Bifunctional protein in carrot contains both aspartokinase and homoserine dehydrogenase activities

• Published in: Plant physiology (Bethesda) Vol. 97; no. 4; pp. 1323 - 1328
• Main Authors: Wilson, B.J. (USDA, ARS, Plant Molecular Laboratory, Beltsville, MD), Gray, A.C, Matthews, B.F
• Format: Journal Article
• Language: English
• Published: Rockville, MD American Society of Plant Physiologists 01.12.1991
• Subjects: ACIDE ASPARTIQUE; ACIDO ASPARTICO; ACTIVIDAD ENZIMATICA; ACTIVITE ENZYMATIQUE; Amino acids; Antibodies; aspartate kinase; Biological and medical sciences; Cell culture techniques; Chemical suspensions; Chromatography; CULTIVO DE CELULAS; CULTURE DE CELLULE; Cultured cells; DAUCUS CAROTA; Dehydrogenases; enzymatic activity; Enzymes; Fundamental and applied biological sciences. Psychology; Gels; homoserine dehydrogenase; Metabolism; OXIDORREDUCTASAS; OXYDOREDUCTASE; Plant physiology and development; PROTEINAS; PROTEINE; Proteins; PURIFICACION; PURIFICATION; SERINA; SERINE; TECHNIQUE ANALYTIQUE; TECNICAS ANALITICAS; TRANSFERASAS; TRANSFERASE; Homoserine dehydrogenase; Purification; Daucus carota; Enzyme; Bifunctional compound; Proteins; Suspension culture; Enzymatic activity; Dicotyledones; Angiospermae; Bacteria; Spermatophyta; Umbelliferae; Aspartate kinase; Comparative study; Aminoacid sequence
• ISSN: 0032-0889; 1532-2548
• DOI: 10.1104/pp.97.4.1323

We have purified homoserine dehydrogenase to homogeneity and subjected polypeptide fragments derived from digests of the protein to amino acid sequencing. The amino acid sequence of homoserine dehydrogenase from carrot (Daucus carota) indicates that in carrot both aspartokinase and homoserine dehydrogenase activities reside on the same protein. Additional evidence that aspartokinase and homoserine dehydrogenase reside on a bifunctional protein is provided by coelution of activities during purification steps and by enzyme-specific gel staining techniques. Highly purified fractions containing aspartokinase activity were stained for aspartokinase activity, homoserine dehydrogenase activity, and protein. These gels confirmed that aspartokinase activity and homoserine dehydrogenase activity were present on the same protein. This arrangement of aspartokinase and homoserine dehydrogenase activities residing on the same protein is also found in Escherichia coli, which has two bifunctional enzymes, aspartokinase I-homoserine dehydrogenase I and aspartokinase II-homoserine dehydrogenase II. The amino acid sequence of the major form of homoserine dehydrogenase from carrot cell suspension cultures most closely resembles that of the E. coli ThrA gene product aspartokinase I-homoserine dehydrogenase I