In vitro reconstitution of ARF-regulated cytoskeletal dynamics on Golgi membranes

Golgi isolation

The Golgi apparatus is a membranous organelle that modifies and packages proteins and lipids into transport carriers and sends them to the proper locations in the cell. The study of Golgi structure and function can be facilitated by the isolation of this organelle from homogenates of tissues or cells. Liver cells have abundant Golgi membranes because they actively secrete proteins and lipids; therefore, liver tissue is often the preferred source. In this protocol, Golgi membranes are purified from rat liver homogenate by two sequential sucrose gradients. The relative yield of the prepared Golgi stacks is then assessed by measuring the increase in activity of a Golgi marker enzyme, β-1,4-galactosyltransferase, over that of the total liver homogenate. A typical preparation can yield Golgi membranes that are purified 80- to 100-fold over the homogenate, and the majority (60%-70%) retain their stacked nature.

ARFGAP1 is dynamically associated with lipid droplets in hepatocytes

The ARF GTPase Activating Protein 1 (ARFGAP1) associates mainly with the cytosolic side of Golgi cisternal membranes where it participates in the formation of both COPI and clathrin-coated vesicles. In this study, we show that ARFGAP1 associates transiently with lipid droplets upon addition of oleate in cultured cells. Also, that addition of cyclic AMP shifts ARFGAP1 from lipid droplets to the Golgi apparatus and that overexpression and knockdown of ARFGAP1 affect lipid droplet formation. Examination of human liver tissue reveals that ARFGAP1 is found associated with lipid droplets at steady state in some but not all hepatocytes.

Uptake and processing of the cytolethal distending toxin by mammalian cells

The cytolethal distending toxin (Cdt) is a heterotrimeric holotoxin produced by a diverse group of Gram-negative pathogenic bacteria. The Cdts expressed by the members of this group comprise a subclass of the AB toxin superfamily. Some AB toxins have hijacked the retrograde transport pathway, carried out by the Golgi apparatus and endoplasmic reticulum (ER), to translocate to cytosolic targets. Those toxins have been used as tools to decipher the roles of the Golgi and ER in intracellular transport and to develop medically useful delivery reagents. In comparison to the other AB toxins, the Cdt exhibits unique properties, such as translocation to the nucleus, that present specific challenges in understanding the precise molecular details of the trafficking pathway in mammalian cells. The purpose of this review is to present current information about the mechanisms of uptake and translocation of the Cdt in relation to standard concepts of endocytosis and retrograde transport. Studies of the Cdt intoxication process to date have led to the discovery of new translocation pathways and components and most likely will continue to reveal unknown features about the mechanisms by which bacterial proteins target the mammalian cell nucleus. Insight gained from these studies has the potential to contribute to the development of novel therapeutic strategies.

Golgin160 recruits the dynein motor to position the Golgi apparatus

Membrane motility is a fundamental characteristic of all eukaryotic cells. One of the best-known examples is that of the mammalian Golgi apparatus, where constant inward movement of Golgi membranes results in its characteristic position near the centrosome. While it is clear that the minus-end-directed motor dynein is required for this process, the mechanism and regulation of dynein recruitment to Golgi membranes remains unknown. Here, we show that the Golgi protein golgin160 recruits dynein to Golgi membranes. This recruitment confers centripetal motility to membranes and is regulated by the GTPase Arf1. Further, during cell division, motor association with membranes is regulated by the dissociation of the receptor-motor complex from membranes. These results identify a cell-cycle-regulated membrane receptor for a molecular motor and suggest a mechanistic basis for achieving the dramatic changes in organelle positioning seen during cell division.

The Sac1 phosphoinositide phosphatase regulates Golgi membrane morphology and mitotic spindle organization in mammals

Phosphoinositides (PIPs) are ubiquitous regulators of signal transduction events in eukaryotic cells. PIPs are degraded by various enzymes, including PIP phosphatases. The integral membrane Sac1 phosphatases represent a major class of such enzymes. The central role of lipid phosphatases in regulating PIP homeostasis notwithstanding, the biological functions of Sac1-phosphatases remain poorly characterized. Herein, we demonstrate that functional ablation of the single murine Sac1 results in preimplantation lethality in the mouse and that Sac1 insufficiencies result in disorganization of mammalian Golgi membranes and mitotic defects characterized by multiple mechanically active spindles. Complementation experiments demonstrate mutant mammalian Sac1 proteins individually defective in either phosphoinositide phosphatase activity, or in recycling of the enzyme from the Golgi system back to the endoplasmic reticulum, are nonfunctional proteins in vivo. The data indicate Sac1 executes an essential household function in mammals that involves organization of both Golgi membranes and mitotic spindles and that both enzymatic activity and endoplasmic reticulum localization are important Sac1 functional properties.

Regulating cytoskeleton-based vesicle motility

During vesicular transport, the assembly of the coat complexes and the selection of cargo proteins must be coordinated with the subsequent translocation of vesicles from the donor to an acceptor compartment. Here, we review recent progress toward uncovering the molecular mechanisms that connect transport vesicles to the protein machinery responsible for cytoskeleton-mediated motility. An emerging theme is that vesicle cargo proteins, either directly or through binding interactions with coat proteins, are able to influence cytoskeletal dynamics and motor protein function. Hence, a vesicle's cargo composition may help direct its intracellular motility and targeting.