Structural basis of TRAPPIII-mediated Rab1 activation.

Area of Science:

• Cell Biology
• Structural Biology
• Molecular Biology

Background:

• Rab1 GTPase regulates the early secretory pathway and autophagy.
• TRAPPIII complex is the guanine nucleotide exchange factor for Rab1 activation.

Purpose of the Study:

• Determine the cryo-EM structure of the Saccharomyces cerevisiae TRAPPIII complex bound to Rab1/Ypt1.
• Elucidate the mechanism of Rab1 activation and membrane interaction by TRAPPIII.

Main Methods:

• 3.7 Å cryo-electron microscopy (cryo-EM) of TRAPPIII-Rab1/Ypt1 complex.
• In vitro and in vivo activation assays.
• Structural analysis of protein-protein and protein-membrane interactions.

Main Results:

• The cryo-EM structure reveals the binding site for the Rab1/Ypt1 hypervariable domain.
• Stable membrane binding of TRAPPIII is necessary for robust Rab1/Ypt1 activation.
• A conserved amphipathic α-helix in the Trs85 subunit mediates membrane anchoring of the TRAPPIII complex.

Conclusions:

• The Trs85 subunit acts as a membrane anchor for the TRAPPIII complex through its amphipathic helix.
• These findings offer a structural basis for understanding Rab activation on organelle and vesicle membranes.
• The study provides critical insights into the regulation of the secretory pathway and autophagy.