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To function in fusion and signaling, Rab GTPases need to be converted into their active GTP form. We previously identified the conserved Mon1-Ccz1 complex as the guanine nucleotide exchange factor (GEF) of the yeast Rab7 GTPase Ypt7. To address the possible GEF mechanism, we generated a homology model of the predicted longin domains of Mon1 and Ccz1 using the Rab-binding surface of the TRAPP complex as a template. On the basis of this, we identified mutations in both yeast Mon1 and Ccz1 that block Ypt7 activation, without affecting heterodimer formation and intracellular localization of Mon1 and Ccz1 at endosomes. Strikingly, the activity of the isolated Mon1-Ccz1 complex for Ypt7 is highly stimulated on membranes, and is promoted by the same anionic phospholipids such as phosphatidylinositol-3-phosphate (PI3P), which also support membrane association of the GEF complex. Our data imply that the GEF activity of the Mon1-Ccz1 complex towards Rab7/Ypt7 requires the interface formed by their longin domains and profits strongly from its association with the organelle surface.

Original publication

DOI

10.1242/jcs.140921

Type

Journal article

Journal

J Cell Sci

Publication Date

01/03/2014

Volume

127

Pages

1043 - 1051

Keywords

Endosome, Guanine nucleotide exchange factor, Membrane fusion, Mon1–Ccz1, Rab GTPase, Guanine Nucleotide Exchange Factors, Intracellular Membranes, Phosphatidylinositol Phosphates, Phosphatidylserines, Protein Binding, Protein Interaction Domains and Motifs, Protein Transport, Saccharomyces cerevisiae, Saccharomyces cerevisiae Proteins, Vacuoles, Vesicular Transport Proteins, rab GTP-Binding Proteins