The Synthesis and Transport of ADPglucose in Cereal Endosperms

概要

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Recent studies of the subcellular compartmentation of starch synthesis have revealed differences between events in monocotyledenous endosperm compared with other starch storing tissues and species . ADPglucose pyrophosphorylase (AGPase) has been found to be located in both amyloplasts and cytosol of barley, rice and maize. We have found a similar distribution in developing endosperm of wheat, although as much as 30% of total cellular AGPase is localized in amyloplasts, a far higher proportion than for other species. Intriguingly the wheat amyloplast enzyme is activated only 2-fold by 3-phosphoglyceric acid (3-PGA) when assayed in the direction of ADPglucose (ADPG) synthesis, and 3-PGA does not reactivate the enzyme when it is inhibited by high concentrations of inorganic orthophosphate . In view of the cytosolic localisation of the majority of AGPase activity, we have studied the transport of ADPG into amyloplasts . The transporter has been reconstituted into artificial lipid bilayers and shown to counter-exchange ADPG with ATP, ADP or AMP, but the protein does not transport UDPglucose (UDPG). The import of ADPG from the cytosol into amyloplasts offers a means of regulating both the flux of carbon to the enzymes of starch synthesis, and the nature of the end product. The role of ADPG transport in controlling the partitioning of carbon between amylose and amylopectin will be discussed.
日本応用糖質科学会の論文
2003-07-14