DIFFERENTIAL EXPRESSION AND PROPERTIES OF STARCH BRANCHING ENZYME ISOFORMS IN DEVELOPING WHEAT ENDOSPERM

Three forms of starch branching enzyme (BE) from developing hexaploid wheat( Triticum aestivum) endosperm have been partially purified and c haracterized. Immunological cross-reactivities indicate that two forms (WBE-I-AD, 88 kD, and WBE-I-B, 87 kD) are related to the maize BE I c lass and that WBE-II (88 kD) is related to maize BE II. Comparison of the N-terminal sequences from WBE-I-AD and WEE-II with maize and rice BEs confirms these relationships. Evidence is presented from the analy sis of nullisomic-tetrasomic wheat lines demonstrating that WEE-I-B is located on chromosome 7B and that the WBE-I-AD fraction contains poly peptides that are encoded on chromosomes 7A and 7D. The wheat endosper m BE classes are differentially expressed during endosperm development . WBE-II is expressed at a constant level throughout mid and late endo sperm development. In contrast, WBE-I-AD and WBE-I-B are preferentiall y expressed in late endosperm development. Differences are also observ ed in the kinetic characteristics of the enzymes. The WBE-I isoforms h ave a 2- to 5-fold higher affinity for amylose than does WBE-II, and t he WEE-I isoforms are activated up to 5-fold by phosphorylated interme diates and inorganic phosphate, whereas WBE-II is activated only 50%. The potential implications of this activation of BE I for starch biosy nthesis are discussed.