Abstract
Small GTPases of the Rab-family are key regulators of intracellular membrane traffic. These proteins constantly cycle between an 'active' GTP-bound and 'inactive' GDP-bound state. In their GTP-bound conformation Rab proteins can engage in complex formation with so called effector proteins. It is at this level that the control of membrane transport is exerted. To date more than 60 Rab-family members, including isoforms, are recognized. To this family also belongs the Golgi-localized Rab6. In the past three different isoforms of this protein were identified: Rab6A, Rab6A'(generated by alternative splicing of a homologues but distinct exon within the Rab6 gene), and a brain specific isoform, Rab6B. Rab6A' is the isoform regulating the entire retrograde pathway from late endosomes to ER, whereas Rab6A seems dispensable for this route. The role of Rab6B is still ill-defined and therefore the main focus of this study. Studies using GFP-Rab6B in neuronal cells revealed the bi-directional movement of Rab6B positive structures in neurites of these cells, possibly belonging to the post-Golgi compartment. This latter finding was corroborated with a tsVSVG-assay, which localized Rab6B on vesicles moving from the Golgi towards the plasma membrane. Furthermore we also found co-localization of Rab6B with vesicles containing internalized GPI-anchored proteins. A regulatory role for Rab6B in the internalization of these proteins can therefore be anticipated. To learn more about the molecular environment of Rab6B we searched for novel Rab6B interacting proteins. Of the newly identified Rab6B interactors two were analyzed in more detail, namely Bicaudal-D1 and DYNLRB1. Whereas Bicaudal-D1 provides an indirect binding of Rab6B to the dynein/dynactin motor protein complex, DYNLRB1 assures direct binding. Dynein/dynactin is the main microtubule based motor protein complex responsible for long range retrograde transport. Based on our findings we expect an important role for Rab6B in regulating this process which, especially in neurons, is important in cell survival and viability.
