binding motif of these for the interaction with other members of the complex. Two alternative models of the localization of Tca17 within TRAPP as well as its potential role in the regulation of TRAPP function by transient integration into the complex are discussed.Structural data are available in the Protein Databank under accession number 3PR6.The transport protein particle (TRAPP) is a hetero-multimeric complex involved in the trafficking of COP II vesicles. The three-dimensional structure of the TRAPP-associated yeast protein Tca17 was determined at a resolution of 1.8 Å. Alternative models of the localization of Tca17 within TRAPP and its potential role in the regulation of TRAPP function by integration into the complex are discussed.