So far, the detailed mechanisms underlying the transition from core TRAPP/TRAPPI and TRAPPIII’s GEF activity for Ypt1 to TRAPPII’s GEF activity for Ypt31/32 and the functions of the specific subunits of TRAPPII in this transition are insufficiently understood.īecause core TRAPP/TRAPPI is the common element of TRAPP complexes, it has been a subject of various in-depth studies on its structure. Core TRAPP/TRAPPI and TRAPPIII are specific to Ypt1 ( 19– 21), whereas TRAPPII activates Ypt31/32 in vivo ( 11, 22– 24). A common core suggests common functions with all complexes acting as GEFs. A proportion of Aspergillus nidulans TRAPPIII contains Tca17, as well as TRAPPC11, TRAPPC12, and TRAPPC13 ( 16), same as metazoan TRAPPIII (table S3) ( 17, 18). TRAPPII contains four additional proteins (Trs120, Trs130, Trs65, and Tca17) ( 14), and TRAPPIII has one additional Trs85 ( 15) in yeast. All the three complexes share a core of six different proteins (Bet3, Bet5, Trs20, Trs23, Trs31, and Trs33) that make up core TRAPP/TRAPPI ( 13). The TRAPP complexes attract particular attention in the vesicle transport because they act as GEFs and play an essential role in catalyzing nucleotide exchange for Ypt/Rab GTPase. Core TRAPP/TRAPPI mediates transport from endoplasmic reticulum to cis-Golgi by tethering COPII (coat protein complex II)-coated vesicles ( 5, 9) TRAPPII interacts with COPI-coated vesicles in mammalian cells and aids in endosome-to-Golgi transport ( 6, 10, 11) TRAPPIII plays a unique role in autophagy ( 12). TRAPP complexes from yeast are well studied among this family and have three forms-core TRAPP/TRAPPI, TRAPPII, and TRAPPIII, which function in various vesicle trafficking pathways ( 8). Transport protein particle (TRAPP) complexes belong to the multisubunit tethering complexes and are a highly conserved family of proteins found in all eukaryotes, from yeast to humans.

How the different types of tethering factors work together to achieve the overall specificity of the tethering process still remains elusive. The tethering is highly regulated by the tethering factors, including the conserved guanosine triphosphatases (GTPases) of the Ypt/Rab family, their guanine nucleotide exchange factors (GEFs), and their downstream effectors ( 4– 7). It promotes the organization of the soluble N-ethylmaleimide–sensitive factor attachment protein receptor (SNARE) proteins in vesicle fusion ( 3) and thus is a crucial step in determining the specificity of vesicle trafficking ( 2). Tethering refers to the initial interaction between a vesicle and its target membrane ( 2). Vesicle transport is a highly sequential process, and each step along the pathway, from the vesicle budding at the donor compartment, via vesicle transporting in cell, to the tethering and fusion with the membrane of the acceptor compartment, is precisely regulated by corresponding factors ( 2). In eukaryotic cells, vesicles act as the cargo carriers by transporting proteins, lipids, and other materials between various membrane-bound compartments ( 1). Combined with functional analysis, the structures provide insights into the assembly of TRAPPII and the mechanism of the specific activation of Ypt31/Ypt32 by TRAPPII. Located in the interior of the monomer, Ypt32 binds with both core TRAPP/TRAPPI and Trs120 via its nucleotide-binding domain and binds with Trs31 via its hypervariable domain.

All the structures show a dimeric architecture assembled by two triangle-shaped monomers, while the monomer in the apo state exhibits both open and closed conformations, and the monomer in the Ypt32-bound form only captures the closed conformation. Here, we present cryo-EM structures of yeast TRAPPII in apo and Ypt32-bound states. TRAPPII activates GTPase Ypt31/Ypt32 as the guanine nucleotide exchange factor in the trans-Golgi network to determine the maturation of Golgi cisternae into post-Golgi carriers in yeast. Transport protein particle (TRAPP) complexes belong to the multiprotein tethering complex and exist in three forms-core TRAPP/TRAPPI, TRAPPII, and TRAPPIII.

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