Abstract
BackgroundThe 60+ members of the mammalian Rab protein family group into subfamilies postulated to share common functionality. The Rab VI subfamily contains 5 Rab proteins, Rab6a/a’, Rab6b, Rab6c and Rab41. High-level knockdown of Rab6a/a’ has little effect on the tightly organized Golgi ribbon in HeLa cells as seen by fluorescence microscopy. In striking contrast, we found Rab41 was strongly required for normal Golgi ribbon organization.Methods/ResultsTreatment of HeLa cells with Rab41 siRNAs scattered the Golgi ribbon into clustered, punctate Golgi elements. Overexpression of GDP-locked Rab41, but not wild type or GTP-locked Rab41, produced a similar Golgi phenotype. By electron microscopy, Rab41 depletion produced short, isolated Golgi stacks. Golgi-associated vesicles accumulated. At low expression levels, wild type and GTP-locked Rab41 showed little concentration in the Golgi region, but puncta were observed and most were in ruffled regions at the cell periphery. There was 25% co-localization of GTP-locked Rab41 with the ER marker, Sec61p. GDP-locked Rab41, as expected, displayed an entirely diffuse cytoplasmic distribution. Depletion of Rab41 or overexpression of GDP-locked Rab41 partially inhibited ER-to-Golgi transport of VSV-G protein. However, Rab41 knockdown had little, if any, effect on endosome-to-Golgi transport of SLTB. Additionally, after a 2-day delay, treatment with Rab41 siRNA inhibited cell growth, while overexpression of GDP-locked Rab41, but not wild type or GTP-locked Rab41, produced a rapid, progressive cell loss. In double knockdown experiments with Rab6, the Golgi ribbon was fragmented, a result consistent with Rab41 and Rab6 acting in parallel.ConclusionWe provide the first evidence for distinctive Rab41 effects on Golgi organization, ER-to-Golgi trafficking and cell growth. When combined with the evidence that Rab6a/a’ and Rab6b have diverse roles in Golgi function, while Rab6c regulates mitotic function, our data indicate that Rab VI subfamily members, although related by homology and structure, share limited functional conservation.
Highlights
In most mammalian cell types, the Golgi apparatus exists as a juxtanuclear ribbon structure
In contrast to Rab6, we found by light microscopy and electron microscopy that Rab41 was strongly required for normal Golgi ribbon organization
In the multiple sequence alignment to determine which portions of Rab41 are shared with other members of the Rab VI subfamily, we found that amino acid segment 29 to 192, the central core sequence of the protein, had the greatest identity with other subfamily members, almost 80%
Summary
In most mammalian cell types, the Golgi apparatus ( known as the Golgi complex) exists as a juxtanuclear ribbon structure. This organized structure is generated by the interconnection of Golgi stacks consisting of a series of flattened, membrane-bound discs termed cisternae (for reviews, see 1,2). Among the 60 or more members of Rab protein family in mammalian cells, several of them including Rab, Rab33b, Rabs and 2, Rab and Rab have been implicated in Golgi organization and trafficking (for review, see 9). After a 2-day delay, treatment with Rab siRNA inhibited cell growth, while overexpression of GDP-locked Rab, but not wild type or GTP-locked Rab, produced a rapid, progressive cell loss. When combined with the evidence that Rab6a/a’ and Rab6b have diverse roles in Golgi function, while Rab6c regulates mitotic function, our data indicate that Rab VI subfamily members, related by homology and structure, share limited functional conservation
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