A cDNA encoding starch branching enzyme I from maize endosperm

ADP-Glc pyrophosphorylase (EC 2.7.7.27), starch synthases (EC 2.4.1.21), and SBEs (EC 2.4.1.18) are the key enzymes in the pathway of plant starch biosynthesis. Starch is a polymer of Glc that exists as two fractions, amylose and amylopectin, in maize (Zea mays L.) kernel amyloplasts. The essentially linear polymer amylose contains alpha -1,4-linked Glc, whereas the branched polymer amylopectin contains 5% alpha -1,6-linked Glc in addition to linear regions of alpha -1,4-linked Glc. Amylopectin synthesis requires the action of SBE, which catalyzes the formation of alpha -1,6-linkages. The branching process involves two steps with the hydrolysis of an internal 1,4-bond and the formation of a 1,6-bond using the linear chain (six to seven Glc units). Thus, branching enzymes are thought to interact with starch synthases in formation of amylopectin. Three SBE isozymes differing in enzymatic, chromatographic, and immunological properties have been resolved in maize endosperm, SBE I, SBE IIa, and SBE IIb. Recently, analysis of SBE I, SBE IIa, and SBE IIb revealed that SBE I may preferentially branch long chains of alpha -glucan, whereas SBE IIa and SBE IIb may play a different role in branching short chains during starch biosynthesis. We previously reported the cloning of a cDNA encoding SBE II from maize endosperm. Using antibodies to purified SBE I protein from maize endosperm, Baba et al. (1991) isolated a partial-length cDNA encoding the SBE I isoform. This cDNA lacks the entire open reading frame, because no ATG codon was found 5' of the known plastid signal peptide cleavage site. Based on similarity to the rice SBE I-like cDNA (rbe1), it was hypothesized that the SBE I cDNA lacked only two bases of the coding region.