Each membrane-bound organelle and its associated transport vesicles have a specific set of Rab proteins. Rabs act in a series where they switch between a cytosolic, GDP-bound inactive state and a membrane-associated, GTP-bound active state with the help of Guanine nucleotide exchange factors or GEFs and GTPase activating proteins or GAPs . An activated Rab recruits the GEF to activate the next Rab, which can recruit the GAP to deactivate the previous Rab. This sequential activation and deactivation of Rabs creates a cascade that builds a specialized Rab domain on the target membrane to guide a vesicle to the correct spot for fusion. Cytosolic Rab5 is activated by its GEF present on the endosome membrane. The active Rab5 binds Rab effectors, such as tethering proteins and Rabaptin5. Rabaptin5 binding stimulates additional GEFs to recruit Rab5 in a cascade, forming a Rab5 domain on the endosome membrane. This helps concentrate the tethering proteins that coordinate with SNARE complexes to capture incoming vesicles and enable membrane fusion.