The remarkable diversity of plant PEPC (phosphoenolpyruvate carboxylase): recent insights into the physiological functions and post-translational controls of non-photosynthetic PEPCs

Abstract

PEPC [PEP (phosphoenolpyruvate) carboxylase] is a tightly controlled enzyme located at the core of plant C-metabolism that catalyses the irreversible β-carboxylation of PEP to form oxaloacetate and Pi. The critical role of PEPC in assimilating atmospheric CO(2) during C(4) and Crassulacean acid metabolism photosynthesis has been studied extensively. PEPC also fulfils a broad spectrum of non-photosynthetic functions, particularly the anaplerotic replenishment of tricarboxylic acid cycle intermediates consumed during biosynthesis and nitrogen assimilation. An impressive array of strategies has evolved to co-ordinate in vivo PEPC activity with cellular demands for C(4)-C(6) carboxylic acids. To achieve its diverse roles and complex regulation, PEPC belongs to a small multigene family encoding several closely related PTPCs (plant-type PEPCs), along with a distantly related BTPC (bacterial-type PEPC). PTPC genes encode ~110-kDa polypeptides containing conserved serine-phosphorylation and lysine-mono-ubiquitination sites, and typically exist as homotetrameric Class-1 PEPCs. In contrast, BTPC genes encode larger ~117-kDa polypeptides owing to a unique intrinsically disordered domain that mediates BTPC's tight interaction with co-expressed PTPC subunits. This association results in the formation of unusual ~900-kDa Class-2 PEPC hetero-octameric complexes that are desensitized to allosteric effectors. BTPC is a catalytic and regulatory subunit of Class-2 PEPC that is subject to multi-site regulatory phosphorylation in vivo. The interaction between divergent PEPC polypeptides within Class-2 PEPCs adds another layer of complexity to the evolution, physiological functions and metabolic control of this essential CO(2)-fixing plant enzyme. The present review summarizes exciting developments concerning the functions, post-translational controls and subcellular location of plant PTPC and BTPC isoenzymes.

Light-dependent activation of phosphoenolpyruvate carboxylase by reversible phosphorylation in cluster roots of white lupin plants: diurnal control in response to photosynthate supply

“…pH 7Á2, 0Á2 mM PEP, 0Á125 mM malate), PEPC-specific activities were progressively and significantly reduced after 6-24 h of darkness (Fig. 3A), which implies that darkness triggered PEPC dephosphorylation (O'Leary et al, 2011). However, re-illumination of 12-h darkened plants for 8 h resulted in a recovery of selective PEPC activity to initial levels (Fig.…”

“…pH 8Á2, 2Á5 mM PEP) (Fig. 3A), which is not influenced by the phosphorylation state of the target enzyme (O'Leary et al, 2011) and the levels of PEPC protein (i.e. immunoreactive p107 subunits) (Fig.…”

“…Preparation of rabbit anti-(developing castor bean PEPC) IgG (anti-PEPC) and the corresponding anti-(phospho-Ser11 site-specific) IgG (anti-pSer11; raised against a synthetic phosphopeptide corresponding to castor bean PEPC's conserved Ser11 phosphorylation site) has been described (Tripodi et al, 2005). This antibody is expected to cross-react with the phosphorylated form of LaPEPC3 (AY663387), the dominant PEPC isozyme expressed in CRs of ÀPi white lupin and whose putative Ser-12 phosphorylation site and flanking residues closely align with the conserved N-terminal phosphorylation domain characteristic of all known plant-type PEPCs (Penaloza et al, 2005;O'Leary et al, 2011). For anti-pSer11 immunoblots, 10 mg mL À1 of the corresponding dephosphopeptide (Tripodi et al, 2005) was used to block any non-specific cross-reaction with non-phosphorylated PEPC.…”

“…It is the major anaplerotic plant enzyme that controls PEP partitioning into tricarboxylic acid cycle C-skeletons needed for biosynthesis and N assimilation (O'Leary et al, 2011). Phosphorylation by a dedicated Ca 2þ -independent PEPC protein kinase (PPCK) at a conserved N-terminal seryl residue activates PEPC in many plant tissues by relieving its allosteric inhibition by malate while enhancing activation by glucose-6-phosphate (O'Leary et al, 2011).…”

“…It is the major anaplerotic plant enzyme that controls PEP partitioning into tricarboxylic acid cycle C-skeletons needed for biosynthesis and N assimilation (O'Leary et al, 2011). Phosphorylation by a dedicated Ca 2þ -independent PEPC protein kinase (PPCK) at a conserved N-terminal seryl residue activates PEPC in many plant tissues by relieving its allosteric inhibition by malate while enhancing activation by glucose-6-phosphate (O'Leary et al, 2011). Amongst its numerous and diverse physiological functions, PEPC plays a critical role during plant acclimation to nutritional Pi deprivation, a common abiotic stress that frequently limits plant growth in natural ecosystems (Vance et al, 2003;Plaxton and Tran, 2011).…”