The detailed localization of the GFP-Rab9 is also telling. Rab GTPases are thought to contribute delivery address information for many trafficking vesicles, but Barbero et al. note that Rab9 is present on the endosome- derived vesicles from budding through docking. This is consistent with the group's earlier experiments suggesting that Rab9 has a function during budding. Perhaps as part of this budding process Rab9 clusters in particular regions, as the authors note that Rab9 and Rab7 (a GTPase involved in late endosome fusion) are present in largely nonoverlapping domains within a given late endosome.
Once Barbero et al. have overcome some protein engineering problems, they hope to visualize MPRs and the Rab9-binding cargo collector TIP47. They predict that MPRs will be present in the transport vesicles, and that TIP47 will be released from the vesicles after budding, perhaps to be replaced by an unknown dockingfactor. ▪