BBMB Research Seminars
September 11, 2008
Tracie Bierwagen
Department of Biochemistry, Biophysics, and Molecular Biology
Iowa State University
"Starch Synthesis in Maize and Arabidopsis"
1414 Molecular Biology Buidling
4:10 p.m.
Biochemical and genetic analyses of multimeric complexes containing starch biosynthetic enzymes in maize endosperm
Abstract
Starch is assembled by the combined actions of multiple starch synthases (SS), starch branching enzymes (SBE), and starch debranching enzymes, however, the mechanisms responsible determining the molecular architecture of the polymer are unknown. Genetic evidence indicates that functional interactions occur between components of the starch metabolic pathway. Our current research has investigated the physical interactions between the SSs and BEs, which are likely to explain these genetic data. Numerous biochemical methods show that specific SSs and BEs exist in multisubunit complexes in maize endosperm extracts, including yeast two-hybrid tests, gel permeation chromatography, immunoprecipitation, and affinity purification. Assembly interdependence analyses revealed that SSIII, in addition to its enzymatic functions, serves as a scaffolding factor in assembly of a high molecular weight complex that contains at least one other SS and two SBEs. Additional proteins that associate with SSIII in the partially purified complex(es) were identified by mass spectrometry. Of particular interest is pyruvate orthophosphate dikinase (PPDK), which is known to function in leaves in C4 photosynthesis. We propose that in the amyloplast the generation of PPi by PPDK serves to inhibit the first committed step in starch biosynthesis catalyzed by ADP glucose pyrophosphorylase (ADPGPP), which also is associated with SSIII in a high molecular weight complex. Thus, the enzyme complexes identified here may function to regulate carbon flux between starch and other metabolic pathways during grain filling. Another function of the complexes may be to provide physical coordination among the biosynthetic enzyme active sites and thus impart architectural specificity to the product.
Biographical data of Dr. Tracie Hennen-Bierwagen
B.S., Biology, Iowa State University
Ph.D., Plant Physiology, Department of BBMB, Iowa State University
Assistant Scientist II, Department of BBMB, Iowa State University
Publications
Edgington, N., Blacketer, M., Bierwagen, T.A. and Myers, A.M. (1998) Control of Saccharomyces cerevisiae filamentous growth by cyclin-dependent kinase CDC28. Mol Cell Biol 19, 1369-1380.
Hennen-Bierwagen, T.A., Liu, F., Marsh, R.S., Kim, S., Gan, Q., Tetlow, I.J., Emes, M.J., James, M.G. and Myers, A.M. (2008). Starch biosynthetic enzymes from developing Zea mays endosperm associate in multisubunit complexes. Plant Physiol 146, 1892-1908.
Hennen-Bierwagen, T.A., Lin, Q., James, M.G. and Myers, A.M. (2008). Maize starch synthase III is a scaffolding protein integrating multiple metabolic pathways. Submitted to Plant Physiol.
