BRITTLE-1, AN ADENYLATE TRANSLOCATOR, FACILITATES TRANSFER OF EXTRAPLASTIDIAL SYNTHESIZED ADP-GLUCOSE INTO AMYLOPLASTS OF MAIZE ENDOSPERMS

Amyloplasts of starchy tissues such as those of maize (Zea mays L.) fu nction in the synthesis and accumulation of starch during kernel devel opment. ADP-glucose pyrophosphorylase (AGPase) is known to be located in chloroplasts, and for many years it was generally accepted that ACP ase was also localized in amyloplasts of starchy tissues. Recent aqueo us fractionation of young maize endosperm led to the conclusion that 9 5% of the cellular AGPase was extraplastidial, but immunolocalization studies at the electron- and light-microscopic levels supported the co nclusion that maize endosperm ACPase was localized in the amyloplasts. We report the results of two nonaqueous procedures that provide evide nce that in maize endosperms in the linear phase of starch accumulatio n, 90% or more of the cellular AGPase is extraplastidial. We also prov ide evidence that the brittle-l protein (BT1), an adenylate translocat or with a KTGGL motif common to the ADP-glucose-binding site of starch synthases and bacterial glycogen synthases, functions in the transfer of ADP-glucose into the amyloplast stroma. The importance of the BT1 translocator in starch accumulation in maize endosperms is demonstrate d by the severely reduced starch content in bt1 mutant kernels.