Abstract
Small GTPase proteins play essential roles in the regulation of vesicular trafficking systems in eukaryotic cells. Two types of small GTPases, secretion-associated Ras-related protein (Sar) and ADP-ribosylation factor (Arf), act in the biogenesis of transport vesicles. Sar/Arf GTPases function as molecular switches by cycling between active, GTP-bound and inactive, GDP-bound forms, catalyzed by guanine nucleotide exchange factors and GTPase-activating proteins, respectively. Activated Sar/Arf GTPases undergo a conformational change, exposing the N-terminal amphipathic α-helix for insertion into membranes. The process triggers the recruitment and assembly of coat proteins to the membranes, followed by coated vesicle formation and scission. In higher plants, Sar/Arf GTPases also play pivotal roles in maintaining the dynamic identity of organelles in the secretory pathway. Sar1 protein strictly controls anterograde transport from the endoplasmic reticulum (ER) through the recruitment of plant COPII coat components onto membranes. COPII vesicle transport is responsible for the organization of highly conserved polygonal ER networks. In contrast, Arf proteins contribute to the regulation of multiple trafficking routes, including transport through the Golgi complex and endocytic transport. These transport systems have diversified in the plant kingdom independently and exhibit several plant-specific features with respect to Golgi organization, endocytic cycling, cell polarity and cytokinesis. The functional diversification of vesicular trafficking systems ensures the multicellular development of higher plants. This review focuses on the current knowledge of Sar/Arf GTPases, highlighting the molecular details of GTPase regulation in vesicle formation in yeast and advances in knowledge of the characteristics of vesicle trafficking in plants.
Highlights
Eukaryotes utilize guanine nucleotides to regulate many intracellular cellular processes, including the endomembrane vesicular trafficking system
These results suggest that the functional diversification of plant COPII components occurred in the regulation of the plant early secretory pathway to maintain the dynamic identity of secretory organelles
Molecular approaches utilizing the transient expression of GDP- or GTP-locked Arabidopsis Arf1A mutants caused an abrogation of endoplasmic reticulum (ER)-to-Golgi transport and a redistribution of green fluorescent protein (GFP)- or Yellow fluorescent protein (YFP)-tagged Golgi membrane marker proteins into the ER in plant cells (Lee et al, 2002; Takeuchi et al, 2002; Stefano et al, 2006). These results suggests that plant Arf1A proteins execute highly conserved functions in the formation of COPI vesicles at the Golgi apparatus by recruiting COPI coat complexes to the membranes (Letourneur et al, 1994; Pimpl et al, 2000; Robinson et al, 2007)
Summary
Eukaryotes utilize guanine nucleotides to regulate many intracellular cellular processes, including the endomembrane vesicular trafficking system. To ensure proper vesicle formation, COPI and COPII proteins consisting of completely different components should be properly recruited to each organelle membrane For this purpose, Sar1/Arf proteins are switched on and off as appropriate on the respective organelle membranes by specific regulators that convert their guanine nucleotide-binding state. In vitro experiments show that when mixed with purified Sar1/Arf protein, liposomes are deformed into a highly curved tubular structure This tubulation process requires the hydrophobicity of the N-terminal amphipathic helix. Further study is necessary for a better understanding of a role of Sar1/Arf1-formed tubular structures in biological processes in vivo Arf family proteins such as Arf possess a modification of a myristoyl moiety on the N-terminal helix, which is required for potential biological activity as well as membrane association (Kahn et al, 1988, 1995; Franco et al, 1996) (Figure 2). In vitro reconstitution experiments have clearly demonstrated that Sec and Gea constitutively facilitate GDP-GTP exchange in Sar and Arf, respectively (Peyroche et al, 1996; Futai et al, 2004), it is unclear whether and how such catalytic activity is controlled in vivo
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