Rab Cascades

Rab proteins act as guanine nucleotide-dependent molecular switches, cycling between inactive GDP-bound and active GTP-bound states. Guanine nucleotide exchange factors (GEFs) promote the exchange of GDP for GTP to activate Rabs, while GTPase activating proteins (GAPs) stimulate GTP hydrolysis to inactivate them. This regulated switching allows Rabs to control membrane traffic by recruiting effector proteins only when active.

A Rab cascade is a sequential activation and deactivation of Rab proteins that builds specialized Rab domains on target membranes. An activated Rab recruits the GEF to activate the next Rab in the pathway, while simultaneously recruiting the GAP that inactivates the previous Rab. This ordered progression establishes the sequence of compartments during cargo movement through the secretory pathway and guides vesicles to correct fusion sites.

Rab effectors are peripheral proteins recruited by active Rab-GTP that mediate vesicle movement and regulate membrane traffic. Key effectors include tethering proteins and Rabaptin5, which coordinate with SNAREs and membrane fusion machinery to capture incoming vesicles. By recruiting these effectors in a specific sequence, Rabs establish distinctive subcellular localization patterns and enable directional cargo progression between organelles.

Cytosolic Rab5 is activated by its GEF on the endosome membrane, then binds effectors like tethering proteins and Rabaptin5. Rabaptin5 binding stimulates additional GEFs to recruit more Rab5 molecules in a cascade, forming a concentrated Rab5 domain. This domain concentrates tethering proteins that coordinate with SNARE complexes to capture incoming vesicles and enable membrane fusion at the endosome.

Sequential Rab activation and deactivation reduces overlap between adjacent Rab domains along a pathway, maintaining compartment identity and preventing cargo misrouting. When one Rab activates the next, it simultaneously recruits the GAP that inactivates the previous Rab, creating a clean transition. This precise ordering ensures cargo moves directionally through the secretory pathway and reaches its intended destination.

Rab cascades establish the order of compartments during cargo progression through the Golgi apparatus. The discovery of Rab cascades and homotypic fusion of Golgi compartments led to a new model for protein transport across the Golgi. Rab domains contact their effectors in sequence, generating directional Rab cascades that guide vesicles through successive Golgi cisternae.

Active Rab-GTP recognizes and binds long or short-range tethering proteins to capture target vesicles. Tethering proteins then coordinate with SNARE complexes on both the vesicle and target membrane to assemble the trans-SNARE complex that locks the bilayers together. Each Rab recruits a unique set of tethering proteins, ensuring specificity in vesicle targeting and membrane fusion.