The apical plasma membrane of pollen tubes contains different PI4P 5-kinases that all produce phosphatidylinositol-4,5-bisphosphate [PtdIns(4,5)P2] but exert distinct cellular effects. In the present example, overexpression of Arabidopsis AtPIP5K5 or tobacco NtPIP5K6-1 caused growth defects previously attributed to increased pectin secretion. In contrast, overexpression of Arabidopsis AtPIP5K2 caused apical tip swelling implicated in altering actin fine structure in the pollen tube apex. AtPIP5K5, NtPIP5K6-1, and AtPIP5K2 share identical domain structures. Domains required for correct membrane association of the enzymes were identified by systematic deletion of N-terminal domains and subsequent expression of fluorescence-tagged enzyme truncations in tobacco pollen tubes. A variable linker region (Lin) contained in all PI4P 5-kinase isoforms of subfamily B, but not conserved in sequence, was recognized to be necessary for correct subcellular localization of AtPIP5K5, NtPIP5K6-1, and AtPIP5K2. Deletion of N-terminal domains including the Lin domain did not impair catalytic activity of recombinant AtPIP5K5, NtPIP5K6-1, or AtPIP5K2 in vitro; however, the presence of the Lin domain was necessary for in vivo effects on pollen tube growth upon overexpression of truncated enzymes. Overexpression of catalytically inactive variants of AtPIP5K5, NtPIP5K6-1, or AtPIP5K2 did not influence pollen tube growth, indicating that PtdIns(4,5)P2 production rather than structural properties of PI4P 5-kinases was relevant for the manifestation of growth phenotypes. When Lin domains were swapped between NtPIP5K6-1 and AtPIP5K2 and the chimeric enzymes overexpressed in pollen tubes, the chimeras reciprocally gained the capabilities to invoke tip swelling or secretion phenotypes, respectively. The data indicate that the Lin domain directed the enzymes into different regulatory contexts, possibly contributing to channeling of PtdIns(4,5)P2 at the interface of secretion and actin cytoskeleton.
Keywords: PtdIns(4,5)P2, PI4P 5-kinase, alternative functions, membrane association, pollen tubes
Citation: Stenzel I, Ischebeck T, Quint M and Heilmann I (2012) Variable regions of PI4P 5-kinases direct PtdIns(4,5)P2 toward alternative regulatory functions in tobacco pollen tubes. Front. Plant Sci. 2:114. doi: 10.3389/fpls.2011.00114
Received: 16 September 2011;
Accepted: 23 December 2011;
Published online: 09 January 2012.
Edited by:Kent D. Chapman, University of North Texas, USA
Reviewed by:Erik Nielsen, University of Michigan, USA
Copyright: © 2012 Stenzel, Ischebeck, Quint and Heilmann. This is an open-access article distributed under the terms of the Creative Commons Attribution Non Commercial License, which permits non-commercial use, distribution, and reproduction in other forums, provided the original authors and source are credited.
*Correspondence: Ingo Heilmann, Department of Cellular Biochemistry, Institute for Biochemistry and Biotechnology, Martin-Luther-University Halle–Wittenberg, Kurt-Mothes-Str. 3, 06120 Halle, Germany. e-mail: email@example.com
†Present address: Irene Stenzel and Ingo Heilmann Department of Cellular Biochemistry, Institute for Biochemistry and Biotechnology, Martin-Luther-University Halle–Wittenberg, Halle, Germany; Till Ischebeck, Department of Molecular Systems Biology, University of Vienna, Vienna, Austria.