The endolysosomal system controls the transport and sorting of proteins and lipids in the cell, and is orchestrated by the Rab GTPases Rab5 and Rab7. Rab7 is found at late endosomes and lysosomes, and although its roles are well established, much less is known regarding its regulation. Only fairly recently, the Mon1–Ccz1 complex has been described as a guanine nucleotide exchange factor (GEF) for Rab7. Because it remains unclear where in the cell Mon1–Ccz1 interacts with and activates Rab7, Takeshi Nakamura and colleagues (p. ) developed a novel Rab7 FRET sensor to examine Rab7 activation in greater detail. Using this sensor, they first show that under steady-state conditions, there is considerable variation in Rab7 activity at individual lysosomes. Surprisingly, Mon1–Ccz1 only regulates Rab7 at late endosomes, because Ccz1 depletion only affects Rab7 activity on late endosomes and not on lysosomes. To investigate Rab7 activity during conversion of a single late endosome to a corresponding lysosome, the authors then imaged macropinosomes that form upon stimulation of cells with epidermal growth factor. They find that Rab7 is initially recruited to macropinosomes in its inactive, GDP-bound form and becomes activated by Mon1–Ccz1 during macropinosome maturation, before the GEF is released upon fusion with the lysosome. Taken together, this work presents new insights into the mechanisms of Rab7 activation and function but also points to the existence of other yet unknown Rab7 GEFs that are responsible for its activation on lysosomes.
