RAB PROTEINS FORM IN-VIVO COMPLEXES WITH 2 ISOFORMS OF THE GDP-DISSOCIATION INHIBITOR PROTEIN (GDI)

GTPases of the Rab family play a key role in the regulation of vesicul ar transport in eukaryotic cells. Several accessory proteins that regu late their GDP/GTP cycle as well as their subcellular localization hav e been identified within the past few years, The best known is Rab3A G DP dissociation inhibitor protein (GDI), originally identified as an i nhibitor of GDP dissociation from Rdb3A, a Rab protein specifically ex pressed in neuronal and neuroendocrine cells. Recent studies have poin ted out a role of Rab3A GDI as a chaperone of several Rab proteins dur ing their cycling between cytosol and membranes and Rab3A GDI has been considered so far as a general regulator of Rab function. However, cD NAs encoding potential isoforms of this protein, called GDI beta and G DI-2, have been recently isolated. In this study,we have characterized cytosolic Rab protein complexes in various cell types and tissues usi ng Mono and chromatography. We show that in rat brain and in insulin-s ecreting RINm5F cells, the majority of Rab proteins are complexed with Rab3A GDI, In contrast, in Chinese hamster ovary cells, they are main ly complexed to a protein that we have identified as GDI beta. In rat liver cytosol, Rab proteins form complexes with both isoforms. We also show that the proportion of Rab proteins complexed with either isofor m depends on the relative abundance of Rab3A GrDI and GDI beta in the cytosol. These findings suggest that GDI isoforms are either redundant or could be involved in the fine control of Rab function.