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

381 The Auxin Perception Network
May Christian 1 , Hartwig Luethen 2
1
University of North Carolina at Chapel Hill, 2 University of Hamburg
During the last years the number of genes identified to be involved in auxin responses raised constantly as did the
number of related mutants. In contrast characterization of these mutants in regard of the most relevant part of auxin action
– the growth response – is still incomplete at best. The reason is that Arabidopsis organs displaying auxin-induced growth
are so tiny and that auxin responses occur on short time scales. To deal with these problems we developed a CCD-based
method in order to carry out high-resolution growth measurements of Arabidopsis hypocotyls and flower stems.
We used this novel technique to investigate the very first part of auxin signaling - auxin perception - and revealed
apparently paradox results.
Back in the 1990ies there was some physiological evidence pointing to an intracellular auxin receptor. Analysis of
the auxin influx carrier mutant aux1 confirmed the belief that auxin perception takes place inside the cell. We were able
to support this theory by observing the growth response of tir1 with the help of the CCD-auxanometer.
On the other hand auxin perception was generally regarded as a process localized at the cell surface. A putative auxin
receptor, auxin binding protein 1 (ABP1), has been identified. Unfortunately knocking out this gene results in embryo
lethality. To by-pass this problem we developed another strategy using signaling mutants to explore if auxin signaling
chains starting with ABP1 are involved in growth control. Here we used a single cell system responsible for auxin and
analyzed the physiological effects of anti-ABP1 antibodies. We found that there is at least some linkage between ABP1
and growth control.
We will propose hypotheses to bring these at first view puzzling results together.
382 MUBS: a Family of Ubiquitin-Fold Proteins that are Plasma Membrane-Anchored by
Prenylation
Brian Downes 1 , Scott Saracco 2 , Sang Sook Lee 2 , Dring Crowell 3 , Richard Vierstra 2
1
Saint Louis University, 2 University of Wisconsin-Madison, 3 Indiana U. Purdue U. Indianapolis
Ubiquitin (Ub)-fold proteins are rapidly emerging as an important class of eukaryotic modifiers, which often exert
their influence by post-translational addition to other intracellular proteins. Despite assuming a common β-grasp threedimensional
structure, their functions are highly diverse owing to distinct surface features and targets, and include tagging
proteins for selective breakdown, nuclear import, autophagic recycling, vesicular trafficking, polarized morphogenesis,
and the stress response. Here, we describe a novel family of Membrane-anchored Ub-fold (MUB) proteins that are present
as single copy genes in animals and filamentous fungi and small gene families in plants. Extending from the C-terminus
of the Ub fold is typically a cysteine-containing CaaX box, a canonical signal for the attachment of either a 15-carbon
farnesyl or a 20-carbon geranylgeranyl moiety.
Using in vitro prenylation assays, we show that representative MUBs from humans, mice, Drosophila, Xenopus,
zebrafish, and Arabidopsis can be prenylated, and the cys of the CaaX box is required for prenylation. Modified forms
of several MUBs were detected in transgenic Arabidopsis, suggesting that these MUBs are prenylated in vivo. Both
cell fractionation and confocal microscopic analyses of Arabidopsis plants expressing GFP-MUB fusions showed that
the modified forms are membrane anchored with a significant enrichment on the plasma membrane. This localization
could be blocked by mevinolin, which inhibits the synthesis of prenyl groups. In addition to the five MUBs with CaaX
boxes, Arabidopsis has one unique MUB variant with a cysteine-rich C-terminus distinct from the CaaX box that is also
membrane anchored, possibly through the attachment of a long-chain acyl group. While the physiological role(s) of
MUBs remain unknown, the discovery of these prenylated forms further expands the diversity and potential functions
of Ub-fold proteins in eukaryotic biology.