Heterotrimeric and Unconventional GTP Binding Proteins in Plant Cell Signaling

Abstract

Signal-transducing GTPases in plants include small G proteins, heterotrimeric G proteins, and, potentially, several unique types of GTP binding proteins that are not members of either of the aforementioned classes. This review will focus on recent discoveries concerning the roles of heterotrimeric and “unconventional” G proteins in plant cell signaling. Small G proteins are reviewed elsewhere in this issue (Yang, 2002). Plant heterotrimeric G proteins have been the subject of several other recent reviews to which the reader is also referred (Ma, 1994; Assmann, 1996; Hooley, 1998; Bischoff et al., 1999; Fujisawa et al., 2001). Ma's review (1994) does a particularly good job of summarizing the early, mostly biochemical and immunological, progress toward identifying plant G proteins, which will not be covered here. Mammalian Heterotrimeric G-Protein Cycle. (A) Classic heterotrimeric GTPase cycle. Ligand (L) binding to a GPCR activates the associated G protein, promoting Gα and/or Gβγ interaction with downstream effectors (E). Intrinsic GTPase activity of the Gα subunit eventually returns the G protein to an inactive state. The Gα subunit remains closely associated with the plasma membrane. (B) Certain Gαs (von Zastrow and Mostov, 2001; Zheng et al., 2001) can show dissociation from the plasma membrane upon GPCR activation, contingent upon the absence or removal of lipid modification and the presence of regulatory (R) proteins. Lipid Modifiers and Effectors of G Proteins. (A) Three types of G protein lipid modifications are shown. N-Myristoylation occurs by the attachment of the saturated fatty acid myristate (blue) to a conserved acceptor Gly (red) next to the initiator Met via a stable amino bond. In palmitoylated proteins, the saturated 16-carbon fatty acid palmitate (blue) is attached to Cys residues (red) via a labile thioester bond. Protein prenylation involves the attachment of the 15- and 20-carbon isoprenes farnesyl and geranylgeranyl (blue), respectively, to conserved Cys residues (red) at the C-terminal ends of proteins via a nonreversible thioester bond. The acceptor Cys residues are part of a conserved CaaX box motif in which C indicates Cys, a represents an aliphatic amino acid, and X is usually Ser, Met, Cys, Ala, Gln, or Leu. Ct and Nt indicate C- and N-terminal amino acid positions, respectively, relative to the acceptor amino acid. Proteolysis removes the final three amino acids, and the new C-terminal Cys is then prenylated. The Arabidopsis prototypical GPA1 contains a conserved myristoylation motif and a putative sequence for palmitoylation in the N-terminal region. The two Arabidopsis G protein γ subunits (AGG1 and AGG2) contain a prenyl group binding site in the C-terminal region. (B) Cleavage sites and hydrolysis products of phospholipases regulated by G proteins. PLA2 is an acylhydrolase that specifically removes the acyl chain from the sn-2 position of the glycerol backbone (black). PLD and PI-PLC are phosphodiesterases that generate similar products, except that the phosphate group (blue) either stays with the lipid moiety (black) or goes with the head group (red). DAG, sn-1-palmitoyl,2-linoleoyl diacylglycerol; Ins(1,4,5)P3, inositol 1,4,5-trisphosphate; LysoPtdCho, sn-1-lysophosphatidylcholine; PtdCho, sn-1-palmitoyl,2-linoleoyl phosphatidylcholine; PtdIns(4,5)P2, phosphatidylinositol 4,5-bisphosphate; PtdOH, sn-1-palmitoyl,2-linoleoyl phosphatidate. Mammals also possess ∼5 distinct Gβ and at least 12 Gγ subunits (Seack et al., 1998; Cook et al., 2001). A hallmark of the ∼35-kD Gβ proteins is the WD-40 motif, consisting of seven or eight tandem repeats and a conserved Trp-Asp (WD) motif. These repeats assemble into a seven-bladed β-propeller structure (Lambright et al., 1996; Sondek et al., 1996). Gγ proteins range in mass from 7 to 10 kD and are not highly conserved. However, all Gγs possess the C-terminal CaaX (where “a” is an aliphatic amino acid) site for isoprenylation (Figure 2A), which confers membrane association. A coiled-coil structure formed between Gβ and Gγ results in a noncovalent but very tight interaction between these two subunits, such that they function as a nondissociable dimer. A nonexhaustive list of some Gα subunit effectors in mammalian systems includes adenylate cyclase, cyclic GMP phosphodiesterase, phosphoinositide 3-kinase, the phosphoinositide (PI)-phospholipase Cβ (PI-PLCβ; Figure 2B) (Singer et al., 1997), phospholipase D (PLD; Figure 2B) (Singer et al., 1997), Na+/H+ exchange transporter (Voyno-Yasenetskaya et al., 1994; Voyno-Yasenetskaya, 1998), the TUBBY transcription factor (Santagata et al., 2001), and K+, Ca2+, and, to a lesser extent, Cl− and Na+ channels (Brown and Birnbaumer, 1990; Morris and Malbon, 1999; Reddy et al., 2001). Gβγ targets include phospholipase A2 (PLA2; Figure 2B), calcium channels, and some isoforms of adenylate cyclase and PI-PLCβ (Jelsema and Axelrod, 1987; Logothetis et al., 1987; Kim et al., 1989; Katz et al., 1992; de Wa