DIFFERENTIAL PROCESSING OF HOMOLOGS OF THE SMALL-SUBUNIT OF ADP-GLUCOSE PYROPHOSPHORYLASE FROM BARLEY (HORDEUM-VULGARE) TISSUES

ADP-glucose pyrophosphorylase (AGPase), a two-gene-encoded enzyme, is the key component of starch synthesis in all plants. In the present st udy, we have used an E. coli expres sion system for the (over)producti on of proteins derived from both full length and specifically truncate d cDNAs encoding small subunits of AGPase from seed endosperm (AGPase- B1) and leaves (AGPase-B2) of barley (Hordeum vulgare). Based on immun oblot analyses, the molecular mass of the expressed AGPase-B1 (52 kD) was similar to that from endosperm extracts, whereas the expressed AGP ase-B2 (56 kD) was larger than that in barley leaves (51 kD). Expressi on of truncated cDNAs for both the seed and leaf proteins has allowed for a direct verification of molecular masses that were earlier propos ed for mature AGPases in barley tissues. The data suggest that seed AG Pase-B1 does not undergo any post-translational proteolytic processing in barley, whereas the leaf homologue is processed to a smaller prote in. Possible implications of these findings are discussed.