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 membranes of wild-type maize endosperm, is required for normal levels of starch accumulation in maize kernels. To gain information about the Br1 locus in control of starch synthesis in plants, we examined Bt1 expression in kernels from different ages and from different genotypes of maize as well as in starchy tissues from different species. Both BT1 protein and Br1 transcripts were detected in kernels from about 10 days post-pollination (DPP) and increased up to 14 DPP. BT1 protein level was similar in kernels after 14 DPP. Furthermore, membranes from amyloplasts 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 accumulation in normal maize endosperm. Unexpectedly, except for a weak band at a higher molecular mass in microsomal proteins extracted from spring 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 roots 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 BT1 homologues in amyloplasts from other tissues or species or amyloplasts from different tissues or species have alternative membrane transporters.