Phosphoenolpyruvate carboxylase (PEPC) activity was detected in the aleurone endosperm of wheat (Triticum aestivum cv Chinese Spring) seeds, and specific anti-Sorghum C, PEPC polyclonal antibodies cross-reacted with 103-and 100-kD polypeptides present in dry seeds and seeds that had imbibed; in addition, a new, 108-kD polypeptide was detected 6 h after imbibition. l h e use of specific anti-phosphorylation-site immunoglobulin C (APS-lgC) identified the presence of a phosphorylation motif equivalent to that found in other plant PEPCs studied so far. The binding of this APS-lgC to the target protein promoted changes in the properties of seed PEPC similar to those produced by phosphorylation, as previously shown for the recombinant Sorghum leaf C, PEPC. In desalted seed extracts, an endogenous PEPC kinase activity catalyzed a bona fide phosphorylation of the target protein, as deduced from the immunoinhibition of the in vitro phosphorylation reaction by the APSIgC. In addition, the major, 103-kD PEPC polypeptide was also shown to be radiolabeled in situ 48 h after imbibition in [32Plorthophosphate. The ratio between optimal (pH 8) and suboptimal (pH 7.3 or 7.1) PEPC activity decreased during germination, thereby suggesting a change in catalytic rate related to an in vivo phosphorylation process. These collective data document that the components needed for the regulatory phosphorylation of PEPC are present and functional during germination of wheat seeds.
Phosphoenolpyruvate carboxylase (PEPC) activity was detected in aleurone-endosperm extracts of barley (Hordeum vulgare) seeds during germination, and specific anti-sorghum (Sorghum bicolor) C 4 PEPC polyclonal antibodies immunodecorated constitutive 103-kD and inducible 108-kD PEPC polypeptides in western analysis. The 103-and 108-kD polypeptides were radiolabeled in situ after imbibition for up to 1.5 d in 32 P-labeled inorganic phosphate. In vitro phosphorylation by a Ca 2؉ -independent PEPC protein kinase (PK) in crude extracts enhanced the enzyme's velocity and decreased its sensitivity to L-malate at suboptimal pH and [PEP]. Isolated aleurone cell protoplasts contained both phosphorylated PEPC and a Ca 2؉ -independent PEPC-PK that was partially purified by affinity chromatography on blue dextran-agarose. This PK activity was present in dry seeds, and PEPC phosphorylation in situ during imbibition was not affected by the cytosolic protein-synthesis inhibitor cycloheximide, by weak acids, or by various pharmacological reagents that had proven to be effective blockers of the light signal transduction chain and PEPC phosphorylation in C 4 mesophyll protoplasts. These collective data support the hypothesis that this Ca 2؉ -independent PEPC-PK was formed during maturation of barley seeds and that its presumed underlying signaling elements were no longer operative during germination.
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