The protein level and regulation of
phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31,
involved in malic acid synthesis) was studied during the fruit development of
two grape (Vitis vinifera L.) varieties, ‘Cabernet
Sauvignon’ and ‘Gora Chirine’, with berries of normal and
low organic acid content, respectively. The protein level and
in vitro activity were higher in the low-acid variety
than in the normal-acid variety for most stages. In vivo
PEPC activity, measured using 14
CO2 labelling, was significantly higher in the low-acid
variety than in the normal-acid variety about 1 week before and 1 week after
veraison (the day which corresponds to the onset of ripening). However,
partitioning into malate was the same for both varieties. Antibodies raised
against the N-terminal part of
SorghumPEPC recognised the grape berry PEPC, indicating
the presence of the consensus phosphorylation site involved in PEPC
regulation. PEPC phosphorylation status was estimated by studying sensitivity
to pH and malate. Grape berry PEPC appeared more sensitive to low pH and
malate during ripening (IC50 malate, 0.2–0.7 mM)
compared to during the earlier stages of development
(IC50 malate, 1.2–2 mM) for both varieties.
Therefore, in the normal-acid variety, PEPC seems to participate in
controlling malic acid accumulation but does not seem to control the
differences in malic acid concentration observed between the two varieties.