Structural and Kinetic Properties of High and Low Molecular Mass Phosphoenolpyruvate Carboxylase Isoforms from the Endosperm of Developing Castor Oilseeds

Phosphoenolpyruvate carboxylase (PEPC) is believed to play an important role in producing malate as a substrate for fatty acid synthesis by leucoplasts of the developing castor oilseed (COS) endosperm. Two kinetically distinct isoforms of COS PEPC were resolved by gel filtration chromatography and purified. PEPC1 is a typical 410-kDa homotetramer composed of 107-kDa subunits (p107). In contrast, PEPC2 exists as an unusual 681-kDa hetero-octamer composed of the same p107 found in PEPC1 and an associated 64-kDa polypeptide (p64) that is structurally and immunologically unrelated to p107. Relative to PEPC1, PEPC2 demonstrated significantly enhanced thermal stability and a much lower sensitivity to allosteric activators (Glc-6-P, Glc-1-P, Fru-6-P, glycerol-3-P) and inhibitors (Asp, Glu, malate) and pH changes within the physiological range. Nondenaturing PAGE of clarified extracts followed by in-gel PEPC activity staining indicated that the ratio of PEPC1:PEPC2 increases during COS development such that only PEPC1 is detected in mature COS. Dissimilar developmental profiles and kinetic properties support the hypotheses that (i) PEPC1 functions to replenish dicarboxylic acids consumed through transamination reactions required for storage protein synthesis, whereas (ii) PEPC2 facilitates PEP flux to malate in support of fatty acid synthesis. Interestingly, the respective physical and kinetic properties of COS PEPC1 and PEPC2 are remarkably comparable with those of the homotetrameric lowMr Class 1 and heteromeric highMr Class 2 PEPC isoforms of unicellular green algae. Phosphoenolpyruvate carboxylase (PEPC) is believed to play an important role in producing malate as a substrate for fatty acid synthesis by leucoplasts of the developing castor oilseed (COS) endosperm. Two kinetically distinct isoforms of COS PEPC were resolved by gel filtration chromatography and purified. PEPC1 is a typical 410-kDa homotetramer composed of 107-kDa subunits (p107). In contrast, PEPC2 exists as an unusual 681-kDa hetero-octamer composed of the same p107 found in PEPC1 and an associated 64-kDa polypeptide (p64) that is structurally and immunologically unrelated to p107. Relative to PEPC1, PEPC2 demonstrated significantly enhanced thermal stability and a much lower sensitivity to allosteric activators (Glc-6-P, Glc-1-P, Fru-6-P, glycerol-3-P) and inhibitors (Asp, Glu, malate) and pH changes within the physiological range. Nondenaturing PAGE of clarified extracts followed by in-gel PEPC activity staining indicated that the ratio of PEPC1:PEPC2 increases during COS development such that only PEPC1 is detected in mature COS. Dissimilar developmental profiles and kinetic properties support the hypotheses that (i) PEPC1 functions to replenish dicarboxylic acids consumed through transamination reactions required for storage protein synthesis, whereas (ii) PEPC2 facilitates PEP flux to malate in support of fatty acid synthesis. Interestingly, the respective physical and kinetic properties of COS PEPC1 and PEPC2 are remarkably comparable with those of the homotetrameric lowMr Class 1 and heteromeric highMr Class 2 PEPC isoforms of unicellular green algae. PEP carboxylase (EC 4.1.1.31) phosphoenolpyruvate crassulacean acid metabolism castor oilseed electrospray quadrupole-time of flight tandem mass spectrometry matrix-assisted laser desorption ionization-time of flight mass spectrometry fast protein liquid chromatography polyethylene glycol 4-morpholineethanesulfonic acid Phosphoenolpyruvate carboxylase (PEPC)1 is a ubiquitous cytosolic enzyme in vascular plants that is also widely distributed in green algae and bacteria (1Chollet R. Vidal J. O' Leary M.H. Annu. Rev. Plant Physiol. Plant Mol. Biol. 1996; 47: 273-298Google Scholar). It catalyzes the irreversible ॆ-carboxylation of PEP in the presence of Mg2+ and HCO3− to yield oxaloacetate and Pi. PEPC is abundant in C4 and crassulacean acid metabolism (CAM) leaves where it participates in photosynthesis by catalyzing the initial fixation of atmospheric CO2. Both allosteric mechanisms and covalent modification are involved in PEPC control in C4 and CAM leaves (1Chollet R. Vidal J. O' Leary M.H. Annu. Rev. Plant Physiol. Plant Mol. Biol. 1996; 47: 273-298Google Scholar, 2Rajagopalan A., V Devi M.T. Raghavendra A.S. Photosynth. Res. 1994; 39: 115-135Google Scholar, 3Nimmo H.G. Batey N.H. Dickinson H.G. Hetherington S.M. Society for Experimental Biology Seminar Series 53: Post Translational Modifications in Plants. Cambridge University Press, Cambridge, United Kingdom1993: 161-170Google Scholar). Early work established that C4 and CAM PEPCs are controlled by a diurnal cycle that modulates their sensitivity to l-malate inhibition (1Chollet R. Vidal J. O' Leary M.H. Annu. Rev. Plant Physiol. Plant Mol. Biol. 1996; 47: 273-298Google Scholar, 2Rajagopalan A., V Devi M.T. Raghavendra A.S. Photosynth. Res. 1994; 39: 115-135Google Scholar, 3Nimmo H.G. Batey N.H. Dickinson H.G. Hetherington S.M. Society for Experimental Biology Seminar Series 53: Post Translational Modifications in Plants. Cambridge University Press, Cambridge, United Kingdom1993: 161-170Google Scholar). This cycle is the result of phosphorylation of the PEPC homotetramer by an endogenous Ca2+-independent PEPC protein kinase and dephosphorylation by a protein phosphatase type 2A at a highly conserved seryl residue localized near the N terminus of the 100–110-kDa PEPC subunit (1Chollet R. Vidal J. O' Leary M.H. Annu. Rev. Plant Physiol. Plant Mol. Biol. 1996; 47: 273-298Google Scholar, 2Rajagopalan A., V Devi M.T. Raghavendra A.S. Photosynth. Res. 1994; 39: 115-135Google Scholar, 3Nimmo H.G. Batey N.H. Dickinson H.G. Hetherington S.M. Society for Experimental Biology Seminar Series 53: Post Translational Modifications in Plants. Cambridge University Press, Cambridge, United Kingdom1993: 161-170Google Scholar).Relative to C4 and CAM PEPCs, the properties of the enzyme from non-green plant tissues are less well understood. Although proposed roles for nonphotosynthetic PEPCs are diverse, a crucial PEPC function is the anaplerotic replenishment of citric acid cycle intermediates consumed during biosynthesis and nitrogen assimilation (1Chollet R. Vidal J. O' Leary M.H. Annu. Rev. Plant Physiol. Plant Mol. Biol. 1996; 47: 273-298Google Scholar). As with C4 and CAM PEPCs, the PEPC of C3leaves and nonphotosynthetic tissues can be controlled by allosteric effectors and reversible phosphorylation (4Duff S.M.G. Chollet R. Plant Physiol. 1995; 107: 775-782Google Scholar, 5Munoz T. Escribano M.I. Merodio C. Phytochemistry. 2001; 58: 1007-1013Google Scholar, 6Schuller K.A. Turpin D.H. Plaxton W.C. Plant Physiol. 1990; 94: 1429-1435Google Scholar, 7Zhang X.Q. Chollet R. Arch. Scholar, Plaxton W.C. J. Scholar, Plaxton W.C. J. 1995; Scholar, Plaxton W.C. J. Scholar). the role of PEPCs in the metabolism of developing and Plaxton W.C. Scholar, Scholar, Physiol. Scholar, C. Vidal J. Plant Physiol. Scholar, C. Scholar, Plaxton W.C. Plant Physiol. PEPC and for as much as of the of mature castor the synthesis of fatty in developing in This the of and intermediates the Res. 2001; of fatty acid synthesis by leucoplasts from developing COS Turpin D.H. Plant Physiol. Scholar). from the the is by a within the COS Plant Physiol. Scholar). and Plaxton W.C. Plant Physiol. that the in PEPC activity and that COS development facilitates malate for fatty acid synthesis. PEP to malate flux also as an anaplerotic of for transamination reactions associated with COS storage protein of to and PEPC from developing COS. for PEPC isoforms from developing COS and their and kinetic Although is a typical PEPC the a PEPC in vascular plants remarkably of Class 2 PEPC isoforms in unicellular green algae J. R. Plaxton W.C. Turpin D.H. Arch. 1996; Scholar, J. Plaxton W.C. Turpin D.H. J. Scholar, J. Plaxton W.C. Turpin D.H. J. Biol. 2001; Scholar, J. Turpin D.H. Plaxton W.C. Plant Physiol. Scholar). that the of a 107-kDa PEPC subunit with an unrelated 64-kDa polypeptide is for the in the physical and kinetic properties the PEPC homotetramer and highMr PEPC of developing in of p107 COS PEPC isoforms that significantly in their physical and kinetic properties were resolved by and highly purified. PEPC are to in vascular plants (1Chollet R. Vidal J. O' Leary M.H. Annu. Rev. Plant Physiol. Plant Mol. Biol. 1996; 47: 273-298Google Scholar, 2Rajagopalan A., V Devi M.T. Raghavendra A.S. Photosynth. Res. 1994; 39: 115-135Google Scholar, 3Nimmo H.G. Batey N.H. Dickinson H.G. Hetherington S.M. Society for Experimental Biology Seminar Series 53: Post Translational Modifications in Plants. Cambridge University Press, Cambridge, United Kingdom1993: 161-170Google and that developing PEPC C. Scholar, T. T. T. Plant Mol. Biol. T. T. Plant Physiol. Scholar). is the of the of PEPC isoforms from the same plant PEPC1 is a p107 typical of plant PEPCs to contrast, PEPC2 to as an unusual hetero-octamer composed of the same p107 found in PEPC1 and an associated that is structurally and immunologically unrelated to p107. of that is highly to PEPCs by of the and Although conserved required for PEPC PEPCs a that the seryl phosphorylation to be conserved plant PEPCs (1Chollet R. Vidal J. O' Leary M.H. Annu. Rev. Plant Physiol. Plant Mol. Biol. 1996; 47: 273-298Google and of and Although it is that an in in of a the that is a polypeptide that with p107 to to the COS PEPC2 heteromeric This the allosteric of p107 an allosteric in p107 such that effectors to their respective PEPC1 and PEPC2 and PEPC the respective properties of COS PEPC1 and PEPC2 are remarkably comparable with those of the homotetrameric Class 1 and heteromeric Class 2 PEPC isoforms of the green and J. R. Plaxton W.C. Turpin D.H. Arch. 1996; Scholar, J. Plaxton W.C. Turpin D.H. J. Scholar, J. Plaxton W.C. Turpin D.H. J. Biol. 2001; Scholar). PEPC isoforms the same subunit to COS the Class 2 PEPCs also associated that are immunologically unrelated to and that the of COS the subunit of Class 2 PEPC a distinct polypeptide that is only to J. Plaxton W.C. Turpin D.H. J. Biol. 2001; Scholar). to COS PEPC1 and Class 1 PEPCs, the COS PEPC2 and Class 2 PEPCs significantly enhanced thermal stability and a much lower sensitivity to allosteric effectors and pH changes within the physiological J. R. Plaxton W.C. Turpin D.H. Arch. 1996; Scholar, J. Plaxton W.C. Turpin D.H. J. Scholar). the that and isoforms in green algae the of vascular with conserved as a of at plant and of COS PEPC1 and to PEPCs from non-green plant tissues K.A. Turpin D.H. Plaxton W.C. Plant Physiol. 1990; 94: 1429-1435Google Scholar, Plaxton W.C. J. Plaxton W.C. J. 1995; Scholar, Plaxton W.C. Scholar, C. COS PEPC1 by and by and at pH This result that PEPC1 a anaplerotic role to replenish dicarboxylic acids consumed through transamination reactions required to support storage protein synthesis. inhibition of PEPC1 by and a control that PEPC1 activity with the of required for assimilation transamination by contrast, PEP flux to malate in support of fatty acid synthesis the malate in developing COS Turpin D.H. Plant Physiol. Scholar). Nondenaturing PAGE of clarified COS extracts followed by in-gel PEPC activity staining that PEPC2 increases during COS at and during COS This in also at J. Scholar). developmental for PEPC1 by contrast, that of storage protein J. J. J. with during the of COS plants inhibitors to the functions of COS PEPC1 and to be and COS PEPC1 and PEPC2 phosphorylation of to be involved in the control and of green Class 2 PEPCs J. Turpin D.H. Plaxton W.C. Plant Physiol. Scholar). COS p107 the seryl phosphorylation of plant PEPCs It be of to COS PEPC1 and PEPC2 are a p107 Phosphoenolpyruvate carboxylase (PEPC)1 is a ubiquitous cytosolic enzyme in vascular plants that is also widely distributed in green algae and bacteria (1Chollet R. Vidal J. O' Leary M.H. Annu. Rev. Plant Physiol. Plant Mol. Biol. 1996; 47: 273-298Google Scholar). It catalyzes the irreversible ॆ-carboxylation of PEP in the presence of Mg2+ and HCO3− to yield oxaloacetate and Pi. PEPC is abundant in C4 and crassulacean acid metabolism (CAM) leaves where it participates in photosynthesis by catalyzing the initial fixation of atmospheric CO2. Both allosteric mechanisms and covalent modification are involved in PEPC control in C4 and CAM leaves (1Chollet R. Vidal J. O' Leary M.H. Annu. Rev. Plant Physiol. Plant Mol. Biol. 1996; 47: 273-298Google Scholar, 2Rajagopalan A., V Devi M.T. Raghavendra A.S. Photosynth. Res. 1994; 39: 115-135Google Scholar, 3Nimmo H.G. Batey N.H. Dickinson H.G. Hetherington S.M. Society for Experimental Biology Seminar Series 53: Post Translational Modifications in Plants. Cambridge University Press, Cambridge, United Kingdom1993: 161-170Google Scholar). Early work established that C4 and CAM PEPCs are controlled by a diurnal cycle that modulates their sensitivity to l-malate inhibition (1Chollet R. Vidal J. O' Leary M.H. Annu. Rev. Plant Physiol. Plant Mol. Biol. 1996; 47: 273-298Google Scholar, 2Rajagopalan A., V Devi M.T. Raghavendra A.S. Photosynth. Res. 1994; 39: 115-135Google Scholar, 3Nimmo H.G. Batey N.H. Dickinson H.G. Hetherington S.M. Society for Experimental Biology Seminar Series 53: Post Translational Modifications in Plants. Cambridge University Press, Cambridge, United Kingdom1993: 161-170Google Scholar). This cycle is the result of phosphorylation of the PEPC homotetramer by an endogenous Ca2+-independent PEPC protein kinase and dephosphorylation by a protein phosphatase type 2A at a highly conserved seryl residue localized near the N terminus of the 100–110-kDa PEPC subunit (1Chollet R. Vidal J. O' Leary M.H. Annu. Rev. Plant Physiol. Plant Mol. Biol. 1996; 47: 273-298Google Scholar, 2Rajagopalan A., V Devi M.T. Raghavendra A.S. Photosynth. Res. 1994; 39: 115-135Google Scholar, 3Nimmo H.G. Batey N.H. Dickinson H.G. Hetherington S.M. Society for Experimental Biology Seminar Series 53: Post Translational Modifications in Plants. Cambridge University Press, Cambridge, United Kingdom1993: 161-170Google Scholar). Relative to C4 and CAM PEPCs, the properties of the enzyme from non-green plant tissues are less well understood. Although proposed roles for nonphotosynthetic PEPCs are diverse, a crucial PEPC function is the anaplerotic replenishment of citric acid cycle intermediates consumed during biosynthesis and nitrogen assimilation (1Chollet R. Vidal J. O' Leary M.H. Annu. Rev. Plant Physiol. Plant Mol. Biol. 1996; 47: 273-298Google Scholar). As with C4 and CAM PEPCs, the PEPC of C3leaves and nonphotosynthetic tissues can be controlled by allosteric effectors and reversible phosphorylation (4Duff S.M.G. Chollet R. Plant Physiol. 1995; 107: 775-782Google Scholar, 5Munoz T. Escribano M.I. Merodio C. Phytochemistry. 2001; 58: 1007-1013Google Scholar, 6Schuller K.A. Turpin D.H. Plaxton W.C. Plant Physiol. 1990; 94: 1429-1435Google Scholar, 7Zhang X.Q. Chollet R. Arch. Scholar, Plaxton W.C. J. Scholar, Plaxton W.C. J. 1995; Scholar, Plaxton W.C. J. Scholar). the role of PEPCs in the metabolism of developing and Plaxton W.C. Scholar, Scholar, Physiol. Scholar, C. Vidal J. Plant Physiol. Scholar, C. Scholar, Plaxton W.C. Plant Physiol. PEPC and for as much as of the of mature castor the synthesis of fatty in developing in This the of and intermediates the Res. 2001; of fatty acid synthesis by leucoplasts from developing COS Turpin D.H. Plant Physiol. Scholar). from the the is by a within the COS Plant Physiol. Scholar). and Plaxton W.C. Plant Physiol. that the in PEPC activity and that COS development facilitates malate for fatty acid synthesis. PEP to malate flux also as an anaplerotic of for transamination reactions associated with COS storage protein synthesis. of to and PEPC from developing COS. for PEPC isoforms from developing COS and their and kinetic Although is a typical PEPC the a PEPC in vascular plants remarkably of Class 2 PEPC isoforms in unicellular green algae J. R. Plaxton W.C. Turpin D.H. Arch. 1996; Scholar, J. Plaxton W.C. Turpin D.H. J. Scholar, J. Plaxton W.C. Turpin D.H. J. Biol. 2001; Scholar, J. Turpin D.H. Plaxton W.C. Plant Physiol. Scholar). that the of a 107-kDa PEPC subunit with an unrelated 64-kDa polypeptide is for the in the physical and kinetic properties the PEPC homotetramer and highMr PEPC of developing COS. in of p107 COS PEPC isoforms that significantly in their physical and kinetic properties were resolved by and highly purified. PEPC are to in vascular plants (1Chollet R. Vidal J. O' Leary M.H. Annu. Rev. Plant Physiol. Plant Mol. Biol. 1996; 47: 273-298Google Scholar, 2Rajagopalan A., V Devi M.T. Raghavendra A.S. Photosynth. Res. 1994; 39: 115-135Google Scholar, 3Nimmo H.G. Batey N.H. Dickinson H.G. Hetherington S.M. Society for Experimental Biology Seminar Series 53: Post Translational Modifications in Plants. Cambridge University Press, Cambridge, United Kingdom1993: 161-170Google and that developing PEPC C. Scholar, T. T. T. Plant Mol. Biol. T. T. Plant Physiol. Scholar). is the of the of PEPC isoforms from the same plant PEPC1 is a p107 typical of plant PEPCs to contrast, PEPC2 to as an unusual hetero-octamer composed of the same p107 found in PEPC1 and an associated that is structurally and immunologically unrelated to p107. of that is highly to PEPCs by of the and Although conserved required for PEPC PEPCs a that the seryl phosphorylation to be conserved plant PEPCs (1Chollet R. Vidal J. O' Leary M.H. Annu. Rev. Plant Physiol. Plant Mol. Biol. 1996; 47: 273-298Google and of and Although it is that an in in of a the that is a polypeptide that with p107 to to the COS PEPC2 heteromeric This the allosteric of p107 an allosteric in p107 such that effectors to their respective PEPC1 and PEPC2 and PEPC the respective properties of COS PEPC1 and PEPC2 are remarkably comparable with those of the homotetrameric Class 1 and heteromeric Class 2 PEPC isoforms of the green and J. R. Plaxton W.C. Turpin D.H. Arch. 1996; Scholar, J. Plaxton W.C. Turpin D.H. J. Scholar, J. Plaxton W.C. Turpin D.H. J. Biol. 2001; Scholar). PEPC isoforms the same subunit to COS the Class 2 PEPCs also associated that are immunologically unrelated to and that the of COS the subunit of Class 2 PEPC a distinct polypeptide that is only to J. Plaxton W.C. Turpin D.H. J. Biol. 2001; Scholar). to COS PEPC1 and Class 1 PEPCs, the COS PEPC2 and Class 2 PEPCs significantly enhanced thermal stability and a much lower sensitivity to allosteric effectors and pH changes within the physiological J. R. Plaxton W.C. Turpin D.H. Arch. 1996; Scholar, J. Plaxton W.C. Turpin D.H. J. Scholar). the that and isoforms in green algae the of vascular with conserved as a of at plant and of COS PEPC1 and to PEPCs from non-green plant tissues K.A. Turpin D.H. Plaxton W.C. Plant Physiol. 1990; 94: 1429-1435Google Scholar, Plaxton W.C. J. Plaxton W.C. J. 1995; Scholar, Plaxton W.C. Scholar, C. COS PEPC1 by and by and at pH This result that PEPC1 a anaplerotic role to replenish dicarboxylic acids consumed through transamination reactions required to support storage protein synthesis. inhibition of PEPC1 by and a control that PEPC1 activity with the of required for assimilation transamination by contrast, PEP flux to malate in support of fatty acid synthesis the malate in developing COS Turpin D.H. Plant Physiol. Scholar). Nondenaturing PAGE of clarified COS extracts followed by in-gel PEPC activity staining that PEPC2 increases during COS at and during COS This in also at J. Scholar). developmental for PEPC1 by contrast, that of storage protein J. J. J. with during the of COS plants inhibitors to the functions of COS PEPC1 and to be and COS PEPC1 and PEPC2 phosphorylation of to be involved in the control and of green Class 2 PEPCs J. Turpin D.H. Plaxton W.C. Plant Physiol. Scholar). COS p107 the seryl phosphorylation of plant PEPCs It be of to COS PEPC1 and PEPC2 are a p107 in of p107 COS PEPC isoforms that significantly in their physical and kinetic properties were resolved by and highly purified. PEPC are to in vascular plants (1Chollet R. Vidal J. O' Leary M.H. Annu. Rev. Plant Physiol. Plant Mol. Biol. 1996; 47: 273-298Google Scholar, 2Rajagopalan A., V Devi M.T. Raghavendra A.S. Photosynth. Res. 1994; 39: 115-135Google Scholar, 3Nimmo H.G. Batey N.H. Dickinson H.G. Hetherington S.M. Society for Experimental Biology Seminar Series 53: Post Translational Modifications in Plants. Cambridge University Press, Cambridge, United Kingdom1993: 161-170Google and that developing PEPC C. Scholar, T. T. T. Plant Mol. Biol. T. T. Plant Physiol. Scholar). is the of the of PEPC isoforms from the same plant COS PEPC1 is a p107 typical of plant PEPCs to contrast, PEPC2 to as an unusual hetero-octamer composed of the same p107 found in PEPC1 and an associated that is structurally and immunologically unrelated to p107. of that is highly to PEPCs by of the and Although conserved required for PEPC PEPCs a that the seryl phosphorylation to be conserved plant PEPCs (1Chollet R. Vidal J. O' Leary M.H. Annu. Rev. Plant Physiol. Plant Mol. Biol. 1996; 47: 273-298Google and of and Although it is that an in in of a the that is a polypeptide that with p107 to to the COS PEPC2 heteromeric This the allosteric of p107 an allosteric in p107 such that effectors to their respective COS PEPC1 and PEPC2 and PEPC the respective properties of COS PEPC1 and PEPC2 are remarkably comparable with those of the homotetrameric Class 1 and heteromeric Class 2 PEPC isoforms of the green and J. R. Plaxton W.C. Turpin D.H. Arch. 1996; Scholar, J. Plaxton W.C. Turpin D.H. J. Scholar, J. Plaxton W.C. Turpin D.H. J. Biol. 2001; Scholar). PEPC isoforms the same subunit to COS the Class 2 PEPCs also associated that are immunologically unrelated to and that the of COS the subunit of Class 2 PEPC a distinct polypeptide that is only to J. Plaxton W.C. Turpin D.H. J. Biol. 2001; Scholar). to COS PEPC1 and Class 1 PEPCs, the COS PEPC2 and Class 2 PEPCs significantly enhanced thermal stability and a much lower sensitivity to allosteric effectors and pH changes within the physiological J. R. Plaxton W.C. Turpin D.H. Arch. 1996; Scholar, J. Plaxton W.C. Turpin D.H. J. Scholar). the that and isoforms in green algae the of vascular with conserved as a of at plant and of COS PEPC1 and to PEPCs from non-green plant tissues K.A. Turpin D.H. Plaxton W.C. Plant Physiol. 1990; 94: 1429-1435Google Scholar, Plaxton W.C. J. Plaxton W.C. J. 1995; Scholar, Plaxton W.C. Scholar, C. COS PEPC1 by and by and at pH This result that PEPC1 a anaplerotic role to replenish dicarboxylic acids consumed through transamination reactions required to support storage protein synthesis. inhibition of PEPC1 by and a control that PEPC1 activity with the of required for assimilation transamination by contrast, PEP flux to malate in support of fatty acid synthesis the malate in developing COS Turpin D.H. Plant Physiol. Scholar). Nondenaturing PAGE of clarified COS extracts followed by in-gel PEPC activity staining that PEPC2 increases during COS at and during COS This in also at J. Scholar). developmental for PEPC1 by contrast, that of storage protein J. J. J. with during the of COS plants inhibitors to the functions of COS PEPC1 and to be and COS PEPC1 and PEPC2 phosphorylation of to be involved in the control and of green Class 2 PEPCs J. Turpin D.H. Plaxton W.C. Plant Physiol. Scholar). COS p107 the seryl phosphorylation of plant PEPCs It be of to COS PEPC1 and PEPC2 are a p107 COS PEPC1 and PEPC2 and PEPC the respective properties of COS PEPC1 and PEPC2 are remarkably comparable with those of the homotetrameric Class 1 and heteromeric Class 2 PEPC isoforms of the green and J. R. Plaxton W.C. Turpin D.H. Arch. 1996; Scholar, J. Plaxton W.C. Turpin D.H. J. Scholar, J. Plaxton W.C. Turpin D.H. J. Biol. 2001; Scholar). PEPC isoforms the same subunit to COS the Class 2 PEPCs also associated that are immunologically unrelated to and that the of COS the subunit of Class 2 PEPC a distinct polypeptide that is only to J. Plaxton W.C. Turpin D.H. J. Biol. 2001; Scholar). to COS PEPC1 and Class 1 PEPCs, the COS PEPC2 and Class 2 PEPCs significantly enhanced thermal stability and a much lower sensitivity to allosteric effectors and pH changes within the physiological J. R. Plaxton W.C. Turpin D.H. Arch. 1996; Scholar, J. Plaxton W.C. Turpin D.H. J. Scholar). the that and isoforms in green algae the of vascular with conserved as a of at plant and of COS PEPC1 and to PEPCs from non-green plant tissues K.A. Turpin D.H. Plaxton W.C. Plant Physiol. 1990; 94: 1429-1435Google Scholar, Plaxton W.C. J. Plaxton W.C. J. 1995; Scholar, Plaxton W.C. Scholar, C. COS PEPC1 by and by and at pH This result that PEPC1 a anaplerotic role to replenish dicarboxylic acids consumed through transamination reactions required to support storage protein synthesis. inhibition of PEPC1 by and a control that PEPC1 activity with the of required for assimilation transamination by contrast, PEP flux to malate in support of fatty acid synthesis the malate in developing COS Turpin D.H. Plant Physiol. Scholar). Nondenaturing PAGE of clarified COS extracts followed by in-gel PEPC activity staining that PEPC2 increases during COS at and during COS This in also at J. Scholar). developmental for PEPC1 by contrast, that of storage protein J. J. J. with during the of COS plants inhibitors to the functions of COS PEPC1 and to be and COS PEPC1 and PEPC2 phosphorylation of to be involved in the control and of green Class 2 PEPCs J. Turpin D.H. Plaxton W.C. Plant Physiol. Scholar). COS p107 the seryl phosphorylation of plant PEPCs It be of to COS PEPC1 and PEPC2 are a p107 COS PEPC1 and PEPC2 and PEPC the respective properties of COS PEPC1 and PEPC2 are remarkably comparable with those of the homotetrameric Class 1 and heteromeric Class 2 PEPC isoforms of the green and J. R. Plaxton W.C. Turpin D.H. Arch. 1996; Scholar, J. Plaxton W.C. Turpin D.H. J. Scholar, J. Plaxton W.C. Turpin D.H. J. Biol. 2001; Scholar). PEPC isoforms the same subunit to COS the Class 2 PEPCs also associated that are immunologically unrelated to and that the of COS the subunit of Class 2 PEPC a distinct polypeptide that is only to J. Plaxton W.C. Turpin D.H. J. Biol. 2001; Scholar). to COS PEPC1 and Class 1 PEPCs, the COS PEPC2 and Class 2 PEPCs significantly enhanced thermal stability and a much lower sensitivity to allosteric effectors and pH changes within the physiological J. R. Plaxton W.C. Turpin D.H. Arch. 1996; Scholar, J. Plaxton W.C. Turpin D.H. J. Scholar). the that and isoforms in green algae the of vascular with conserved as a of at plant Interestingly, the respective properties of COS PEPC1 and PEPC2 are remarkably comparable with those of the homotetrameric Class 1 and heteromeric Class 2 PEPC isoforms of the green and J. R. Plaxton W.C. Turpin D.H. Arch. 1996; Scholar, J. Plaxton W.C. Turpin D.H. J. Scholar, J. Plaxton W.C. Turpin D.H. J. Biol. 2001; Scholar). PEPC isoforms the same subunit to COS the Class 2 PEPCs also associated that are immunologically unrelated to and that the of COS the subunit of Class 2 PEPC a distinct polypeptide that is only to J. Plaxton W.C. Turpin D.H. J. Biol. 2001; Scholar). to COS PEPC1 and Class 1 PEPCs, the COS PEPC2 and Class 2 PEPCs significantly enhanced thermal stability and a much lower sensitivity to allosteric effectors and pH changes within the physiological J. R. Plaxton W.C. Turpin D.H. Arch. 1996; Scholar, J. Plaxton W.C. Turpin D.H. J. Scholar). the that and isoforms in green algae the of vascular with conserved as a of at plant and of COS PEPC1 and to PEPCs from non-green plant tissues K.A. Turpin D.H. Plaxton W.C. Plant Physiol. 1990; 94: 1429-1435Google Scholar, Plaxton W.C. J. Plaxton W.C. J. 1995; Scholar, Plaxton W.C. Scholar, C. COS PEPC1 by and by and at pH This result that PEPC1 a anaplerotic role to replenish dicarboxylic acids consumed through transamination reactions required to support storage protein synthesis. inhibition of PEPC1 by and a control that PEPC1 activity with the of required for assimilation transamination by contrast, PEP flux to malate in support of fatty acid synthesis the malate in developing COS Turpin D.H. Plant Physiol. Scholar). Nondenaturing PAGE of clarified COS extracts followed by in-gel PEPC activity staining that PEPC2 increases during COS at and during COS This in also at J. Scholar). developmental for PEPC1 by contrast, that of storage protein J. J. J. with during the of COS plants inhibitors to the functions of COS PEPC1 and to be and COS PEPC1 and PEPC2 phosphorylation of to be involved in the control and of green Class 2 PEPCs J. Turpin D.H. Plaxton W.C. Plant Physiol. Scholar). COS p107 the seryl phosphorylation of plant PEPCs It be of to COS PEPC1 and PEPC2 are a p107 to PEPCs from non-green plant tissues K.A. Turpin D.H. Plaxton W.C. Plant Physiol. 1990; 94: 1429-1435Google Scholar, Plaxton W.C. J. Plaxton W.C. J. 1995; Scholar, Plaxton W.C. Scholar, C. COS PEPC1 by and by and at pH This result that PEPC1 a anaplerotic role to replenish dicarboxylic acids consumed through transamination reactions required to support storage protein synthesis. inhibition of PEPC1 by and a control that PEPC1 activity with the of required for assimilation transamination by contrast, PEP flux to malate in support of fatty acid synthesis the malate in developing COS Turpin D.H. Plant Physiol. Scholar). Nondenaturing PAGE of clarified COS extracts followed by in-gel PEPC activity staining that PEPC2 increases during COS at and during COS This in also at J. Scholar). developmental for PEPC1 by contrast, that of storage protein J. J. J. with during the of COS plants inhibitors to the functions of COS PEPC1 and It to be and COS PEPC1 and PEPC2 phosphorylation of to be involved in the control and of green Class 2 PEPCs J. Turpin D.H. Plaxton W.C. Plant Physiol. Scholar). COS p107 the seryl phosphorylation of plant PEPCs It be of to COS PEPC1 and PEPC2 are a p107 of for also of the and for with the of p107 and

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