A role for the RabA4b effector protein PI-4Kβ1 in polarized expansion of root hair cells in Arabidopsis thaliana
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The RabA4b GTPase labels a novel, trans-Golgi network compartment displaying a developmentally regulated polar distribution in growing Arabidopsis thaliana root hair cells. GTP bound RabA4b selectively recruits the plant phosphatidylinositol 4-OH kinase, PI-4Kβ1, but not members of other PI-4K families. PI-4Kβ1 colocalizes with RabA4b on tip-localized membranes in growing root hairs, and mutant plants in which both the PI-4Kβ1 and -4Kβ2 genes are disrupted display aberrant root hair morphologies. PI-4Kβ1 interacts with RabA4b through a novel homology domain, specific to eukaryotic type IIIβ PI-4Ks, and PI-4Kβ1 also interacts with a Ca2+ sensor, AtCBL1, through its NH2 terminus. We propose that RabA4b recruitment of PI-4Kβ1 results in Ca2+-dependent generation of PI-4P on this compartment, providing a link between Ca2+ and PI-4,5P2–dependent signals during the polarized secretion of cell wall components in tip-growing root hair cells.Keywords:
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The production of Golgi complexes was investigated in Amoeba proteus by introducing a nucleus into cells that had been enucleated for 5 days. Golgi complexes were not detected in 5 day enucleates, nor were they observed in amebae fixed 15 min after renucleation. Samples taken at longer intervals after the introduction of a nucleus exhibited an increase in the size and abundance of Golgi complexes. Small curved smooth cisternae, some of which were aligned in parallel to form small Golgi complexes, were observed 30 min after the operation. Aggregations of small Golgi complexes increased in number in amebae fixed 1 to 6 hr after renucleation. Golgi complexes of normal size were present 6 hr after the operation and became more abundant in samples fixed 12 hr, and 1, 2, and 3 days after renucleation. The possible participation of the granular endoplasmic reticulum in the development of Golgi complexes was suggested by two observations. First, the Golgi complexes in renucleates contained a dense material similar to the content of the endoplasmic reticulum in enucleates and early renucleates. Second, examples of continuity between the endoplasmic reticulum and Golgi cisternae were present in renucleates. The possibility that Golgi complexes can be produced in the absence of preexisting Golgi complexes is discussed.
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The Golgi complex is characterized by its unique morphology of closely apposed flattened cisternae that persists despite the large quantity of lipids and proteins that transit bidirectionally. Whether such a structure is maintained through endoplasmic reticulum (ER)-based recycling and auto-organization or whether it depends on a permanent Golgi structure is strongly debated. To further study Golgi maintenance in interphase cells, we developed a method allowing for a drug-free inactivation of Golgi dynamics and function in living cells. After Golgi inactivation, a new Golgi-like structure, containing only certain Golgi markers and newly synthesized cargos, was produced. However, this structure did not acquire a normal Golgi architecture and was unable to ensure a normal trafficking activity. This suggests an integrative model for Golgi maintenance in interphase where the ER is able to autonomously produce Golgi-like structures that need pre-existing Golgi complexes to be organized as morphologically normal and active Golgi elements.
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GTPases are important regulatory proteins that hydrolyze GTP to GDP. A novel GTP-hydrolysis mechanism is employed by MnmE, YqeH and FeoB, where a potassium ion plays a role analogous to the Arginine finger of the Ras-RasGAP system, to accelerate otherwise slow GTP hydrolysis rates. In these proteins, two conserved asparagines and a 'K-loop' present in switch-I, were suggested as attributes of GTPases employing a K(+)-mediated mechanism. Based on their conservation, a similar mechanism was suggested for TEES family GTPases. Recently, in Dynamin, Fzo1 and RbgA, which also conserve these attributes, a similar mechanism was shown to be operative. Here, we probe K(+)-activated GTP hydrolysis in TEES (TrmE-Era-EngA-YihA-Septin) GTPases - Era, EngB and the two contiguous G-domains, GD1 and GD2 of YphC (EngA homologue) - and also in HflX, another GTPase that also conserves the same attributes. While GD1-YphC and Era exhibit a K(+)-mediated activation of GTP hydrolysis, surprisingly GD2-YphC, EngB and HflX do not. Therefore, the attributes identified thus far, do not necessarily predict a K(+)-mechanism in GTPases and hence warrant extensive structural investigations.
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History of the Golgi Apparatus.- Morphology of the Golgi Apparatus (Architecture/Structure).- Isolation and Subfractionation.- Golgi Apparatus Tubules.- Golgi Apparatus and Membrane Biogenesis.- Golgi Apparatus Function in the Flow-Differentiation of Membranes.- Biochemistry of the Golgi Apparatus.- Golgi Apparatus Function in Secretion.- Golgi Apparatus Replication.- Cell Free Systems for Study of Golgi Apparatus Function.- Golgi Apparatus Function in Growth and Cell Enlargement.- The Golgi Apparatus and Cancer.- The Golgi Apparatus: A Look Ahead
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Abstract The study of Golgi structure and function has been greatly facilitated by the purification of this membrane organelle from cellular homogenates. Purified Golgi membranes have been used in a variety of cell‐free assays to investigate sugar modifications, vesicle transport, Golgi structure formation and Golgi–cytoskeleton interactions. Golgi membranes can be purified from cells and tissues using a number of different methods, with liver as a preferred source. Highly purified Golgi stacks can be obtained after two sequential density gradient centrifugations of rat liver homogenate. The relative purity of the prepared Golgi stacks is assessed by measuring the increase in activity of a Golgi enzyme, β‐1,4‐galactosyltransferase (GalT), over that of the total liver homogenate. A typical preparation can yield milligrams of Golgi membranes that are purified 80‐ to 100‐fold over the homogenate and 60–70% in stacks, which provides abundant material for both structural and functional studies of this organelle. Key Concepts: The Golgi apparatus is an essential membrane‐bound organelle in the centre of the secretory pathway in almost all eukaryotic cells. The primary function of the Golgi is to modify and package proteins and lipids into transport carriers and send them to the proper locations. Golgi can be separated from other membranous organelles by density gradient centrifugation. Glycosyl transferases are used as marker enzymes to monitor the yield and purity of the purified Golgi apparatus during Golgi preparation. Purified Golgi membranes can be used in a variety of cell‐free assays to investigate sugar modifications, vesicle transport, Golgi structure formation and Golgi–cytoskeleton interactions.
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