Endocytosis is regulated through the pH-dependent phosphorylation of Rab GTPases by Parkinson's kinase LRRK2

While it has been known for decades that luminal acidification is required for normal traffic along the endocytic pathway, the precise underlying mechanism(s) remain unknown. We found that dissipation of the endomembrane pH gradient resulted in acute formation of large Rab5- or Rab7-positive vacuoles. Vacuole formation was associated with and required hyperactivation of the Rabs, which was attributable to impaired GTPase activity, despite normal recruitment of cognate GAPs. Surprisingly, LRRK2 -a kinase linked to Parkinsons disease-was recruited to endomembranes and markedly activated upon dissipation of luminal acidification. LRRK2 phosphorylated Rab GTPases, rendering them insensitive to deactivation. Importantly, genetic deletion of LRRK2 prevented the {Delta}pH-induced vacuolation, implying that the kinase is required to modulate vesicular traffic. We propose that by dictating the state of activation of LRRK2 and in turn that of Rab GTPases, the development of a progressive luminal acidification serves as a timing device to control endocytic maturation.