BT1, A POSSIBLE ADENYLATE TRANSLOCATOR, IS DEVELOPMENTALLY EXPRESSED IN MAIZE ENDOSPERM BUT NOT DETECTED IN STARCHY TISSUES FROM SEVERAL OTHER SPECIES

Brittlel protein (BTI), the major protein in amyloplast envelope membr anes of wild-type maize endosperm, is required for normal levels of st arch accumulation in maize kernels. To gain information about the Br1 locus in control of starch synthesis in plants, we examined Bt1 expres sion in kernels from different ages and from different genotypes of ma ize as well as in starchy tissues from different species. Both BT1 pro tein and Br1 transcripts were detected in kernels from about 10 days p ost-pollination (DPP) and increased up to 14 DPP. BT1 protein level wa s similar in kernels after 14 DPP. Furthermore, membranes from amylopl asts containing larger starch granules contained more BT1 protein than those containing smaller starch granules from the same age of kernels . These results indicate that Br1 gene expression was developmentally regulated and that BT1 protein level was correlated with starch accumu lation in normal maize endosperm. Unexpectedly, except for a weak band at a higher molecular mass in microsomal proteins extracted from spri ng barley grain which reacted with maize BT1 antibody, no BT1 protein was observed in starchy tissues from several other species, including grains of sorghum, wheat. winter barley and oats, potato tubers. pea r oots and cauliflower buds. Thus either the BT1 antibody raised against a fusion protein including the 56 amino acid residues in the carboxyl terminal sequence of maize endosperm BT1 protein does not recognize B T1 homologues in amyloplasts from other tissues or species or amylopla sts from different tissues or species have alternative membrane transp orters.