Reference "Alteration of the structural properties of starch components by the lack of an isoform of starch branching enzyme in rice seeds"

Reference ID

6500

Title

Alteration of the structural properties of starch components by the lack of an isoform of starch branching enzyme in rice seeds

Source

The Journal of biological chemistry, 1993, vol. 268, pp. 19084-19091

Authors (8)

Mizuno-K	Kawasaki-T
Shimada-H	Satoh-H
Kobayashi-E	Okumura-S
Arai-Y	Baba-T

Abstract

This study describes the effect of starch-synthesizing enzymes on biosynthesis
of storage starch in rice amylose-extender mutants, which contain branched D-
glucans with abnormal structures. Western blot analysis indicated that two out
of five amylose-extender mutant lines lacked an isoform of starch branching
enzyme, termed RBE3, although the levels of granule-bound starch synthase and a
major form of branching enzyme, RBE1, were normal in these two mutants. Proteins
corresponding to the 87-kDa RBE3 molecule were present in the three other amylose-
extender mutants as well as in the wild type. However, the level of branching
enzyme activity significantly decreased in all amylose-extender mutants,
suggesting that the 87-kDa proteins in these three mutants are inactive forms of
RBE3. Therefore, we conclude that formation of the abnormal branched glucans in
the amylose-extender mutant of rice is due to the lack of the RBE3 activity. The
cDNA clones encoding RBE3 have been identified from a normal rice seed cDNA
library in lambda gt11, using a synthetic oligonucleotide as a probe. The
deduced amino acid sequence of RBE3 indicates that this protein is initially
synthesized as a precursor of 825 amino acids, including a 65-residue transit
peptide at the NH2 terminus. The sequences of the catalytic regions in
amylolytic enzymes are highly conserved in the sequence of RBE3. Thus, the
branching enzyme isoform belongs to a family of the amylolytic enzymes. RBE3
also shares a noticeable degree of sequence identity with RBE1, especially at
the central portion of the protein molecule. However, RBE3 possesses an
approximately 70-residue extra sequence at the NH2 terminus and lacks a COOH-
terminal sequence of almost 50 residues as compared with RBE1. The structural
differences at both termini may explain the distinct role in starch synthesis
for RBE1 and RBE3.