Specificity of starch synthase isoforms from potato.
In higher plants several isoforms of starch synthase contribute to the extension of glucan chains in the synthesis of starch. Different isoforms are responsible for the synthesis of essentially linear amylose chains and branched, amylopectin chains. The activity of granule-bound starch synthase I from potato has been compared with that of starch synthase II from potato following expression of both isoforms in Escherichia coli. Significant differences in their activities are apparent which may be important in determining their specificities in vivo. These differences include affinities for ADPglucose and glucan substrates, activation by amylopectin, response to citrate, thermosensitivity and the processivity of glucan chain extension. To define regions of the isoforms determining these characteristic traits, chimeric proteins have been produced by expression in E. coli. These experiments reveal that the C-terminal region of granule-bound starch synthase I confers most of the specific properties of this isoform, except its processive elongation of glucan chains. This region of granule-bound starch synthase I is distinct from the C-terminal region of other starch synthases. The specific properties it confers may be important in defining the specificity of granule-bound starch synthase I in producing amylose in vivo.
Edwards
A
A
John Innes Centre, Norwich, UK.
Borthakur
A
A
Bornemann
S
S
Venail
J
J
Denyer
K
K
Waite
D
D
Fulton
D
D
Smith
A
A
Martin
C
C
D000595:Amino Acid Sequence; D000687:Amylopectin / Q000494:pharmacology; D001483:Base Sequence; D019343:Citric Acid / Q000494:pharmacology; D018076:DNA, Complementary / Q000235:genetics; D018744:DNA, Plant / Q000235:genetics; D004789:Enzyme Activation / Q000187:drug effects; D004926:Escherichia coli / Q000235:genetics; D005964:Glucosyltransferases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D006006:Glycogen Synthase / Q000235:genetics; D007527:Isoenzymes / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D007700:Kinetics; D008969:Molecular Sequence Data; D009844:Oligosaccharides / Q000378:metabolism; D010940:Plant Proteins; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D011198:Solanum tuberosum / Q000201:enzymology* / Q000235:genetics; D012995:Solubility; D013214:Starch Synthase / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D013379:Substrate Specificity; D013696:Temperature
D018076:DNA, Complementary; D018744:DNA, Plant; D007527:Isoenzymes; D009844:Oligosaccharides; D010940:Plant Proteins; D011993:Recombinant Fusion Proteins; C021705:maltooligosaccharides; D019343:Citric Acid; D000687:Amylopectin; D005964:Glucosyltransferases; C111377:starch synthase II; D006006:Glycogen Synthase; D013214:Starch Synthase