TBC1D18, a novel Rab5-GAP, coordinates endosome maturation together with Mon1

Endosome maturation is essential for efficient degradation of internalized extracellular molecules and plasma membrane proteins. Two Rab GTPases, Rab5 and Rab7, are known to regulate endosome maturation, and a Rab5-to-Rab7 conversion mediated by a Rab7 activator, Mon1-Ccz1, is essential for progression of the maturation process. However, the importance and mechanism of Rab5 inactivation during endosome maturation is poorly understood. Here we report a novel Rab5 inactivator (Rab5-GTPase activating protein [Rab5-GAP]), TBC1D18, which is associated with Mon1 and mediates endosome maturation. We found that Rab5 hyperactivation in addition to Rab7 inactivation occurs in the absence of Mon1. We present evidence showing that the severe defects in endosome maturation observed in Mon1-KO cells are attributable to Rab5 hyperactivation rather than to Rab7 inactivation. We then identified TBC1D18 as a Rab5-GAP by comprehensive screening of TBC-domain-containing Rab-GAPs. Expression of TBC1D18 in Mon1-KO cells rescued the defects in endosome maturation, whereas its depletion attenuated endosome formation and degradation of endocytosed cargos. Moreover, TBC1D18 was found to be able to interact with Mon1, and it localized in close proximity to lysosomes in a Mon1-dependent manner. Thus, TBC1D18 is a crucial regulator of endosome maturation that functions together with Mon1.