Question 1

What are Rab proteins and what role do they play in cells?

Accepted Answer

Rab proteins are the largest family of monomeric GTPases, with 70 members in humans. They regulate consecutive stages of vesicle transport, including docking and fusion to target membranes. Rab proteins switch between inactive GDP-bound and active GTP-bound states to guide vesicles through introduction to membrane traffic pathways and ensure cargo reaches the correct cellular destination.

Question 2

How do Rab proteins switch between active and inactive states?

Accepted Answer

Rab proteins reversibly switch between GDP-bound inactive and GTP-bound active states with help from regulatory proteins. Rab-GEFs activate Rabs by replacing GDP with GTP and inducing conformational changes that insert Rabs into membranes. Rab-GAPs deactivate Rabs by facilitating GTP hydrolysis, returning them to their inactive state.

Question 3

What is the role of GDI in Rab protein regulation?

Accepted Answer

The GDP dissociation inhibitor, or GDI, keeps Rab-GDP inactive and associated in the cytosol. GDI prevents premature activation and maintains Rabs in their inactive state until they are needed. GDI displacement factors help release GDI from Rab, presenting the protein to membrane-bound GEF for activation.

Question 4

How are Rab proteins anchored to membranes?

Accepted Answer

Once activated by GEF, Rab-GTP is localized to the membrane by Rab Escort Proteins. These escort proteins allow geranyl transferases to add a prenyl group to the Rab, covalently anchoring it to the membrane. This anchoring is essential for Rab effectors to bind and coordinate vesicle delivery pathways to the lysosome and other destinations.

Question 5

Why are Rab proteins considered incomplete enzymes?

Accepted Answer

Rab proteins show slow rates of GDP/GTP exchange and GTP hydrolysis by themselves, making them incomplete enzymes. They require additional regulatory proteins called Rab regulators, including GEFs, GAPs, and GDIs, to efficiently catalyze nucleotide exchange and hydrolysis. This dependence allows cells to precisely control vesicle transport timing and specificity.

Question 6

What happens when Rab-GTP binds to a Rab effector?

Accepted Answer

When Rab-GTP binds to a Rab effector, the complex becomes anchored to the membrane. This interaction stabilizes the active Rab state and coordinates downstream processes in vesicle transport. If Rab-GTP remains unbound to an effector, Rab-GAP can facilitate GTP hydrolysis, deactivating the Rab and terminating its signaling function.

Question 7

How do defects in Rab regulation affect human health?

Accepted Answer

Biochemical defects in Rab regulation can lead to infections and congenital diseases. Understanding molecular interactions between Rab effectors and regulatory proteins like GEFs, GAPs, and GDIs reveals how cellular compartment distribution depends on proper vesicle movement. These insights identify transient protein interfaces that drugs can target to treat Rab-related disorders.

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