GAPex 5 plays a role in several processes such as endocytosis, insulin receptor internalisation and LC2A4/GLUT4 trafficking. It acts as both a GTPase-activating protein (GAP) and a guanine nucleotide exchange factor (GEF). GAPex 5 is a GEF for the small G protein Rab31, promoting the exchange of GDP bound to Rab31 for GTP and thus regulating LC2A4/GLUT4 trafficking. In the absence of insulin, GAPex 5 maintains Rab31 in an active (GTP-bound) state, thereby promoting a futile cycle between LC2A4/GLUT4 storage vesicles and early endosomes, keeping LC2A4/GLUT4 inside the cells. Following insulin stimulation, GAPex 5 is translocated to the plasma membrane, enabling the release of LC2A4/GLUT4 from the intracellular storage vesicles. GAPex 5 is also involved in EGFR trafficking and degradation, and also has GEF activity for Rab5 and GAP activity for Ras.