Differential processing of homologues of the small subunit of ADP-glucose pyrophosphorylase from barley (Hordeum vulgare) tissues

Abstract

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