Rab33b and Rab6 are Functionally Overlapping Regulators of Golgi Homeostasis and Trafficking

Abstract

We used multiple approaches to investigate the coordination of trans and medial Rab proteins in the regulation of intra-Golgi retrograde trafficking. We reasoned that medially located Rab33b might act downstream of the trans Golgi Rab, Rab6, in regulating intra-Golgi retrograde trafficking. We found that knockdown of Rab33b, like Rab6, suppressed conserved oligomeric Golgi (COG) complex-or Zeste White 10 (ZW10)-depletion induced disruption of the Golgi ribbon in HeLa cells. Moreover, efficient GTP-restricted Rab6 induced relocation of Golgi enzymes to the endoplasmic reticulum (ER) was Rab33b-dependent, but not vice versa, suggesting that the two Rabs act sequentially in an intra-Golgi Rab cascade. In support of this hypothesis, we found that overexpression of GTP-Rab33b induced the dissociation of Rab6 from Golgi membranes in vivo. In addition, the transport of Shiga-like toxin B fragment (SLTB) from the trans to cis Golgi and ER required Rab33b. Surprisingly, depletion of Rab33b had little, if any, immediate effect on cell growth and multiplication. Furthermore, anterograde trafficking of tsO45G protein through the Golgi apparatus was normal. We suggest that the Rab33b/Rab6 regulated intra-Golgi retrograde trafficking pathway must coexist with other Golgi trafficking pathways. In conclusion, we provide the first evidence that Rab33b and Rab6 act to coordinate a major intra-Golgi retrograde trafficking pathway. This coordination may have parallels with Rab conversion/cascade events that regulate endosome, phagosome and exocytic processes.

Rab GAP cascade regulates dynamics of Ypt6 in the Golgi traffic

“…Ypt6 is the yeast homolog of the mammalian Rab6, which functions in the fusion of endosome-derived vesicles to the Golgi by recruiting the GARP complex as its effector (12,15,16). In mammals and plants, it is generally agreed that Rab6 functions in trans-Golgi/TGN (13,14). However, our results showed significant colocalization of GFP-Ypt6 with mRuby-Sed5, suggesting that Ypt6 also localizes to cis-Golgi cisternae in yeast.…”

“…In mammalian cells, accumulating evidence suggests that Rab6 localizes at the trans-Golgi region and functions in retrograde protein transport and in Golgi structural organization (13,14). Ypt6 may very well function in the yeast Golgi maintenance.…”

RUTBC1 Protein, a Rab9A Effector That Activates GTP Hydrolysis by Rab32 and Rab33B Proteins

“…Depletion of Rab33B impairs Shiga-like toxin B trafficking from the Golgi to the ER (53), and its function is somehow related to that of Rab6 in COPI-independent retrograde trafficking (54). Interestingly, Rab33B also rescues a dispersed Golgi phenotype observed upon depletion of ZW10 (human homolog of yeast Dsl1p) or Cog3 (human homolog of yeast Sec34), two distinct tethering complexes involved in Golgi to ER retrograde traffic (53). These data suggest that Rab33B might regulate flux of material through the medial Golgi.…”

Autophagy and proteins involved in vesicular trafficking

“…RAB33B is present in the cis-Golgi where it participates in Golgi-to-ER retrograde membrane trafficking [61]. Mutations in RAB33B have been investigated in the pathogenesis of the Dyggve-Melchior-Clausen's syndrome [62] and the Smith-McCort's dysplasia [63], thus establishing the important role of the retrograde Golgi traffic in both human syndromes.…”