The function of RAB40B (RAB40B, member RAS oncogene family, ENSG00000141542) is as follows. RAB40B small GTPase acts as substrate-recognition components of the ECS(RAB40B) E3 ubiquitin ligase complex which mediates the ubiquitination of target proteins (PubMed:33999101, PubMed:35293963). The Rab40 subfamily belongs to the Rab family that 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 sets of downstream effectors directly responsible for vesicle formation, movement, tethering and fusion (PubMed:27789576). As part of the ECS(RAB40B) complex, GTP-bound RAB40B promotes LIMA1/EPLIN ubiquitination and degradation, thereby regulating leading-edge actin dynamics during cell migration (PubMed:33999101). As part of the ECS(RAB40B) complex, GTP-bound RAB40B also ubiquitinates RAP2A GTPase which promotes its localization to lamellipodia and activation to drive cell migration. The ECS(RAB40B) complex does not mediate canonical ubiquitin-dependent degradation of RAP2 (PubMed:35293963). RAB40B also binds TKS5/SH3PXD2A effector independently from ECS complex to promote invadopodia-mediated extracellular matrix degradation (PubMed:27789576). {ECO:0000269|PubMed:27789576, ECO:0000269|PubMed:33999101, ECO:0000269|PubMed:35293963}.