STARCH BRANCHING ENZYMES BELONGING TO DISTINCT ENZYME FAMILIES ARE DIFFERENTIALLY EXPRESSED DURING PEA EMBRYO DEVELOPMENT

cDNA clones for two isoforms of starch branching enzyme (SBEI and SBEI I) have been isolated from pea embryos and sequenced. The deduced amin o acid sequences of pea SBEI and SBEII are closely related to starch b ranching enzymes of maize, rice, potato and cassava and a number of gl ycogen branching enzymes from yeast, mammals and several prokaryotic s pecies. In comparison with SBEI, the deduced amino acid sequence of SB EII lacks a flexible domain at the N-terminus of the mature protein. T his domain is also present in maize SBEII and rice SBEIII and resemble s one previously reported for pea granule-bound starch synthase II (GB SSII). However, in each case it is missing from the other isoform of S EE from the same species. On the basis of this structural feature (whi ch exists in some isoforms from both monocots and dicots) and other di fferences in sequence, SBEs from plants may be divided into two distin ct enzyme families. There is strong evidence from our own and other wo rk that the amylopectin products of the enzymes from these two familie s are qualitatively different. Pea SBEI and SBEII are differentially e xpressed during embryo development. SBEI is relatively highly expresse d in young embryos whilst maximum expression of SBEII occurs in older embryos. The differential expression of isoforms which have distinct c atalytic properties means that the contribution of each SEE isoform to starch biosynthesis changes during embryo development. Qualitative me asurement of amyiopectin from developing and maturing embryos confirms that the nature of amylopectin changes during pea embryo development and that this correlates with the differential expression of SEE isofo rms.