Northern Illinois University
Department of Biological Sciences
College of Liberal Arts and Sciences

Stafstrom, JP (2000). Regulation of growth and dormancy in pea axillary buds. IN: Dormancy in Plants, J.-D. Viémont and J. Crabbé, eds.  CAB International, Wallingford, UK.  pp. 331-346.

Devitt, ML, Maas, KJ, and Stafstrom, JP (1999). Characterization of DRGs, developmentally regulated GTP-binding proteins, from pea and Arabidopsis. Plant Molecular Biology 39: 75-82.

Stafstrom, JP, Ripley, BD, Devitt, ML, and Drake, B (1998). Dormancy-associated gene expression in pea axillary buds. Cloning and expression of PsDRM1 and PsDRM2. Planta 205:547-552.

Research

GTP binding proteins (G proteins) are molecular switches that use the GTPase cycle to regulate a wide variety of fundamental cellular processes.  Well-characterized proteins such as Ras and heterotrimeric G proteins use the GTPase cycle to transduce signals across the plasma membrane.  Other G proteins assure the fidelity of physiological activities, including the accuracy of protein synthesis steps (initiation and elongation factors IF2, EF-G, EF-Tu) and targeting of vesicles to the proper organelle (Rabs, Arf).  Many other families of G proteins are far less well understood, despite very high levels of sequence conservation and an evolution history that can be traced to the last universal common ancestor of all life (Caldon et al., 2001; Leipe et al., 2002).  Among this latter group are DRGs and OBGs, which are the focus of our research. 

DRGs occur in archaea (one gene; COG 1163) and eukaryotes (two orthologous groups in all sequenced organisms; KOGs 1486 and 1487; Li and Trueb, 2000).  OBGs (COG 0536), which are closely related to DRGs, occur in bacteria and endosymbiont organelles of eukaryotes (Czyz and Wegrzyn, 2005; Datta et al., 2005).  Both DRG1 and DRG2 from most organisms contain about 365-370 amino acid residues and have molecular masses of about 43 kDa.  Amino acid identity among plant, animal and fungal representatives within an orthologous group is about 65-70%, whereas paralogs from a single species share about 55-60% identity.  The guanine nucleotide binding pocket is within the N-terminal two-thirds of the protein.  In addition, DRGs contain a TGS domain (pfam02824), which is suggested to be responsible for binding RNA (Wolf et al., 1999).  DRG2 proteins of land plants and green algae contain a distinctive 32 residue domain at their C-termini, giving them a total length of about 399 amino acids. 

Arabidopsis has three DRG genes, AtDRG1 (At4g39520), AtDRG2 (At1g17470) and AtDRG3 (At1g72660).  AtDRG2 and AtDRG3, which are DRG2 orthologues, encode proteins that are 95% identical.  In earlier work, we showed that pea and Arabidopsis DRG2 genes are broadly expressed, but are more highly expressed in cells and tissues that are growing, dividing or metabolically active (Devitt et al., 1999).  More recently, expression of Arabidopsis DRG genes was studied by:  1) determining patterns of promoter::GUS expression in transgenic plants; 2) assessing patterns of mRNA accumulation using qRT-PCR; and 3) analyzing protein accumulation using DRG1- and DRG2-specific antibodies (Stafstrom, in press).  DRG1 and DRG2 promoters show similar but not identical spatial activity in seedlings and mature organs.  qRT-PCR experiments indicate similar levels of DRG1 and DRG2 mRNA accumulation in most tissues.  DRG3 transcripts are almost undetectable in nearly all tissues, but increase in abundance about 1000-fold after exposure to 37°C for 3 hours.  DRG1 antibodies recognize a 43 kDa protein and DRG2 antibodies recognize 45, 43 and 30 kDa proteins (the latter are breakdown products of 45 kDa).  Exposure of plants to a variety of environmental conditions (including salt, heat, cold, UV light, and osmotic stresses) did not have much effect on levels of DRG1 or DRG2 accumulation.  The exception was heat stress at 37°C.  Heat did not affect DRG1 levels, but increases in protein bands recognized by DRG2 antibodies were seen.  The modest changes in DRG mRNA and protein levels suggest that other types of regulation, such as altered subcellular localization, may be important for the cellular functions of DRGs.

Many G proteins function through transient associations with various organelles.  For example, Ran helps to shuttle cargos between the cytoplasm and nucleoplasm.  Using differential centrifugation, we showed that pea DRG2 occurs predominantly in P-150 and S-150 fractions (150,000 x g pellet and supernatant, respectively; Devitt et al. 1999).  Ethridge and co-workers (Ethridge et al. 1999) showed using immunocytochemistry that Arabidopsis DRG2 occurred in cytoplasmic granules or bodies (these authors called this protein AtDRG1, but its sequence is more related to the DRG2 orthologous group; Li and Trueb 2000; Stafstrom, in press).  The organelles responsible for DRG2 association with the P-150 fraction or with these cytoplasmic granules are not known.  Neither DRG1 nor DRG2 appears to be an integral membrane protein and neither contains conserved sequence motifs that are responsible for prenylation or alkylation.  OBGs associate with ribosomes (Scott et al., 2000; Datta et al., 2005).  We have found recently that pea and Arabidopsis DRGs also associate with various forms of ribosomes (40S and 60S subunits, 80S monosomes and polysomes; Nelson, Maas, Dekeyser and Stafstrom, in preparation).

References cited:

• Caldon CE, Yoong P and March PE (2001).  Evolution of a molecular switch: Universal bacterial GTPases regulate ribosome function.  Molecular Microbiology 41:289-297.
• Czyz A and Wegrzyn G (2005).  The Obg subfamily of bacterial GTP-binding proteins: essential proteins of largely unknown functions that are evolutionarily conserved from bacteria to humans.  Acta Biochim Polonica 52:35-43.
• Datta K, Fuentes JL and Maddock JR (2005).  The yeast GTPase Mtg2p is required for mitochondrial translation and partially suppresses an rRNA methyltransferase mutant, mrm2.  Mol Biol Cell 16:954-963.
• Devitt ML, Maas KJ and Stafstrom JP (1999).  Characterization of DRGs, developmentally regulated GTP-binding proteins, from pea and Arabidopsis.  Plant Molec Biol 39:75-82.
• Etheridge N, Trusov Y, Verbelen JP and Botella JR (1999).  Characterization of ATDRG1, a member of a new class of GTP-binding proteins in plants.  Plant Molec Biol 39:1113-1126.
• Leipe DD, Wolf YI, Koonin EV and Aravind L (2002).  Classification and evolution of P-loop GTPases and related ATPases.  J Molec Biol 317:41-72.
• Li B and Trueb B (2000).  DRG represents a family of two closely related GTP-binding proteins.  Biochim Biophys Acta 1491:196-204.
• Scott JM, Ju J, Mitchell T and Haldenwang WG (2000).  The Bacillus subtilis GTP binding protein obg and regulators of the sigma(B) stress response transcription factor cofractionate with ribosomes.  J Bacteriol 182:2771-2777.
• Stafstrom JP.  Expression Patterns of Arabidopsis DRG genes:  Promoter::GUS fusions, quantitative RT-PCR and patterns of protein accumulation in response to environmental stresses.  Intl J Plant Sci in press.
• Wolf YI, Aravind L, Grishin NV and Koonin EV (1999). Evolution of aminoacyl-tRNA synthetases--analysis of unique domain architectures and phylogenetic trees reveals a complex history of horizontal gene transfer events.  Genome Research 9:689-710.