This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The multimeric membrane-tethering complexes TRAPPI and TRAPPII function in vesicle traffic to the Golgi apparatus, playing a role in recognition as the secretory vesicle arrives at the Golgi and before it fuses to release its cargo. TRAPPI and TRAPPII share seven subunits, of which four (Bet3p, Bet5p, Trs23p, and Trs31p) are minimally required to activate the Rab GTPase Ypt1p, an event required for subsequent membrane fusion. We have determined the structure of a heteropentameric TRAPP subassembly sufficient for activation complexed with Ypt1p. This view of a multimeric tethering assembly in complex with a Rab provides a framework for understanding events preceding membrane fusion at a molecular level.. Based on the structure, we propose that TRAPPI promotes nucleotide exchange by stabilizing the nucleotide binding pocket of Ypt1p in an open, accessible conformation. Three of the subunits interact directly with Ypt1p to stabilize this form, while the C-terminus of one subunit (Bet3p) invades the nucleotide binding pocket and may initiate the remodeling. One of the subunits (Trs31p) does not interact directly with Ypt1p but allosterically regulates the TRAPP interface with Ypt1p. TRAPPII likely activates Ypt1p by the same mechanism as TRAPPI.
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