Signal transduction involving heterotrimeric G proteins is universal among fungi, animals, and plants. In plants and fungi, the best understood function for the G protein complex is its modulation of cell proliferation and one of several important signals that are known to modulate the rate at which these cells proliferate is D-glucose. Arabidopsis thaliana seedlings lacking the  subunit (AGB1) of the G protein complex have altered cell division in the hypocotyl and are D-glucose hypersensitive. With the aim to discover new elements in G protein signaling, we screened for gain-of-function suppressors of altered cell proliferation during early development in the agb1-2 mutant background. One agb1-2-dependent suppressor, designated sgb1-1 D for suppressor of G protein beta1 (agb1-2), restored to wild type the altered cell division in the hypocotyl and sugar hypersensitivity of the agb1-2 mutant. Consistent with AGB1 localization, SGB1 is found at the highest steady-state level in tissues with active cell division, and this level increases in hypocotyls when grown on D-glucose and sucrose. SGB1 is shown here to be a Golgi-localized hexose transporter and acts genetically with AGB1 in early seedling development.
INTRODUCTIONAn evolutionarily ancient mechanism for sensing extracellular signals involves the heterotrimeric G proteins, composed of ␣, , and ␥ subunits. Heterotrimeric G protein complexes link ligand perception via seven-transmembrane (7TM), G protein-coupled receptors (GPCRs) to downstream effectors. Genes that encode G protein signaling elements have been identified in amoebae, fungi, plants, and animals, but among all multicellular eukaryotes, plants have the simplest repertoire of G protein elements to date. Specifically, the Arabidopsis genome encodes a single canonical G␣ and G (AGB1) subunit and two G␥ subunits and a single regulator of G signaling (RGS1) protein (Jones and Assmann, 2004). There are as yet no plant GPCRs having confirmed ligands, although plants do have a limited set of predicted 7TM proteins (Moriyama and Jones, unpublished data). Similarly, there are few known downstream effectors that physically interact with either the plant G␣ subunit or the G␥ dimer. One example is a pirin protein (Lapik and Kaufman, 2003), known to serve as a transcriptional cofactor in humans, but with unknown function in Arabidopsis. Based on either genetic or biochemical tests, G␣ effectors in plants also include phospholipase D (Mishra et al., 2006) and ion channels (Aharon et al., 1998;Wang et al., 2001). Recently, we reported that a plant interactor and putative effector to G␣ is an outer membrane plastid protein designated THF1, and this protein together with G␣ comprises part of a dglucose signaling network (Huang et al., 2006).In animals and yeast, heterotrimeric G proteins couple a diverse set of signals such as photons, ions, small molecules, sugars, peptides, and protein ligands (Jones and Assmann, 2004) to control a broad range of physiology (Csaszar and Abel, 2001;Rosenkilde et al., 2001;Roc...