Rab7b was recently identified as a small GTPase that shares ~50% identity with Rab7. Previous work suggested that, similar to Rab7, Rab7b regulates transport from early endosomes to late endosomes and/or lysosomes. However, Cecilia Bucci and colleagues (p. ) now report that Rab7b is not directly involved in endocytic transport and instead has a distinct role in mediating transport from endosomes to the Golgi and/or trans-Golgi network (TGN). They first show that Rab7b localises to the Golgi and TGN, as well as to late endosomes and lysosomes, in many cell types. Second, unlike Rab7, Rab7b is not involved in the degradation of EGF or EGFR. Third, Rab7b is involved in the transport of lysosomal enzymes: inhibiting Rab7b expression or function causes a block in the transport of the lysosomal enzyme hexosaminidase. Fourth, the same treatment also impairs cathepsin-D maturation, suggesting that normal transport of this protease from the TGN to lysosomes via endosomes is disrupted in the absence of Rab7b. Fifth, Rab7b is required for transport of the TGN protein TGN46 as well as for the proper transport and subcellular localisation of the late endosomal marker CI-MPR. Finally, the authors show that Rab7b is required for retrograde transport of the cholera toxin B subunit from endosomes to the Golgi. Together, these data clarify the unique roles of Rab7b in pathways of intracellular transport.
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