The small GTPases Rab are key regulators of intracellular membrane trafficking, from the formation of transport vesicles to their fusion with membranes (PubMed:20545908, PubMed:9437002). 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 (PubMed:9437002). Plays a role in the initial events of the autophagic vacuole development which take place at specialized regions of the endoplasmic reticulum (PubMed:20545908). Regulates vesicular transport between the endoplasmic reticulum and successive Golgi compartments (By similarity). Required to modulate the compacted morphology of the Golgi (PubMed:26209634). 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 (PubMed:15694364, PubMed:27552051). Interacts (in GTP-bound form) with MICALCL, MICAL1 and MILCAL3 (PubMed:15694364, PubMed:27552051). Interacts with GDI1; the interaction requires the GDP-bound state (PubMed:8836150, PubMed:11389151). Interacts with CHM/REP1; the interaction requires the GDP-bound form and is necessary for prenylation by GGTase II (PubMed:9437002, PubMed:11389151). Interacts with RabGAP TBC1D20 (PubMed:23236136). 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 (By similarity). (Microbial infection) Interacts with L.pneumophila AnkX (PubMed:21822290, PubMed:22307087). Interacts with L.pneumophila Lem3 (PubMed:22158903, PubMed:22307087). Interacts with L.pneumophila SidD (PubMed:21734656, PubMed:21680813). Interacts with L.pneumophila DrrA (PubMed:20064470, PubMed:20651120). Targeted by REP1 to membranes of specific subcellular compartments including endoplasmic reticulum, Golgi apparatus, and intermediate vesicles between these two compartments (PubMed:11389151). In the GDP-form, colocalizes with GDI in the cytoplasm (PubMed:11389151). 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). (Microbial infection) AMPylation at Tyr-77 by L.pneumophila DrrA occurs in the switch 2 region and leads to moderate inactivation of the GTPase activity. It appears to prolong the lifetime of the GTP state of RAB1B by restricting access of GTPase effectors to switch 2 and blocking effector-stimulated GTP hydrolysis, thereby rendering RAB1B constitutively active. It is later de-AMPylated by L.pneumophila SidD, releasing RAB1B from bacterial phagosomes. (Microbial infection) Phosphocholinated at Ser-76 by L.pneumophila AnkX, leading to displace GDP dissociation inhibitors (GDI) (PubMed:21822290, PubMed:22307087). Both GDP-bound and GTP-bound forms can be phosphocholinated. Dephosphocholinated by L.pneumophila Lem3, restoring accessibility to L.pneumophila GTPase effector LepB (PubMed:22158903, PubMed:22307087). (Microbial infection) Glycosylated by S.typhimurium protein Ssek3: arginine GlcNAcylation prevents GTPase activity, thereby disrupting vesicular protein transport from the endoplasmic reticulum (ER) to the Golgi compartment. Rab-1B binds GTP and GDP and possesses low intrinsic GTPase activity. Belongs to the small GTPase superfamily. Rab family.