Extra offprints can be ordered before the press date. Reprints may be purchased at a later stage but are more expensive. Policies Publication in Current Biology implies that all authors of the paper have read and agreed to its content, and that readily replaceable material described in the paper will be freely distributed to academic colleagues. Nucleic acid sequences, protein sequences and atomic coordinates should be deposited in an appropriate database in time for the accession number to be included in the paper. Correspondence concerning papers published in Current Biology is encouraged.
Correspondence containing data or scientific argument is subject to review.
In eukaryotic cells, there is a complex system of traffic between distinct vesicular compartments, and a number of the biochemical components of this system have been identified. There is evidence that small GTPases related to the Ras family, particularly members of the Rab family, are important in directing membrane traffic. We have previously reported evidence that Rab5 is involved in the fusion of early endosomes with each other, but the precise scope of the function of Rab5 in vivo has not been defined. Surprisingly, overexpression of wild-type Rab5 accelerated the uptake of endocytic markers and led to the appearance of atypically large early endosomes. We conclude that Rab5 is a rate-limiting component of the machinery regulating the kinetics of membrane traffic in the early endocytic pathway.