Optineurin minds Rab8′s GAP

Rab GTPases are key components of membrane-trafficking pathways. Each Rab exists in two states in the cell: a GTP-bound active state and a GDP-bound inactive state. Guanine nucleotide exchange factors, which activate Rabs, and GTPase activating proteins (GAPs), which inactive Rabs, regulate Rab cycling and thus membrane trafficking. To date, more than 40 Rab GAPs have been identified, but the mechanisms that target Rabs to their GAPs are unclear. Now, on page 5026, Ghanshyam Swarup and co-workers report that the effector protein optineurin mediates the interaction between Rab8 (which controls several trafficking pathways including endocytic trafficking of the transferrin receptor) with the Rab GAP TBC1D17. The authors show that optineurin, which interacts preferentially with activated Rab8, binds directly to a non-catalytic region of TBC1D17, that Rab8 is linked to TBC1D17 through optineurin, and that this interaction leads to the inactivation of Rab8. Overexpression of TBC1D17 in HeLa cells, they report, reduces recruitment of Rab8 to tubules emanating from the endocytic recycling compartment, and inhibits trafficking of the transferrin receptor. Conversely, knockdown of optineurin or TBC1D17 enhances recruitment of Rab8 to the tubules. Thus, the authors suggest, optineurin negatively regulates Rab8 function by bringing together Rab8 and its GAP TBC1D17.