Purification and Characterization of Maize Starch Synthase I and Its Truncated Forms

• Published in: Archives of biochemistry and biophysics 1998-05, Vol.353 (1), p.64-72
• Main Authors: Imparl-Radosevich, Jennifer M., Li, Ping, Zhang, Lei, McKean, Angela L., Keeling, Peter L., Guan, Hanping
• Format: Article
• Language: English
• Subjects: ACTIVIDAD ENZIMATICA; ACTIVITE ENZYMATIQUE; ADN; Amino Acid Sequence; Base Sequence; Chromatography, Affinity; Chromatography, Ion Exchange; Cloning, Molecular; COMPLEMENTARY DNA; Cytoplasmic Granules - enzymology; DNA; enzyme kinetics; enzyme purification; ENZYMIC ACTIVITY; Escherichia coli; Genes, Plant; GLICOSILTRANSFERASAS; GLYCOSYLTRANSFERASE; GLYCOSYLTRANSFERASES; HEXOSYLTRANSFERASES; KINETICS; maize; Molecular Sequence Data; PURIFICACION; PURIFICATION; Recombinant Proteins - chemistry; Recombinant Proteins - isolation & purification; Recombinant Proteins - metabolism; starch; starch synthase; Starch Synthase - chemistry; Starch Synthase - isolation & purification; Starch Synthase - metabolism; Substrate Specificity; ZEA MAYS; Zea mays - enzymology
• ISSN: 0003-9861; 1096-0384
• DOI: 10.1006/abbi.1998.0613

Comparison of the protein sequences deduced from the cDNAs of maize granule-bound starch synthase,Escherichia coliglycogen synthase, and maize starch synthase I (SSI) reveals that maize SSI contains an N-terminal extension of 93 amino acids. In order to study the properties of maize SSI and to understand the functions of the maize SSI N-terminal extension, the gene coding for full-length SSI (SSI-1) and genes coding for N-terminally truncated SSI (SSI-2 and SSI-3) were individually expressed inE. coli.Here we describe for the first time the purification of a higher plant starch synthase to apparent homogeneity. Its kinetic properties were therefore studied in the absence of interfering amylolytic enzymes. The specific activities of the purified SSI-1, SSI-2, and SSI-3 were 22.5, 33.4, and 26.3 μmol Glc/min/mg of protein, respectively, which are eight times higher than those of partially purified SSI from developing maize endosperm. The full-length recombinant enzyme SSI-1 exhibited properties similar to those of the enzyme from maize endosperm. As observed for native maize enzyme, recombinant SSI-1 exhibited “unprimed” activity without added primer in the presence of 0.5 M citrate. Our results have clearly indicated that the catalytic center of SSI is not located in its N-terminal extension. However, N-terminal truncation decreased the enzyme affinity for amylopectin, with theKmfor amylopectin of the truncated SSI-3 being about 60–90% higher than that of the full-length SSI-1. These results suggest that the N-terminal extension in SSI may not be directly involved in enzyme catalysis, but may instead regulate the enzyme binding of α-glucans. Additionally, the N-terminal extension may play a role in determining the localization of SSI to specific portions of the starch granule or it may regulate its interactions with other enzymes involved in starch synthesis.