RAB40B belongs to the Rab family within the small GTPase superfamily. The Rab40 subfamily members are key regulators of intracellular membrane trafficking, from the formation of transport vesicles to their fusion with membranes. Like other Rabs, RAB40B cycles between an inactive GDP-bound form and an active GTP-bound form. The GTP-bound form recruits sets of downstream effectors to membranes, and these effectors are directly responsible for vesicle formation, movement, tethering and fusion.

RAB40B also acts as a substrate-recognition component of the ECS(RAB40B) E3 ubiquitin ligase complex, which mediates ubiquitination of target proteins. In this complex, GTP-bound RAB40B promotes ubiquitination and degradation of LIMA1/EPLIN, thereby regulating leading-edge actin dynamics during cell migration. The complex also ubiquitinates the GTPase RAP2A, which promotes its localisation to lamellipodia and its activation to drive cell migration. This does not lead to canonical ubiquitin-dependent degradation of RAP2. Independently of the ECS complex, RAB40B binds the effector TKS5/SH3PXD2A to promote invadopodia-mediated degradation of the extracellular matrix.

The RAB40B gene is classed by HGNC within the RAB, member RAS oncogene GTPases group and also among microRNA protein coding host genes. The RAB40B protein is annotated at the cell membrane, in the cytosol, and in the lamellipodium membrane and membrane ruffles. NCBI notes similarity to a yeast protein, which suggests a role in regulating secretory vesicles. The mouse ortholog is Rab40b.