Identification of Rab30 as a novel regulator of tubular endosomes

Tubular-shaped recycling endosomes, known as tubular endosomes, are present in certain types of cells, including HeLa cells, and they regulate the recycling of clathrin-independent endocytosed (CIE) cargo proteins to the plasma membrane. Several key regulators of tubular endosomes, including Rab small GTPases and related proteins, have been identified thus far, but the entire process of tubular endosome formation is not yet fully understood. We previously showed that expression of a Golgi-related Rab-GTPase-activating protein TBC1D22B in HeLa cells caused their tubular structures to disappear, suggesting a possible link between tubular endosome formation and a certain Golgi function(s). However, nothing is known about the target Rab(s) of TBC1D22B in tubular endosome formation or about the functional relationship between tubular endosomes and the Golgi apparatus. Here, we performed comprehensive Rab-knockdown screening in combination with dominant-negative Rab expression and succeeded in identifying 12 Rabs as regulators of tubular endosome formation. One of them, Rab30, a Golgi-resident Rab, is a novel target of TBC1D22B and involved in both tubular endosome formation and CIE cargo trafficking. We also showed that a Rab30-BICD2-KIF5B axis is likely to be involved in tubular endosome formation. Our findings suggest the importance of Rab30-mediated post-Golgi trafficking in tubular endosome formation.Key words: Golgi, GTPase-activating protein (GAP), Rab30, siRNA screening, tubular endosome.