75 Integrating Membrane Transport with Male Gametophyte ... - TAIR

107 Ubiquitin Lys 63 Chain Forming Ligases RGL51 and RGL52 Mediate Apical Dominance,
Hormone Balance and Cell Fate Decisions in Arabidopsis
Xiao-Jun Yin 1 , Sara Volk 2 , Karin Ljung 3 , Karel Dolezal 3 , Shigeru Hanano 1 , Seth Davis 1 , Elmon Schmelzer 1 , Goran
Sandberg 3 , Cecile Pickart 2 , Andreas Bachmair 1
1
Max Planck Institute for Plant Breeding Research, D-50829 Cologne, Germany, 2 Johns Hopkins University,
Baltimore, MD, USA 21205, 3 Umea Plant Science Centre, S-90183 Umea, Sweden
Polyubiquitin chains assembled through Lys 48 of ubiquitin are recognition signals for degradation of the modified
substrate protein. In contrast, chains assembled through Lys 63 of ubiquitin have no function in proteolysis. A known
role for ubiquitin Lys 63 chains in animals and fungi is in DNA repair. The function of these chains in plants is unclear
at present.
In order to investigate ubiquitin Lys 63 chain formation in Arabidopsis, we carried out a yeast two-hybrid experiment,
using the plant homologs of two conserved cofactors of ubiquitin Lys 63 chain formation, MMS2 and UBC13, as a bait.
Two closely related ubiquitin protein ligases were identified, the RGL51 and RGL52. They reside at the plasma membrane
and on the endo-membrane system. In vitro, RGL52 can attach ubiquitin Lys 63 chains onto itself. Double mutants rgl51
rgl52 have altered levels of the mobile plant growth regulators, auxin and cytokinin. Response to exogenously added
auxin is decreased compared to wild type plants. Mutants are bushy, and differ from wild type in a variety of additional
traits such as phyllotaxy, circadian rhythm and cell size. RGL51 / 52 may be involved in signal transduction, for instance
by influencing auxin and / or cytokinin transport and distribution.
108 The V-ATPase and its role in cell elongation
Angela Bruex, Matthias Grauer, Karin Schumacher
ZMBP Plant Physiology, University of Tuebingen, Germany
The vacuolar H+ ATPase (V-ATPase), a highly conserved eukaryotic proton pump present in the endomembrane
compartments, plays a crucial role in establishing proton gradients, which are needed for secondary active transport and
turgor regulation. The V-ATPase also functions in regulating the pH homeostasis of cellular compartments, which is
important for protein targeting, enzyme activity, and vesicle trafficking. Inhibition of the V-ATPase with Concanamycin,
a V-ATPase specific inhibitor leads to reduced hypocotyl growth, an effect also observed in the det3 (de-etiolated3)
mutant. The det3 phenotype is caused by a mutation in the V-ATPase subunit C (VHA-C) leading to a reduced V-ATPase
activity of approximately 50%. These pharmacological and genetical evidences indicate that the V-ATPase is important
for cell elongation. To clarify the role of the V-ATPase in cell elongation, we further investigated the det3 mutant, an
excellent tool due to its conditional phenotype: The det3 phenotype is inducible by nitrate and lower temperatures causing
a short hypocotyl, whereas det3 seedlings grown under permissive conditions exhibit normal hypocotyl length. Further
experiments like growth studies on different inhibitors, cellulose measurements and the investigation of the transcriptome
revealed a similarity between det3 and cell wall synthesis mutants and a misfunction in vesicle trafficking.