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 (By similarity). Plays a role in the initial events of the autophagic vacuole development which take place at specialized regions of the endoplasmic reticulum (By similarity). Regulates vesicular transport between the endoplasmic reticulum and successive Golgi compartments. Required to modulate the compacted morphology of the Golgi. Promotes the recruitment of lipid phosphatase MTMR6 to the endoplasmic reticulum-Golgi intermediate compartment (By similarity). GTP + H2O = GDP + H(+) + phosphate 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). Interacts with MICAL1 and MICAL2. Interacts (GTP-bound form) with MICALCL, MICAL1 and MILCAL3. Interacts with GDI1; the interaction requires the GDP-bound state. Interacts with CHM/REP1; the interaction requires the GDP-bound form and is necessary for prenylation by GGTase II. Interacts with RabGAP TBC1D20 (By similarity). Interacts (in GDP-bound form) with lipid phosphatase MTMR6 (via GRAM domain); the interaction regulates MTMR6 recruitment to the endoplasmic reticulum-Golgi intermediate compartment (PubMed:23188820). Interacts (in GDP-bound form) with lipid phosphatase MTMR7 (PubMed:23188820). Targeted by REP1 to membranes of specific subcellular compartments including endoplasmic reticulum, Golgi apparatus, and intermediate vesicles between these two compartments. In the GDP-form, colocalizes with GDI in the cytoplasm (By similarity). Co-localizes with MTMR6 to the endoplasmic reticulum-Golgi intermediate compartment and to the peri-Golgi region (By similarity). Prenylated; by GGTase II, only after interaction of the substrate with Rab escort protein 1 (REP1). Rab-1B binds GTP and GDP and possesses intrinsic GTPase activity. Belongs to the small GTPase superfamily. Rab family.