title: Differential characteristics and subcellular localization of two starch-branching enzyme isoforms encoded by a single gene in Phaseolus vulgaris L.
abstract: Starch-branching enzymes (SBE) have a dominant role for amylopectin structure as they define chain length and frequency of branch points. We have previously shown that one of the SBE isoforms of kidney bean (Phaseolus vulgaris L.), designated PvSBE2, has a molecular mass (82 kDa) significantly smaller than those reported for isologous SBEs from pea (SBEI), maize (BEIIb), and rice (RBE3). Additionally, in contrast to the dual location of the pea SBEI in both the soluble and starch granule fractions, PvSBE2 was found only in the soluble fraction during seed development. Analysis of a pvsbe2 cDNA suggested that PvSBE2 is generated from a larger precursor with a putative plastid targeting sequence of 156 residues. Here we describe the occurrence of a larger 100-kDa form (LF-PvSBE2) of PvSBE2 found both in the soluble and starch granule fractions of the developing seeds. The determined N-terminal sequence, VKSSHDSD, of LF-PvSBE2 corresponded to a peptide sequence located 111 amino acids upstream from the N terminus of purified PvSBE2, suggesting that LF-PvSBE2 and PvSBE2 are products of the same gene. Analysis of the products by 5'-RACE (rapid amplification of cDNA ends) and reverse transcription PCR indicated that the two transcripts for pre-LF-PvSBE2 and pre-PvSBE2 are generated by alternative splicing. Recombinant LF-PvSBE2 (rLF-PvSBE2) was purified from Escherichia coli and the kinetic properties were compared with those of recombinant PvSBE2 (rPvSBE2). rLF-PvSBE2 had much higher affinity for amylopectin (K(m) = 4.4 mg/ml) than rPvSBE2 (18.4 mg/ml), whereas the V(max) of rLF-PvSBE2 (135 units/mg) for this substrate was much lower than that of rPvSBE2 (561 units/mg). These results suggest that the N-terminal extension of LF-PvSBE2 plays a critical role for localization in starch granules by altering its enzymatic properties.
authors.affiliation: Department of Applied Bioscience, Graduate School of Agriculture, Hokkaido University, Sapporo 060-8589, Japan.
authors.forename: Shigeki
authors.initials: S
authors.lastname: Hamada
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Ito
authors.forename: Susumu
authors.initials: S
authors.lastname: Hiraga
authors.forename: Keisuke
authors.initials: K
authors.lastname: Inagaki
authors.forename: Kouichi
authors.initials: K
authors.lastname: Nozaki
authors.forename: Naoto
authors.initials: N
authors.lastname: Isono
authors.forename: Yasushi
authors.initials: Y
authors.lastname: Yoshimoto
authors.forename: Yasuhito
authors.initials: Y
authors.lastname: Takeda
authors.forename: Hirokazu
authors.initials: H
authors.lastname: Matsui
mesh_terms: D015061:1,4-alpha-Glucan Branching Enzyme / Q000737:chemistry*; D017398:Alternative Splicing; D000595:Amino Acid Sequence; D000687:Amylopectin / Q000096:biosynthesis; D001483:Base Sequence; D015152:Blotting, Northern; D018076:DNA, Complementary / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D004926:Escherichia coli / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D015151:Immunoblotting; D007700:Kinetics; D008954:Models, Biological; D008969:Molecular Sequence Data; D010455:Peptides / Q000737:chemistry; D027805:Phaseolus / Q000201:enzymology* / Q000235:genetics*; D011485:Protein Binding; D020033:Protein Isoforms; D017434:Protein Structure, Tertiary; D012333:RNA, Messenger / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D017386:Sequence Homology, Amino Acid; D013696:Temperature; D013997:Time Factors
chemical_list: D018076:DNA, Complementary; D010455:Peptides; D020033:Protein Isoforms; D012333:RNA, Messenger; D000687:Amylopectin; D015061:1,4-alpha-Glucan Branching Enzyme