Expression of wild-type and mutant green fluorescent protein-Rab1 for fluorescence microscopy analysis

Differential Regulation of Lipoprotein and Hepatitis C Virus Secretion by Rab1b

Secretory cells produce diverse cargoes, yet how they regulate concomitant secretory traffic remains insufficiently explored. Rab GTPases control intracellular vesicular transport. To map secretion pathways, we generated a library of lentivirus-expressed dominant-negative Rab mutants and used it in a large-scale screen to identify regulators of hepatic lipoprotein secretion. We identified several candidate pathways, including those mediated by Rab11 and Rab8. Surprisingly, inhibition of Rab1b, the major regulator of transport from the endoplasmic reticulum to the Golgi, differently affected the secretion of the very-low-density lipoprotein components ApoE and ApoB100, despite their final association on mature secreted lipoprotein particles. Since hepatitis C virus (HCV) incorporates ApoE and ApoB100 into its virus particle, we also investigated infectious HCV secretion and show that its regulation by Rab1b mirrors that of ApoB100. These observations reveal differential regulation of hepatocyte secretion by Rab1b and advance our understanding of lipoprotein assembly and lipoprotein and HCV secretion.

Legionella pneumophila replication vacuole formation involves rapid recruitment of proteins of the early secretory system

Legionella pneumophila vacuole biogenesis was analyzed by using a cell-free system. We show that calnexin, Sec22b, and Rab1 are recruited to the vacuole very shortly after bacterial uptake, and we have identified Rab1 as a potential host factor involved in the endoplasmic reticulum recruitment process.

Rab GTPases are recruited to chlamydial inclusions in both a species-dependent and species-independent manner

Chlamydiae are obligate intracellular bacteria that replicate within an inclusion that is trafficked to the peri-Golgi region where it fuses with exocytic vesicles. The host and chlamydial proteins that regulate the trafficking of the inclusion have not been identified. Since Rab GTPases are key regulators of membrane trafficking, we examined the intracellular localization of several green fluorescent protein (GFP)-tagged Rab GTPases in chlamydia-infected HeLa cells. GFP-Rab4 and GFP-Rab11, which function in receptor recycling, and GFP-Rab1, which functions in endoplasmic reticulum (ER)-to-Golgi trafficking, are recruited to Chlamydia trachomatis, Chlamydia muridarum, and Chlamydia pneumoniae inclusions, whereas GFP-Rab5, GFP-Rab7, and GFP-Rab9, markers of early and late endosomes, are not. In contrast, GFP-Rab6, which functions in Golgi-to-ER and endosome-to-Golgi trafficking, is associated with C. trachomatis inclusions but not with C. pneumoniae or C. muridarum inclusions, while the opposite was observed for the Golgi-localized GFP-Rab10. Colocalization studies between transferrin and GFP-Rab11 demonstrate that a portion of GFP-Rab11 that localizes to inclusions does not colocalize with transferrin, which suggests that GFP-Rab11's association with the inclusion is not mediated solely through Rab11's association with transferrin-containing recycling endosomes. Finally, GFP-Rab GTPases remain associated with the inclusion even after disassembly of microtubules, which disperses recycling endosomes and the Golgi apparatus within the cytoplasm, suggesting a specific interaction with the inclusion membrane. Consistent with this, GFP-Rab11 colocalizes with C. trachomatis IncG at the inclusion membrane. Therefore, chlamydiae recruit key regulators of membrane trafficking to the inclusion, which may function to regulate the trafficking or fusogenic properties of the inclusion.

COPI recruitment is modulated by a Rab1b-dependent mechanism

The small GTPase Rab1b is essential for endoplasmic reticulum (ER) to Golgi transport, but its exact function remains unclear. We have examined the effects of wild-type and three mutant forms of Rab1b in vivo. We show that the inactive form of Rab1b (the N121I mutant with impaired guanine nucleotide binding) blocks forward transport of cargo and induces Golgi disruption. The phenotype is analogous to that induced by brefeldin A (BFA): it causes resident Golgi proteins to relocate to the ER and induces redistribution of ER-Golgi intermediate compartment proteins to punctate structures. The COPII exit machinery seems to be functional in cells expressing the N121I mutant, but COPI is compromised, as shown by the release of beta-COP into the cytosol. Our results suggest that Rab1b function influences COPI recruitment. In support of this, we show that the disruptive effects of N121I can be reversed by expressing known mediators of COPI recruitment, the GTPase ARF1 and its guanine nucleotide exchange factor GBF1. Further evidence is provided by the finding that cells expressing the active form of Rab1b (the Q67L mutant with impaired GTPase activity) are resistant to BFA. Our data suggest a novel role for Rab1b in ARF1- and GBF1-mediated COPI recruitment pathway.