The atypical Rab GTPase associated with Parkinson’s disease, Rab29, is localized to membranes

Several genetic studies have shown that the small GTPase Rab29 is involved in the pathogenesis of Parkinson's disease (PD). It has also been shown that overexpression of Rab29 increases the activity of leucine-rich repeat kinase 2 (LRRK2), a protein kinase often mutated in familial PD, although the mechanism underlying this activation remains unclear. Here we employed biochemical analyses to characterize the localization of Rab29 and found that unlike general Rab proteins, Rab29 is predominantly fractionated into the membrane fraction by ultracentrifugation. We also found that Rab29 is resistant to extraction from membranes by GDP-dissociation inhibitors (GDIs) in vitro. Furthermore, Rab29 failed to interact with GDIs, and its membrane localization was not affected by the knockout of GDIs in cells. We show that knockout of Rab geranylgeranyltransferase decreased the hydrophobicity of Rab29, suggesting that Rab29 is geranylgeranylated at its carboxyl terminus as is with typical Rab proteins. Notably, we demonstrated that membrane-bound Rab29 retains some hydrophilicity, indicating that mechanisms other than geranylgeranylation might also be involved in the membrane localization of Rab29. Taken together, these findings uncover the atypical nature of Rab29 among Rab proteins, which will provide important clues for understanding how Rab29 is involved in the molecular pathomechanism of PD.