The compartmentalisation of eukaryotic cells has its advantages - the separation of mutually exclusive processes, for example. But it means that cells must be able to maintain each compartment correctly, which in turn means each organelle must be identifiable. Daniel Cutler and coworkers report that for the Weibel-Palade body (a lysosome-related organelle) Rab27a may be important for establishing organelle identity (see p. 3939). They show that the Weibel-Palade body, which stores von Willebrand Factor (VWF) in human umbilical vein endothelial cells, contains Rab27a and that similar structures induced in nonendothelial cells by VWF expression also contain Rab27a. Furthermore, by tagging VWF with GFP, the researchers demonstrate that newly formed Weibel-Palade bodies in endothelial cells acquire Rab27a only during maturation. They conclude that, in the case of Weibel-Palade bodies, recruitment of Rab27a to the organelle membrane is indirectly driven by the lumenal cargo protein in a maturation-dependent process, which is...

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