Molecular characterization of BET1, a gene expressed in the endosperm transfer cells of maize

A cDNA clone, BET1 (for basal endosperm transfer layer), was isolated from a cDNA bank prepared from 10-days after pollination (DAP) maize endosperm mRNA. BET1 mRNA was shown to encode a 7-kD cell wall polypeptide. Both the mRNA and protein were restricted in their distribution to the basal endosperm transfer layer and were not expressed elsewhere in the plant. BET1 expression commenced at 9 DAP, reached a maximum between 12 and 16 DAP, and declined after 16 DAP. The initial accumulation of the BET1 polypeptide reached a plateau by 16 DAP and declined thereafter, becoming undetectable by 20 DAP. The antibody raised against the BET1 protein reacted with a number of polypeptides of higher molecular mass than the BET1 monomer. Most of these were present in cytosolic fractions and were found in nonbasal cell endosperm extracts, but three species appeared to be basal cell specific. This result and the reactivity of exhaustively extracted cell wall material with the BET1 antibody suggest that a fraction of the protein is deposited in a covalently bound form in the extracellular matrix. We propose that the BET1 protein plays a role in the structural specialization of the transfer cells. In addition, BET1 provides a new molecular marker for the development of this endosperm domain.