ADP-ribosylation of thylakoid membrane polypeptides by cholera toxin is correlated with inhibition of thylakoid GTPase activity and protein phosphorylation

Moving beyond the arabidopsis-centric view of G-protein signaling in plants

Heterotrimeric G-protein-mediated signaling is a key mechanism to transduce a multitude of endogenous and environmental signals in diverse organisms. The scope and expectations of plant G-protein research were set by pioneering work in metazoans. Given the similarity of the core constituents, G-protein-signaling mechanisms were presumed to be universally conserved. However, because of the enormous diversity of survival strategies and endless forms among eukaryotes, the signal, its interpretation, and responses vary even among different plant groups. Earlier G-protein research in arabidopsis (Arabidopsis thaliana) has emphasized its divergence from Metazoa. Here, we compare recent evidence from diverse plant lineages with the available arabidopsis G-protein model and discuss the conserved and novel protein components, signaling mechanisms, and response regulation.

Emerging themes in heterotrimeric G-protein signaling in plants

Heterotrimeric G-proteins are key signaling components involved during the regulation of a multitude of growth and developmental pathways in all eukaryotes. Although the core proteins (Gα, Gβ, Gγ subunits) and their basic biochemistries are conserved between plants and non-plant systems, seemingly different inherent properties of specific components, altered wirings of G-protein network architectures, and the presence of novel receptors and effector proteins make plant G-protein signaling mechanisms somewhat distinct from the well-established animal paradigm. G-protein research in plants is getting a lot of attention recently due to the emerging roles of these proteins in controlling many agronomically important traits. New findings on both canonical and novel G-protein components and their conserved and unique signaling mechanisms are expected to improve our understanding of this important module in affecting critical plant growth and development pathways and eventually their utilization to produce plants for the future needs. In this review, we briefly summarize what is currently known in plant G-protein research, describe new findings and how they are changing our perceptions of the field, and discuss important issues that still need to be addressed.

ADP-ribosylation reactions in plants

Poly ADP-ribosylation is a post-translational modification of protein structure and function that occurs in the nucleus of most eukaryotic cells. Although its function has not been fully elucidated it is thought to have a role in the processing DNA strand breaks. Poly(ADP-ribose) polymerase, a highly conserved enzyme, is well studied in animal cell systems but is less well characterised in plants. Our present understanding of mono and poly ADP-ribosylation reactions in plants is reviewed in this article.

Molecular cloning, sequence determination and heterologous expression of nucleoside diphosphate kinase from Pisum sativum

Protein sequence data derived from the N-terminal region of a 17 kDa polypeptide associated with the microsomal membrane fraction from Pisum sativum was used to design degenerate oligonucleotides which were used to amplify P. sativum cDNA via the polymerase chain reaction (PCR). Amplified cDNA was used as a probe to screen a P. sativum cDNA library and a cDNA clone, NDK-P1 was isolated and sequenced. The protein encoded by NDK-P1 had a calculated molecular mass of 16,485 Da and possessed substantial homology with nucleoside diphosphate kinases (NDKs) isolated and cloned from other sources. High levels of expression of NDK-P1 protein were achieved in Escherichia coli using a T7-driven expression system. Recombinant NDK-P1 protein was shown to possess NDK activity and had similar biochemical characteristics to NDKs isolated from other sources. The Michaelis constants for a variety of nucleoside diphosphate (NDP) substrates were found to be broadly similar to those reported for other NDKs, with thymidine nucleotides being the substrates of greatest affinity.

NAD turnover and utilisation of metabolites for RNA synthesis in a reaction sensing the redox state of the cytochrome b6f complex in isolated chloroplasts

NAD is normally regarded as a redox molecule or as the substrate for ADP-ribosylation reactions. In this study, we describe the rapid metabolism of NAD by Percoll-gradient-purified lettuce chloroplasts and show that the adenine moiety can be incorporated into RNA in a dark-activated reaction that senses the redox state of the cytochrome b6f complex. Isolated chloroplasts rapidly metabolised radiolabelled NAD+ to 5'-AMP (within seconds) and adenosine during a 60-min incubation in vitro; the products were analysed by high-performance liquid chromatography. No radiolabelled ADP-ribose was detected. Radioactivity was incorporated into trichloroacetic-acid-insoluble material during this period, with approximately 2-4-fold more incorporation occurring in the dark. Most of this radiolabel was rendered acid-soluble by dilute alkaline digestion at 37 degrees C, yielding an approximately equal mixture of 2'-AMP and 3'-AMP, and by RNase digestion, identifying the acid-insoluble radioactive material as RNA. Protein-bound ADP-ribose would have yielded 5'-AMP and/or oligomeric/polymeric ADP-ribose after alkali digestion. The utilisation of NAD metabolites for RNA synthesis was restricted to the thylakoid compartment of the chloroplast. The use of a variety of electron-transport inhibitors such as 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone, bromanil (tetrabromo-1,4-benzoquinone), electron donors (dithiothreitol), electron acceptors (ferricyanide) and an uncoupler showed that the incorporation of radiolabel from NAD into acid-insoluble material was favoured when the cytochrome b6f complex was in the oxidised state (as pertaining to incubations in the dark).