Isolation of a putative receptor from Zea mays microsomal membranes that interacts with the G-protein, GP alpha 1

Are retinal and retinal-binding proteins involved in stomatal response to blue light?

Stomata respond to blue light and it is generally believed that the photoreceptor for this response is located inside the guard cells. Only a small number of blue light photoreceptors have been identified so far, namely cryptochromes and phototropins, and they show overlapping functions in regulating many different responses to light. The possibility that plants may possess other receptors regulating blue light responses under different light conditions cannot be excluded. In this paper we show the presence of two retinal-binding proteins in Commelina communis and we report the identification of retinal, a chromophore usually bound to the photoreceptor rhodopsin and previously identified in algae and other higher plants. We show that, under our experimental conditions, stomata open promptly when exposed to blue light and we demonstrated that this response is dependent on retinal. We hypothesise that rhodopsin-like retinal-binding proteins might be involved in stomatal response to blue light.

Heterotrimeric G-proteins in plant cell signaling

Heterotrimeric G-proteins, which couple cell surface receptors with internal effectors, are evident in all eukaryotes. Their operation involves receptor activation, GTP/GDP exchange and modulation of effector activity; deactivation occurs by an intrinsic GTPase activity. Structurally, G-proteins comprise three dissimilar subunits; Gα, Gβ and Gγ. The Gα subunit consists of an α-helical and a GTPase domain, the latter is responsible for interaction with Gβγ, receptor and effector. Gβ and Gγ form a tightly associated heterodimer which can also modulate effector activity when released by the activated Gα. Genome sequence and other data suggest that, in plants, there are several (~8-10?) Gα, one or two Gβ and one Gγ. These proteins are expressed throughout the plant, mainly in the plasma membrane and endoplasmic reticulum. In vivo, there is strong evidence for G-protein control of ion channels, particularly K⁺ , in the response pathways to fungal and bacterial pathogens as well as in some aspects of gibberellin, abscisic acid and auxin signaling pathways. Finally, future prospects for understanding plant G-protein linked signaling will rely on new and emerging technologies; these include antisense suppression, gene knockouts, yeast two-hybrid and phage display molecular approaches, intracellular immunization using recombinant single chain antibodies and expression of peptide encoding minigenes.

Completing the heterotrimer: isolation and characterization of an Arabidopsis thaliana G protein gamma-subunit cDNA

Heterotrimeric G proteins consist of three subunits (alpha, beta, and gamma). alpha- and beta- subunits have been previously cloned in plants, but the gamma-subunit has remained elusive. To isolate the gamma-subunit of a plant heterotrimeric G protein an Arabidopsis thaliana yeast two-hybrid library was screened by using a tobacco G-beta-subunit as the bait protein. One positive clone (AGG1) was isolated several times; it displays significant homology to the conserved domains of mammalian gamma-subunits. The predicted AGG1 protein sequence contains all of the typical characteristics of mammalian gamma-subunits such as small size (98 amino acids, 10.8 kDa), presence of a C-terminal CAAX box to direct isoprenyl modification, and an N-terminal alpha-helix region capable of forming a coiled-coil interaction with the beta-subunit. Northern and Southern analyses showed that AGG1 is a single-copy gene in Arabidopsis with a similar expression pattern to the Arabidopsis beta-subunit, AGB1 [Weiss, C. A., Garnaat, C. W., Mukai, K., Hu, Y. & Ma, H. (1994) Proc. Natl. Acad. Sci. USA 91, 9554-9558]. By using the yeast two-hybrid system, we show that AGG1 strongly interacts with tobacco and Arabidopsis beta-subunits. The in vivo results have been confirmed by using in vitro methods to prove the interaction between AGG1 and the Arabidopsis beta-subunit. As previously observed in mammalian systems, both the coiled-coil domain and the WD repeat regions of the beta-subunit are essential for AGG1 interaction. Also in agreement with previous observations, the removal of the N-terminal alpha-helix of the AGG1 greatly reduces but does not completely block the interaction.

Biochemical characteristics of a rice (Oryza sativa L., IR36) G-protein alpha-subunit expressed in Escherichia coli

A cDNA encoding the alpha-subunit of the heterotrimeric G-protein in rice (RGA1) was overexpressed in Escherichia coli and then isolated by Ni2+-nitrilotriacetic acid affinity chromatography. The molecular mass of RGA1 bearing a His tag was approx. 49 kDa. Immunoblot analysis using anti-RGA1 revealed that the RGA1 protein is most abundant in seedling leaves and least abundant in mature roots. It exists at particularly high levels in the immature embryo after pellicle extrusion. In addition, the RGA1 antiserum exhibited a difference in binding affinity for Galpha proteins from monocots (maize and rice) and dicots (Arabidopsis, pea, soya bean and tomato); whereas it cross-reacted with Galpha proteins of monocots, it did not with those of dicot plants. When bound to guanosine 5'-(gamma-thio)triphosphate (GTP[S]), the RGA1 protein was partially protected from tryptic proteolysis. In the presence of GTP[S], trypsin cleaved the RGA1 protein into four fragments 24, 14, 11 and 5 kDa in size. When RGA1 was bound to GDP, only the 5 kDa polypeptide was seen on SDS/PAGE after trypsin digestion. Photoaffinity labelling with [alpha-32P]GTP and a GTP[S]-binding assay revealed that RGA1 incorporated 32P and showed specific binding to a guanine nucleotide. Guanidine binding of RGA1 was affected by the concentration of MgCl2 (maximum at 2 mM). The rate of guanine nucleotide binding of RGA1 (kon,GTP[S]=0.0141+/-0.0014 min-1) and, at steady state, the kcat value for GTP hydrolysis (0.0075+/-0.0001 min-1) were very low even at 2 mM MgCl2. The binding affinity for the nucleotides examined was in the order GTP-S- >/= GTP > GDP > CTP > ATP >/= dTTP.