The small GTPases Rab are key regulators of intracellular membrane trafficking, from the formation of transport vesicles to their fusion with membranes. Rabs cycle between an inactive GDP-bound form and an active GTP-bound form that is able to recruit to membranes different set of downstream effectors directly responsible for vesicle formation, movement, tethering and fusion. YPT1 regulates the trafficking of secretory vesicles from the endoplasmic reticulum (ER) to the Golgi. Vesicular transport depends on shuttling of YPT1 between membrane and cytosol by GDI1, probably by recycling it to its membrane of origin after a vesicle fusion event. Plays a role in the initial events of the autophagic vacuole development which take place at specialized regions of the endoplasmic reticulum. Also involved in the recycling of membrane proteins. Rab activation is generally mediated by a guanine exchange factor (GEF), while inactivation through hydrolysis of bound GTP is catalyzed by a GTPase activating protein (GAP). YPT1 is activated by the GEFs DSS4 and TRAPP complex, and inactivated by GAPs GYP1, GYP5 and GYP8. Forms a complex with the Rab escort protein (REP) MRS6, which is recognized by Rab geranylgeranyltransferase BET2-BET4. Interacts with the Rab GDP dissociation inhibitor GDI1, which can retrieve from and deliver to membranes the GDP-bound and prenylated form of YPT1. Interacts with YIP1, which is required for proper membrane targeting of prenylated YPT1. Interacts with YIF1, YIP3, YIP4 and YIP5. ER and Golgi when GTP-bound. Cytoplasmic when bound to GDI1. Prenylation is required for interaction with GDI1 and YIP1. Belongs to the small GTPase superfamily. Rab family. In PubMed:3042385 the location of any palmitoylation was not determined. Mutagenesis of either Cys-205 or Cys-206 would disrupt normal geranylgeranylation, and there would be no primary membrane association for secondary S-palmitoylation to occur at some other position, for example Cys-23. Mutagenesis of both Cys-205 and Cys-206 is lethal, so protein production could not have been observed.