Mon1-Ccz1 activates Rab7 only on late endosome and dissociates from lysosome in mammalian cells

Abstract

Rab GTPases act as molecular switches regulating various aspects of membrane trafficking. Among them, Rab5 and Rab7 play central roles in the endolysosomal network. Although many effectors downstream of Rab7 have been elucidated, our present understanding of the mechanism regulating Rab7 activity is extremely limited. It has only recently been accepted that Mon1-Ccz1 is a Rab7 guanine nucleotide exchange factor, but the question where Mon1-Ccz1 works with Rab7 is hardly answered. To address what kind of change/switch exists in the regulatory mechanism upstream of Rab7 during the transition from late endosome to lysosome, we examined Rab7 activity in steady-state cells and EGF-induced macropinocytosis using a newly developed FRET sensor. A combination of a Rab7 sensor and confocal FRET imaging techniques revealed that the activation of Rab7 on late endosomes depends on Mon1-Ccz1 and is implicated in late endosome-lysosome fusion. In contrast, Rab7 activity on lysosomes was independent of Mon1-Ccz1 and active Rab7 played a role in perinuclear clustering of lysosomes.