Enhancement of hypocotyl elongation by LOV KELCH PROTEIN2 production is mediated by auxin and phytochrome-interacting factors in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>

Key message

Auxin and two phytochrome-interacting factors, PHYTOCHROME-INTERACTING FACTOR4 (PIF4) and PIF5, play crucial roles in the enhancement of hypocotyl elongation in transgenic Arabidopsis thaliana plants that overproduce LOV KELCH PROTEIN2 (LKP2).

Abstract

LOV KELCH PROTEIN2 (LKP2) is a positive regulator of hypocotyl elongation under white light in Arabidopsis thaliana. In this study, using microarray analysis, we compared the gene expression profiles of hypocotyls of wild-type Arabidopsis (Columbia accession), a transgenic line that produces green fluorescent protein (GFP), and two lines that produce GFP-tagged LKP2 (GFP-LKP2). We found that, in GFP-LKP2 hypocotyls, 775 genes were up-regulated, including 36 auxin-responsive genes, such as 27 SMALL AUXIN UP RNA (SAUR) and 6 AUXIN/INDOLE-3-ACETIC ACID (AUX/IAA) genes, and 21 genes involved in responses to red or far-red light, including PHYTOCHROME-INTERACTING FACTOR4 (PIF4) and PIF5; and 725 genes were down-regulated, including 15 flavonoid biosynthesis genes. Hypocotyls of GFP-LKP2 seedlings, but not cotyledons or roots, contained a higher level of indole-3-acetic acid (IAA) than those of control seedlings. Auxin inhibitors reduced the enhancement of hypocotyl elongation in GFP-LKP2 seedlings by inhibiting the increase in cortical cell number and elongation of the epidermal and cortical cells. The enhancement of hypocotyl elongation was completely suppressed in progeny of the crosses between GFP-LKP2 lines and dominant gain-of-function auxin-resistant mutants (axr2-1 and axr3-1) or loss-of-function mutants pif4, pif5, and pif4 pif5. Our results suggest that the enhancement of hypocotyl elongation in GFP-LKP2 seedlings is due to the elevated level of IAA and to the up-regulated expression of PIF4 and PIF5 in hypocotyls.

     

The expression of tobacco knotted1-type class 1 homeobox genes correspond to regions predicted by the cytohistological zonation model

We have isolated and characterized four tobacco homeobox genes, NTH1, NTH9, NTH20, NTH22 (Nicotiana tabacum homeobox) which belong to the class 1 knotted1-type family of homeobox genes. Comparison of the inferred amino acid sequences of the ELK homeodomains of these genes and previously reported kn1-type class 1 proteins has revealed that the four new tobacco genes belong to distinct subclasses, suggesting that each NTH gene may have distinct functions. Using in situ hybridization and by analysing the distribution of GUS activity in tobacco plants transformed with NTH promoter::GUS constructs, localized expression of the three NTH genes was observed in the shoot apical meristem (SAM). In the vegetative SAM, NTH1 and NTH15 showed overlapping expression in the corpus, NTH20 was expressed in the peripheral zone, and NTH9 was predominantly expressed in the rib zone. The expression patterns of the different NTH genes correspond to regions predicted by the cytohistological zonation model, suggesting that each NTH gene specifies the function of the SAM zone with which it is associated.     

Domain-domain interaction of P-Rex1 is essential for the activation and inhibition by G protein betagamma subunits and PKA

PtdIns(3, 4, 5)P(3)-dependent Rac exchanger (P-Rex) 1 is a guanine nucleotide exchange factor (GEF) for the small GTPase Rac. P-Rex1 is activated by G protein betagamma subunits (Gbetagamma), and the Gbetagamma-induced activation is inhibited by cAMP-dependent protein kinase A (PKA). However, the details of regulatory mechanism of P-Rex1 remain to be clarified. In the present study, we investigated the mechanism of activation and inhibition of P-Rex1 using various truncated and alanine-substituted mutants and found that the domain-domain interaction of P-Rex1 is important for Gbetagamma-induced activation and PKA-induced inhibition. Immunoprecipitation analysis showed that the second Disheveled/EGL-10/Pleckstrin (DEP) and first PSD-95/Dlg/ZO-1 (PDZ) domains of P-Rex1 associate with the inositol polyphosphate-4-phosphatase (IP4P) domain. Carboxyl-terminal truncation on the IP4P domain or mutations in the protein-binding pocket of the first PDZ domain abolished the association. Analysis of in vitro guanine nucleotide exchange assay, PAK1/2 phosphorylation, and Rac-specific actin reorganization revealed that Gbetagamma could activate a complex of the P-Rex1 mutant lacking the IP4P domain and the isolated IP4P domain as well as full-length P-Rex1. Moreover, PKA phosphorylation prevented the domain-domain interaction and Gbetagamma-binding. These results provide a new insight into the regulation of other Rho-family GEFs and cell responses induced by the heterotrimeric G protein.     

Mannose binding lectin and lung collectins interact with Toll-like receptor 4 and MD-2 by different mechanisms

Background

We have previously shown that lung collectins, surfactant protein A (SP-A) and surfactant protein D, interact with Toll-like receptor (TLR) 2, TLR4, or MD-2. Bindings of lung collectins to TLR2 and TLR4/MD-2 result in the alterations of signaling through these receptors, suggesting the immunomodulatory functions of lung collectins. Mannose binding lectin (MBL) is another collectin molecule which has structural homology to SP-A. The interaction between MBL and TLRs has not yet been determined.

Methods

We prepared recombinant MBL, and analyzed its bindings to recombinant soluble forms of TLR4 (sTLR4) and MD-2.

Results

MBL bound to sTLR4 and MD-2. The interactions were Ca²⁺-dependent and inhibited by mannose or monoclonal antibody against the carbohydrate-recognition domain of MBL. Treatment of sTLR4 or MD-2 by peptide N-glycosidase F significantly decreased the binding of MBL. SP-A bound to deglycosylated sTLR4, and this property did not change in chimeric molecules of SP-A/MBL in which Glu¹⁹⁵–Phe²²⁸ or Thr¹⁷⁴–Gly¹⁹⁴ of SP-A were replaced with the corresponding MBL sequences.

General Significance

These results suggested that MBL binds to TLR4 and MD-2 through the carbohydrate-recognition domain, and that oligosaccharide moieties of TLR4 and MD-2 are important for recognition by MBL. Since our previous studies indicated that lung collectins bind to the peptide portions of TLRs, MBL and lung collectins interact with TLRs by different mechanisms. These direct interactions between MBL and TLR4 or MD-2 suggest that MBL may modulate cellular responses by altering signals through TLRs.     

Mal3 Masks Catastrophe Events in Schizosaccharomyces pombe Microtubules by Inhibiting Shrinkage and Promoting Rescue

Schizosaccharomyces pombe Mal3 is a member of the EB family of proteins, which are proposed to be core elements in a tip-tracking network that regulates microtubule dynamics in cells. How Mal3 itself influences microtubule dynamics is unclear. We tested the effects of full-length recombinant Mal3 on dynamic microtubules assembled in vitro from purified S. pombe tubulin, using dark field video microscopy to avoid fluorescent tagging and data-averaging techniques to improve spatiotemporal resolution. We find that catastrophe occurs stochastically as a fast (<2.2 s) transition from constant speed growth to constant speed shrinkage with a constant probability that is independent of the Mal3 concentration. This implies that Mal3 neither stabilizes nor destabilizes microtubule tips. Mal3 does, however, stabilize the main part of the microtubule lattice, inhibiting shrinkage and increasing the frequency of rescues, consistent with recent models in which Mal3 on the lattice forms stabilizing lateral links between neighboring protofilaments. At high concentrations, Mal3 can entirely block shrinkage and induce very rapid rescue, making catastrophes impossible to detect, which may account for the apparent suppression of catastrophe by Mal3 and other EBs in vivo. Overall, we find that Mal3 stabilizes microtubules not by preventing catastrophe at the microtubule tip but by inhibiting lattice depolymerization and enhancing rescue. We argue that this implies that Mal3 binds microtubules in different modes at the tip and on the lattice.Microtubules are intrinsically dynamic self-assembling structures of tubulin subunits (1) whose polymerization is subject to extensive spatial and temporal control in cells partly through the activity of microtubule-associated proteins (2). In cells, the EB family of microtubule plus end-tracking proteins (+TIPs)² localizes at the plus end of growing but not shrinking microtubules. EB depletion increases catastrophe frequency and reduces microtubule length in many species (3–5), suggesting that EBs suppress microtubule catastrophes. It is, however, unclear from these cellular studies whether this activity is direct or indirect because the dynamic binding of EBs to other +TIPs proteins enhances the localization of all EB complex proteins, including EB1, to microtubule ends (6).To determine the direct effect of EB family proteins on microtubule dynamics, in vitro experiments are necessary. These have established that microtubule end tracking is an intrinsic property of the EB proteins and that other +TIP proteins such as CLIP170 are dependent upon EBs for their microtubule end localization (7–9). However, EB1 binding also directly alters the structure of growing microtubule tips (10). In vitro studies show that Mal3, the EB1 homologue in Schizosaccharomyces pombe, can also affect the structure of microtubules. Sandblad et al. (11) found localization of Mal3 along the (A-lattice) seam of B-lattice microtubules and proposed this as a potential mechanism for direct microtubule stabilization by the EBs. Des Georges et al. (12) showed that Mal3 binds to and specifically stabilizes the A-lattice protofilament overlap, promoting nucleation and assembly of A-lattice-containing microtubules.Several studies in vitro have all shown that EBs can affect microtubule dynamics (4, 7, 10, 13) but conflict over which parameter is affected. Thus although Bieling et al. (7) and Manna et al. (13) observed no effect on microtubule growth rates, Komarova et al. (4) and Vitre et al. (10) found an acceleration of growth. Manna et al. (13) found that EB1 inhibits catastrophe, yet the other studies observed that EBs trigger catastrophe events. There is clearly a need to resolve these apparent conflicts, especially as the same proteins in vivo appear to suppress catastrophe.To try to elucidate the mechanism by which EB proteins influence microtubule assembly, we developed a minimalist approach in which the potential for confounding factors to affect the data is reduced or eliminated. Our assay uses proteins from a single organism, S. pombe, and GMPCPP-stabilized microtubule seeds assembled from purified tubulin with only the seeds attached to the chamber surface. We used this system to measure the effects of unlabeled full-length Mal3 on the polymerization dynamics of unlabeled S. pombe microtubules. Microtubules were imaged using dark field microscopy to avoid fluorescent labeling (see Fig. 1A). We also developed a semiautomated analysis system that allows us to digitize a large number of events, which can then be processed by data averaging and filtering. This reduces noise, allowing us to examine the detailed kinetics of the catastrophic switch from growth to shrinkage. Using this system, we find that Mal3 has no direct effect upon the frequency or kinetics of catastrophe events but that it does reduce shrinkage rates and increase rescue frequency in a dose-dependent manner.Open in a separate windowFIGURE 1.In vitro S. pombe microtubule dynamics assay. A, schematic diagram of S. pombe microtubule dynamics assay. GMPCPP stabilized polarity-marked microtubule seed assembled from Alexa Fluor 488- and Alexa Fluor 680-labeled pig brain tubulin. Only the center of the seed is attached to the surface by anti-Alexa Fluor 488 antibody. Dynamic non-fluorescently labeled S. pombe microtubules grown from seeds were observed by dark field illumination. B, merged fluorescence images of GMPCPP stabilized, polarity-marked pig microtubule seed (pig Alexa-MT). Green, Alexa Fluor 488; red, Alexa Fluor 680. Polarity is indicated by − or +. The plus end of the seed has a longer Alexa Fluor 680-labeled region (upper panel), a dark field image showing pig microtubule seed plus elongated S. pombe microtubules (middle panel), and the merged images (lower panel). Red broken lines show the ends of the seed, and yellow broken lines show the ends of the elongated S. pombe microtubules. Arrows indicate the dynamic S. pombe microtubule elongated from the stabilized microtubule seed. Scale bar: 10 μm. C, kymographs of microtubule length change over time. The left panel shows a diagram of a typical example. Time is indicated by the vertical axis, and length is indicated by the horizontal axis. Rescue (r) and catastrophe (c) events are labeled. Regrowth of shrinking microtubules from the seed (yellow arrow) were not counted as rescues. Scale bars: vertical, 5 min; horizontal, 20 μm. + and − ends of microtubule are indicated. D, enlargement of catastrophe events from the yellow rectangle in C. Scale bars: vertical, 30 s; horizontal, 5 μm.     

Reconstitution in vitro of the catalytic portion (NtpA3-B3-D-G complex) of Enterococcus hirae V-type Na-ATPase

Enterococcus hirae vacuolar ATPase (V-ATPase) is composed of a soluble catalytic domain (V₁; NtpA₃-B₃-D-G) and an integral membrane domain (V₀; NtpI-K₁₀) connected by a central and peripheral stalk(s) (NtpC and NtpE-F). Here we examined the nucleotide binding of NtpA monomer, NtpB monomer or NtpD-G heterodimer purified by using Escherichia coli expression system in vivo or in vitro, and the reconstitution of the V₁ portion with these polypeptides. The affinity of nucleotide binding to NtpA was 6.6 μM for ADP or 3.1 μM for ATP, while NtpB or NtpD-G did not show any binding. The NtpA and NtpB monomers assembled into NtpA₃-B₃ heterohexamer in nucleotide binding-dependent manner. NtpD-G bound NtpA₃-B₃ forming V₁ (NtpA₃-B₃-D-G) complex independent of nucleotides. The V₁ formation from individual NtpA and NtpB monomers with NtpD-G heterodimer was absolutely dependent on nucleotides. The ATPase activity of reconstituted V₁ complex was as high as that of native V₁-ATPase purified from the V₀V₁ complex by EDTA treatment of cell membrane. This in vitro reconstitution system of E. hirae V₁ complex will be valuable for characterizing the subunit-subunit interactions and assembly mechanism of the V₁-ATPase complex.