Two distinct effectors of the small GTPase Rab5 cooperate in endocytic membrane fusion.

Using the yeast two-hybrid system, we have identified a novel 62 kDa coiled-coil protein that specifically interacts with the GTP-bound form of Rab5, a small GTPase that regulates membrane traffic in the early endocytic pathway. This protein shares 42% sequence identity with Rabaptin-5, a previously identified effector of Rab5, and ...
we therefore named it Rabaptin-5beta. Like Rabaptin-5, Rabaptin-5beta displays heptad repeats characteristic of coiled-coil proteins and is recruited on the endosomal membrane by Rab5 in a GTP-dependent manner. However, Rabaptin-5beta has features that distinguish it from Rabaptin-5. The relative expression levels of the two proteins varies in different cell types. Rabaptin-5beta does not heterodimerize with Rabaptin-5, and forms a distinct complex with Rabex-5, the GDP/GTP exchange factor for Rab5. Immunodepletion of the Rabaptin-5beta complex from cytosol only partially inhibits early endosome fusion in vitro, whereas the additional depletion of the Rabaptin-5 complex has a stronger inhibitory effect. Fusion activity can mostly be recovered by addition of the Rabaptin-5 complex alone, but maximal fusion efficiency requires the presence of both Rabaptin-5 and Rabaptin-5beta complexes. Our results suggest that Rab5 binds to at least two distinct effectors which cooperate for optimal endocytic membrane docking and fusion.
Mesh Terms:
Amino Acid Sequence, Animals, Carrier Proteins, Cell-Free System, Cloning, Molecular, Cytosol, Dimerization, Endosomes, GTP Phosphohydrolases, GTP-Binding Proteins, Guanine Nucleotide Exchange Factors, HeLa Cells, Humans, Membrane Fusion, Membrane Proteins, Mice, Molecular Sequence Data, Recombinant Fusion Proteins, Sequence Homology, Amino Acid, Species Specificity, Vesicular Transport Proteins, Yeasts, rab5 GTP-Binding Proteins
EMBO J.
Date: Apr. 01, 1998
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