THE EXPRESSION OF SERINE CARBOXYPEPTIDASES DURING MATURATION AND GERMINATION OF THE BARLEY-GRAIN

cDNA clones encoding three additional serine carboxypeptidases (Ser-CP s) have been isolated from a gibberellic acid-induced barley aleurone cDNA library. The three deduced Ser-CPs belong to the two-chain subfam ily of Ser-CPs; they are synthesized as precursors with a putative sig nal peptide, propeptide, and linker peptide between the A and B chains . Their identification provides the proof for the existence of more th an three Ser-CPs in cereal grains, and, based on their sequences, they mag exhibit new substrate specificities. The expression of these and of the three previously isolated Ser-CPs from barley grains (CP-MI, CP -MII, and CP-MIII) has been investigated by Northern and Western analy sis and RNA PCR. CP-MII is the only Ser-CP to be expressed and accumul ate in the developing grain and is stored in its active form in the ma ture grain. All six Ser-CPs are expressed de novo in the germinating g rain, in the scutellum, and/or in the aleurone. Furthermore, at least CP-MI, CP-MIII, and CP-MIII are secreted into the endosperm. In additi on, all Ser-CPs (except CP-MI) are also expressed in the roots and sho ots of the growing seedling. This enzyme family thus appears to be ubi quitous in the barley plant, which suggests that Ser-CPs play addition al roles besides their participation in the mobilization of storage pr oteins.