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PLANT PHYSIOLOGY , Vol 111, Issue 2 551-558, Copyright © 1996 by American Society of Plant Biologists
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BIOCHEMISTRY AND ENZYMOLOGY |
In Vivo and in Vitro Phosphorylation of the Phosphoenolpyruvate Carboxylase from Wheat Seeds during Germination
L. Osuna, M. C. Gonzalez, F. J. Cejudo, J. Vidal and C. Echevarria
Departamento de Biologia Vegetal, Facultad de Biologia, Universidad de Sevilla, Avenida Reina Mercedes no. 6, 41012 Seville, Spain (L.O., C.E.)
Phosphoenolpyruvate carboxylase (PEPC) activity was detected in the
aleurone endosperm of wheat (Triticum aestivum cv Chinese Spring) seeds,
and specific anti-Sorghum C4 PEPC polyclonal anti-bodies 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. The use of specific anti-phosphorylation-site immunoglobulin G
(APS-IgG) identified the presence of a phosphorylation motif equivalent to
that found in other plant PEPCs studied so far. The binding of this APS-IgG
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 C4 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 APS- IgG. In addition, the major, 103-kD
PEPC polypeptide was also shown to be radiolabeled in situ 48 h after
imbibition in [32P]orthophosphate. 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.
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