Journal
CELL
Volume 133, Issue 7, Pages 1202-1213Publisher
CELL PRESS
DOI: 10.1016/j.cell.2008.04.049
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Funding
- Howard Hughes Medical Institute Funding Source: Medline
- NCRR NIH HHS [RR020171, P20 RR020171] Funding Source: Medline
- NIDA NIH HHS [R01 DA002243, DA02243] Funding Source: Medline
- NIDCD NIH HHS [F31 DC009143, 1F31DC009143-01, F31 DC009143-01] Funding Source: Medline
- NIGMS NIH HHS [R01 GM071688, T32-GM07223, GM071688, R01 GM071688-02, R01 GM080616-01, R01 GM080616, GM080616] Funding Source: Medline
- NINDS NIH HHS [R01 NS038041, NS38041, R01 NS038041-04, R56 NS038041] Funding Source: Medline
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The multimeric membrane-tethering complexes TRAPPI and TRAPPII share seven subunits, of which four (Bet3p, Bet5p, Trs23p, and Trs31p) are minimally needed to activate the Rab GTPase Ypt1p in an event preceding membrane fusion. Here, we present the structure of a heteropentameric TRAPPI assembly complexed with Ypt1p. We propose that TRAPPI facilitates nucleotide exchange primarily by stabilizing the nucleotide-binding pocket of Ypt1p in an open, solvent-accessible form. Bet3p, Bet5p, and Trs23p interact directly with Ypt1p to stabilize this form, while the C terminus of Bet3p invades the pocket to participate in its remodeling. The Trs31p subunit does not interact directly with the GTPase but allosterically regulates the TRAPPI interface with Ypt1p. Our findings imply that TRAPPII activates Ypt1p by an identical mechanism. This view of a multimeric membrane-tethering assembly complexed with a Rab provides a framework for understanding events preceding membrane fusion at the molecular level.
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