Elevated proportions of a
mylose in cereals are co
mmonly associated with either the loss of starch branching or starch synthase activity. Goa
mi 2 is a high-a
mylose
mutant of the
temperate japonica rice variety Ilpu
mbyeo. Genotyping revealed that Goa
mi 2 and Ilpu
mbyeo carry the sa
me alleles for
starch synthase IIa and
granule-bound starch synthase I genes. Analyses of granule-bound proteins revealed that SSI and SSIIa accu
mulate inside the
mature starch granules of Goa
mi 2, which is si
milar to the
amylose extender mutant IR36ae. However, unlike the
amylose extender mutants, SBEIIb was still detectable inside the starch granules of Goa
mi 2. Detection of SBEIIb after protein fractionation revealed that
most of the SBEIIb in Goa
mi 2 accu
mulates inside the starch granules, whereas
most of it accu
mulates at the granule surface in Ilpu
mbyeo. Exhaustive
mass spectro
metric characterisations of granule-bound proteins failed to detect any peptide sequence
mutation or
major post-translational
modifications in Goa
mi 2. Moreover, the signal peptide was found to be cleaved nor
mally fro
m the precursor protein, and there is no apparent N-linked glycosylation. Finally, no difference was found in the
SBEIIb structural gene sequence of Goa
mi 2 co
mpared with Ilpu
mbyeo. In contrast, a G-to-A
mutation was detected in the
SBEIIb gene of IR36ae located at the splice site between exon and intron 11, which could potentially introduce a pre
mature stop codon and produce a truncated for
m of SBEIIb. It is suggested that the
mutation responsible for producing high a
mylose in Goa
mi 2 is not due to a defect in
SBEIIb gene as was observed in IR36ae, even though it produces a phenotype analogous to the
amylose extender mutation. Understanding the
molecular genetic basis of this
mutation will be i
mportant in identifying novel targets for increasing a
mylose and resistant starch contents in rice and other cereals.
Keywords:
mylopectin&qsSearchArea=searchText">amylopectin; m+MS&qsSearchArea=searchText">ion trap tandem MS; my+2&qsSearchArea=searchText">Goamy 2; mass+spectrometry&qsSearchArea=searchText">mass spectrometry; matrix%5C-assisted+laser+desorption%2Fionization+MS&qsSearchArea=searchText">matrix-assisted laser desorption/ionization MS; resistant starch; morphism&qsSearchArea=searchText">single-nucleotide polymorphism; Suweon 464