ResearchCrossroads

Summary
This basic research project is focused on this laboratory's long-standing interest in plant metabolic enzymes and their regulation by the ubiquitous process of reversible protein phosphorylation. The overall investigation deals specifically with phosphoenolpyruvate carboxylase (PEPC) and sucrose synthase (SuSy), two target enzymes that undergo phosphorylation in photosynthesizing leaves and/or nitrogen-fixing root nodules of soybeans, peas, and other legumes. While PEPC is best known for its cardinal role in the initial fixation of atmospheric carbon dioxide during photosynthesis by such crop plants as sorghum, corn, sugarcane and pineapple, it also functions in a diverse array of non-photosynthetic contexts, e.g., carbon/nitrogen-metabolism in legume root nodules and unicellular green algae. This multifaceted nature of plant PEPC makes it an extremely attractive subject for continued study of its (a) intricate control by PEPC-kinase (PpcK), the novel, highly regulated protein-kinase enzyme that phosphorylates/activates PEPC, and (b) emerging biochemical structure-function relationships. Along these lines, two of the three specific experimental objectives of this renewal project focus on the molecular/biochemical analysis of plant PpcK, and sorghum-leaf and green-algal PEPC. The third and final focus of this investigation is SuSy, likely the major sucrose-metabolizing enzyme in a variety of non-photosynthetic sink tissues in plants, including legume root nodules, developing leaves, fruits and seeds, and tubers. Continued interest in this metabolic enzyme largely stems from the fact that, like PEPC, it is one of but a handful of phosphorylated plant enzymes with a specific target amino-acid residue(s) identified both in planta and in vitro. In addition, this target enzyme has the documented potential for phosphorylation at multiple sites by at least two distinct subfamilies of plant protein kinases. However, neither the exact role of this phosphorylation event(s) nor its dynamic reversibility in planta is established. These uncertainties will be addressed experimentally during this project by a combination of biochemical, immunological and mass-spectroscopic approaches. The significance of this multi-pronged, basic research and discovery project is that it will (a) enhance the fundamental understanding of two important enzymes involved in plant primary metabolism in terms of their intricate regulation and structure-function relationships, and (b) provide detailed molecular insight into the unique protein kinase that targets one of them exclusively. As such, the results from this research will contribute significantly to the requisite underpinning for the constructive manipulation of plant carbon/nitrogen-metabolism and its control mechanisms in leaves and legume root nodules by biotechnological approaches.

Principal Investigator	Raymond Chollet
Co-Principal Investigator(s)
Recipient Organization	University of Nebraska-Lincoln
Granting Organization	Division of Molecular and Cellular Biosciences (MCB - NSF)

Reference	Dates	Fiscal Year	Funded Amount
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