THE SPATIAL-DISTRIBUTION OF SUCROSE SYNTHASE ISOZYMES IN BARLEY

The sucrose (Sue) synthase enzyme purified from barley (Hordeum vulgar e L.) roots is a homotetramer that is composed of 90-kD type 1: Suc sy nthase (SS1) subunits. K-m values for Suc and UDP were 30 mM and 5 mu M, respectively. This enzyme can also utilize ADP at 25% of the UDP ra te. Anti-SS1 polyclonal antibodies, which recognized both SS1 and type 2 Suc synthase (SS2) (88-kD) subunits, and antibodies raised against a synthetic peptide, LANGSTDNNFV, which were specific for SS2, were us ed to study the spatial distribution of these subunits by immunoblot a nalysis and immunolocalization. Both SS1 and SS2 were abundantly expre ssed in endosperm, where they polymerize to form the five possible hom o- and heterotetramers. Only SS1 homotetramers were detected in young leaves, where they appeared exclusively in phloem cells, and in roots, where expression was associated with cap cells and the Vascular bundl e. In the seed both SS1 and SS2 were present in endosperm, but only SS 1 was apparent in the chalazal region, the nucellar projection, and th e vascular bundle. The physiological implications for the difference i n expression patterns observed are discussed with respect to the maize (Zea mays L.) model.