Rab proteins are the largest subfamily of the Ras superfamily of small GTPases, with more than 60 known members, that are involved in a multitude of different processes regulating membrane traffic. Rab proteins cycle between the cytosol and association with membranes, whereby each Rab exhibits a characteristic and specific subcellular localisation. It remains obscure how Rab proteins, in spite of high sequence and structure identity, distinguish between different membranes in the cell with such specificity. Membrane biophysical properties, such as stored curvature elastic stress and bending rigidity, are increasingly found to be determinants for protein recruitment and activity, and other Ras related proteins have recently been shown to exhibit sensitivity towards lipid species and elastic membrane properties.
In this study Rab membrane binding is for the first time correlated to membrane bending rigidity, suggesting that biophysical properties of lipid membranes may play a role in the regulation of Rab targeting. Furthermore, all Rab proteins tested were observed to bind membranes in the absence of other protein factors, questioning the function of protein targeting factors for the Rab membrane recruitment process. Another aspect of Rab membrane interaction is Rab extraction from membranes by GDI. A large scale in vitro screening of 17 Rab proteins revealed a broad range of extractability from membranes with GDI. No correlation was found between extractability and the C-terminal prenylation motif, and no difference in extractability was observed in direct comparison of the extraction potential with GDIα and β. However, Rab proteins that exhibited low extractability from membranes are involved in secretory processes, suggesting a functional correlation to extractability. Furthermore, Rab40c as the first mammalian Rab protein to date was shown to be palmitoylated.