Characterization of SU1 isoamylase, a determinant of storage starch structure in maize.
Function of the maize (Zea mays) gene sugary1 (su1) is required for normal starch biosynthesis in endosperm. Homozygous su1- mutant endosperms accumulate a highly branched polysaccharide, phytoglycogen, at the expense of the normal branched component of starch, amylopectin. These data suggest that both branched polysaccharides share a common precursor, and that the product of the su1 gene, designated SU1, participates in kernel starch biosynthesis. SU1 is similar in sequence to alpha-(1-->6) glucan hydrolases (starch-debranching enzymes [DBEs]). Specific antibodies were produced and used to demonstrate that SU1 is a 79-kD protein that accumulates in endosperm coincident with the time of starch biosynthesis. Nearly full-length SU1 was expressed in Escherichia coli and purified to apparent homogeneity. Two biochemical assays confirmed that SU1 hydrolyzes alpha-(1-->6) linkages in branched polysaccharides. Determination of the specific activity of SU1 toward various substrates enabled its classification as an isoamylase. Previous studies had shown, however, that su1- mutant endosperms are deficient in a different type of DBE, a pullulanase (or R enzyme). Immunoblot analyses revealed that both SU1 and a protein detected by antibodies specific for the rice (Oryza sativa) R enzyme are missing from su1- mutant kernels. These data support the hypothesis that DBEs are directly involved in starch biosynthesis.
Rahman
A
A
Department of Biochemistry and Biophysics, Iowa State University, Ames, Iowa 50011, USA.
Wong
K s
Ks
Jane
J l
Jl
Myers
A M
AM
James
M G
MG
D007517:Isoamylase / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000096:biosynthesis; D012639:Seeds / Q000201:enzymology; D013213:Starch / Q000737:chemistry / Q000378:metabolism*; D013379:Substrate Specificity; D003313:Zea mays / Q000201:enzymology* / Q000235:genetics
D011993:Recombinant Fusion Proteins; D013213:Starch; D007517:Isoamylase