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First published online April 10, 2009
doi: 10.1242/10.1242/dev.030098

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Phosphoinositide-dependent regulation of VAN3 ARF-GAP localization and activity essential for vascular tissue continuity in plants

Satoshi Naramoto^1,2,3,, Shinichiro Sawa¹, Koji Koizumi^1,*, Tomohiro Uemura¹, Takashi Ueda¹, Jíí Friml³, Akihiko Nakano^1,2 and Hiroo Fukuda^1,

¹ Department of Biological Sciences, Graduate School of Science, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan.
² Molecular Membrane Biology Laboratory, RIKEN Advanced Science Institute, Wako, Saitama 351-0198, Japan.
³ Department of Plant Systems Biology, VIB and Department of Plant Biotechnology and Genetics, Ghent University, Technologiepark 927, 9052 Gent, Belgium.

Authors for correspondence (e-mails: sanar{at}psb.vib-ugent.be; fukuda{at}biol.s.u-tokyo.ac.jp)

Accepted 27 February 2009

ACAP-type ARF GTPase activating proteins (ARF-GAPs) regulate multiple cellular processes, including endocytosis, secretion, phagocytosis, cell adhesion and cell migration. However, the regulation of ACAP functions by other cellular proteins is poorly understood. We have reported previously that a plant ACAP, VAN3, plays a pivotal role in plant venation continuity. Here, we report on newly identified VAN3 regulators: the CVP2 (cotyledon vascular pattern 2) 5 PTase, which is considered to degrade IP₃ and also to produce PtdIns(4)P from PtdIns(4,5)P₂; and a PH domain-containing protein, VAB (VAN3 binding protein). Combinational mutations of both CVP2 and its closest homologue CVL1 (CVP2 like 1) phenocopied the strong allele of van3 mutants, showing severe vascular continuity. The phenotype of double mutants between van3, cvp2 and vab suggested that VAN3, CVP2 and VAB function in vascular pattern formation in the same pathway. Localization analysis revealed that both CVP2 and VAB colocalize with VAN3 in the trans-Golgi network (TGN), supporting their functions in the same pathway. The subcellular localization of VAN3 was dependent on its PH domain, and mislocalization of VAN3 was induced in cvp2 or vab mutants. These results suggest that CVP2 and VAB cooperatively regulate the subcellular localization of VAN3 through the interaction between its PH domain and phosphoinositides and/or inositol phosphates. In addition, PtdIns(4)P, to which VAN3 binds preferentially, enhanced the ARF-GAP activity of VAN3, whereas IP₃ inhibited it. These results suggest the existence of PtdIns(4)P and/or IP₃-dependent subcellular targeting and regulation of VAN3 ACAP activity that governs plant vascular tissue continuity.

Key words: Arabidopsis, VAN3, Vein, ARF-GAP, CVP2, VAB

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