A cDNA clone, BETl (for basal gndosperm jransfer layer), was isolated from a cDNA bank prepared from 10-days after pollination (DAP) maize endosperm mRNA. BE7Y mRNA was shown to encode a 7-kD cell wall polypeptlde. Both the mRNA and protein were restricted in their distrlbution to the basal endosperm transfer layer and were not expressed elsewhere in the plant. BE7Y expression commenced at 9 DAP, reached a maximum between 12 and 16 DAP, and declined after 16 DAP. The initial accumulation of the

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... BETl polypeptlde reached a plateau by 16 DAP and declined thereafter, becoming undetectable by 20 DAP. The antlbody raised against the BETl protein reacted with a number of polypeptides of higher molecular mass than the BETl monomer. Most of these were present in cytosollc fractlons and were found in nonbasal cell endosperm extracts, but three species appeared to be basal cell specific. Thls result and the reactivity of exhaustively extracted cell wall material with the BETl antibody suggest that a fractlon of the protein is deposlted in a covalently bound form in the extracellular matrix. We propose that the BETl protein plays a role in the structural specialization of the transfer cells. In addition, BE7Y provides a new molecular marker for the development of this endosperm domain.