2110 Molecular Biology Building
Dept. of Biochemistry, Biophysics and Molecular Biology
Iowa State University
Ames, IA 50011
Phone: (515) 294-9548
B.S., Biological Science, Carnegie Mellon University, 1977
Ph.D., Genetics, Duke University, 1983
Postdoctoral Fellow, Columbia University, 1983-1986
The overall goal of the joint laboratory of Alan Myers and Martha James is to provide a comprehensive understanding of starch metabolism in higher plants. This problem is important owing to the facts that more than half of the calories in the human diet worldwide originate from starch, and that starch offers a renewable source of chemical energy and industrial raw material. Starch biosynthesis also is of fundamental interest considering that the process is central in the conversion of electromagnetic energy to chemical energy utilized by our biosphere. Also of interest is how enzymes are able to catalyze the formation of a homopolymer with a specific architecture.
The molecular architecture of starch is integrally related to its biologica functions. In the glucose homopolymers of starch, amylopectin and amylose, glucose units are linked in linear chains by alpha-1,4 glycoside bonds, and these are joined to each other at branch points formed by alpha-1,6 glycoside bonds. About 5% of the glucose residues in amylopectin have branch linkages. The branches are clustered in specific regions that are separated by areas that lack such linkages. This architecture allows packing of linear chains into semicrystalline regions, which in turn form starch granules. This close packing allows large amounts of glucose to be stored for later use, for example, in leaves at night or seeds during germination. The central question for our laboratory is how the biosynthetic enzymes can catalyze chain growth and branch linkage formation so that the highly specific architecture of amylopectin arises.
Our research involves all of the enzymes coded for by plant genomes that can be identified as factors involved in starch assembly or disassembly. The enzymes involved include: 1) starch synthases (SS), which form linear glucose chains, 2) starch branching enzymes (BE), which form branch linkages, 3) starch debranching enzymes (DBE), which eliminate branch linkages, 4) alpha- and beta- amylases, which hydrolyze alpha-1,4 glycoside bonds, 5) starch phosphorylases, which catalyze phosphorolysis of alpha-1,4 bonds, and 6) disproportionating enzymes (DE), which exchange portions of linear chains. All together about 30 such enzymes are potentially involved in starch metabolism, because multiple isoforms of each catalytic activity are conserved in plants.
The model organisms used in the lab are maize and Arabidopsis, with the former serving to study storage starch metabolism and the latter for leaf starch. Earlier work in this project revealed that genetic elimination of one of the four conserved DBE genes caused a decrease in starch content, and accumulation of an abnormal glucan polymer in which the architectural specificity was lost. This observation is not intuitive, because a DBE would be expected to be required for degradation of starch, not for starch biosynthesis. Following up on this work, one major goal in the lab is to discover the specific function of DBEs in the assembly of starch.
Most recently the lab has undertaken a functional genomics project making use of the complete sequence of the Arabidopsis genome to study starch metabolism in comprehensive sense. The Arabidopsis 2010 Program of the NSF supports this work. Project goals include collecting loss of function mutations for each of the 30 genes referred to in the preceding paragraph, generation of isoform specific antibodies capable of detecting any of the proteins, detailed descriptions of the effects of each mutation on starch accumulation and degradation, recombination and purification of all 30 enzymes, characterization of temporal and spatial gene expression patters for each gene in the study set, protein-protein interaction studies among members of the pathway, and global mRNA profiling in each of the mutants. This set of comprehensive tools is a necessary step towards a clear mechanistic understanding of the complex and critical processes of starch assembly and disassembly.
1: Lin Q, Huang B, Zhang M, Zhang X, Rivenbark J, Lappe RL, James MG, Myers AM, Hennen-Bierwagen TA. Functional interactions between starch synthase III and isoamylase-type starch-debranching enzyme in maize endosperm. Plant Physiol. 2012 Feb;158(2):679-92. doi: 10.1104/pp.111.189704. Epub 2011 Dec 22. PubMed PMID: 22193705; PubMed Central PMCID: PMC3271759.
2: Myers AM, James MG, Lin Q, Yi G, Stinard PS, Hennen-Bierwagen TA, Becraft PW. Maize opaque5 encodes monogalactosyldiacylglycerol synthase and specifically affects galactolipids necessary for amyloplast and chloroplast function. Plant Cell. 2011 Jun;23(6):2331-47. doi: 10.1105/tpc.111.087205. Epub 2011 Jun 17. PubMed PMID: 21685260; PubMed Central PMCID: PMC3160020.
3: Kubo A, Colleoni C, Dinges JR, Lin Q, Lappe RR, Rivenbark JG, Meyer AJ, Ball SG, James MG, Hennen-Bierwagen TA, Myers AM. Functions of heteromeric and homomeric isoamylase-type starch-debranching enzymes in developing maize endosperm. Plant Physiol. 2010 Jul;153(3):956-69. doi: 10.1104/pp.110.155259. Epub 2010 May 6. PubMed PMID: 20448101; PubMed Central PMCID: PMC2899900.
4: Keeling PL, Myers AM. Biochemistry and genetics of starch synthesis. Annu Rev Food Sci Technol. 2010;1:271-303. doi: 10.1146/annurev.food.102308.124214. Review. PubMed PMID: 22129338.
5: Schnable PS, et al. The B73 maize genome: complexity, diversity, and dynamics. Science. 2009 Nov 20;326(5956):1112-5. doi: 10.1126/science.1178534. Erratum in: Science. 2012 Aug 31;337(6098):1040. PubMed PMID: 19965430.
6: Hennen-Bierwagen TA, Lin Q, Grimaud F, Planchot V, Keeling PL, James MG, Myers AM. Proteins from multiple metabolic pathways associate with starch biosynthetic enzymes in high molecular weight complexes: a model for regulation of carbon allocation in maize amyloplasts. Plant Physiol. 2009 Mar;149(3):1541-59. doi: 10.1104/pp.109.135293. Epub 2009 Jan 23. PubMed PMID: 19168640; PubMed Central
7: Li L, Foster CM, Gan Q, Nettleton D, James MG, Myers AM, Wurtele ES. Identification of the novel protein QQS as a component of the starch metabolic network in Arabidopsis leaves. Plant J. 2009 May;58(3):485-98. doi: 10.1111/j.1365 313X.2009.03793.x. Epub 2008 Jan 18. PubMed PMID: 19154206.
8: Hennen-Bierwagen TA, Liu F, Marsh RS, Kim S, Gan Q, Tetlow IJ, Emes MJ, James MG, Myers AM. Starch biosynthetic enzymes from developing maize endosperm associate in multisubunit complexes. Plant Physiol. 2008 Apr;146(4):1892-908. doi: 10.1104/pp.108.116285. Epub 2008 Feb 15. PubMed PMID: 18281416; PubMed Central PMCID: PMC2287357.
9: Li L, Ilarslan H, James MG, Myers AM, Wurtele ES. Genome wide co-expression among the starch debranching enzyme genes AtISA1, AtISA2, and AtISA3 in Arabidopsis thaliana. J Exp Bot. 2007;58(12):3323-42. Epub 2007 Sep 20. PubMed PMID: 17890231.
10: Zhang X, Myers AM, James MG. Mutations affecting starch synthase III in Arabidopsis alter leaf starch structure and increase the rate of starch synthesis. Plant Physiol. 2005 Jun;138(2):663-74. Epub 2005 May 20. PubMed PMID: 15908598; PubMed Central PMCID: PMC1150387.
11: Zhang X, Colleoni C, Ratushna V, Sirghie-Colleoni M, James MG, Myers AM. Molecular characterization demonstrates that the Zea mays gene sugary2 codes for the starch synthase isoform SSIIa. Plant Mol Biol. 2004 Apr;54(6):865-79. PubMed PMID: 15604657.
12: Thomas CL, Blacketer MJ, Edgington NP, Myers AM. Assembly interdependence among the S. cerevisiae bud neck ring proteins Elm1p, Hsl1p and Cdc12p. Yeast. 2003 Jul 15;20(9):813-26. PubMed PMID: 12845607.
13: James MG, Denyer K, Myers AM. Starch synthesis in the cereal endosperm. Curr Opin Plant Biol. 2003 Jun;6(3):215-22. Review. PubMed PMID: 12753970.
14: Dinges JR, Colleoni C, James MG, Myers AM. Mutational analysis of the pullulanase-type debranching enzyme of maize indicates multiple functions in starch metabolism. Plant Cell. 2003 Mar;15(3):666-80. PubMed PMID: 12615940; PubMed Central PMCID: PMC150021.
15: Wu C, Colleoni C, Myers AM, James MG. Enzymatic properties and regulation of ZPU1, the maize pullulanase-type starch debranching enzyme. Arch Biochem Biophys. 2002 Oct 1;406(1):21-32. PubMed PMID: 12234486.
16: Seo BS, Kim S, Scott MP, Singletary GW, Wong KS, James MG, Myers AM. Functional interactions between heterologously expressed starch-branching enzymes of maize and the glycogen synthases of Brewer’s yeast. Plant Physiol. 2002 Apr;128(4):1189-99. PubMed MID: 11950968; PubMed Central PMCID: PMC154247.
17: Dinges JR, Colleoni C, Myers AM, James MG. Molecular structure of three mutations at the maize sugary1 locus and their allele-specific phenotypic effects. Plant Physiol. 2001 Mar;125(3):1406-18. PubMed PMID: 11244120; PubMed Central PMCID: PMC65619.
18: Myers AM, Morell MK, James MG, Ball SG. Recent progress toward understanding biosynthesis of the amylopectin crystal. Plant Physiol. 2000 Apr;122(4):989-97. Review. PubMed PMID: 10759494; PubMed Central PMCID: PMC1539245.
19: Cao H, James MG, Myers AM. Purification and characterization of soluble starch synthases from maize endosperm. Arch Biochem Biophys. 2000 Jan 1;373(1):135-46. PubMed PMID: 10620332.
20: Cao H, Imparl-Radosevich J, Guan H, Keeling PL, James MG, Myers AM. Identification of the soluble starch synthase activities of maize endosperm. Plant Physiol. 1999 May;120(1):205-16. PubMed PMID: 10318698; PubMed Central PMCID: PMC59252.