The Nodulin 26 Family of Plant Aquaglyceroporins: Transport Properties and Regulation

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Major Intrinsic Proteins are an ancient family of ubiquitous membrane channels that serve as the "molecular plumbing" of cells and tissues, mediating the rapid transport of water and solutes across biological membranes. The MIP family is particularly diverse in plants, reflecting the importance of water relations in plant growth and development and the need to adapt to osmotic challenge (drought and salinity). In this project the structural, functional and regulatory properties of a unique subset of plant MIPs, the Nodulin-like Instrinsic Proteins (NIPs) will be investigated. The archetype of this family is nodulin 26, which is a major component of the symbiosome, an organelle formed during the nodulation of legume roots with symbiotic, nitrogen-fixing bacteria of the Rhizobaceae family. Nodulin 26 forms a multiselective channel that transports water and uncharged solutes across the symbiosome membrane, and may engage in gas exchange (fixed ammonia and molecular oxygen) that is critical to the symbiosis. Nodulin 26 transport is activated by calcium-regulated phosphorylation in response to developmental and water stress signals. The goals of this project are to investigate: 1. the transport selectivity of nodulin 26 and its ability to mediate gas permeability across the symbiosome membrane; 2. the role of calcium-dependent phosphorylation and other regulatory factors on nodulin 26 transport and selectivity; 3. the structures of phosphorylated and unphosphorylated nodulin 26 by crystallography and molecular dynamics techniques, and 4. the transport properties and biological functions of NIP orthologs in the model plant Arabidopsis thaliana. The proposed work will aid in establishing a structural and functional profile for this conserved family of membrane transporters, as well as shedding light on the environmental and developmental factors that regulate their function. Besides contributing to the understanding of transport processes associated with nitrogen-fixing legume-microbe symbiosis, the work will also provide insight into additional fundamental roles of NIPs in water relations and stress adapation by utilizing the powerful Arabidopsis molecular genetic model system.Broader Impact: From the perspective of infrastructure, the work will impact our understanding of the molecular basis of stress regulation and adaptation in plant systems as well as provide a structural framework for plant MIP protein research in general. The understanding of the molecular basis of plant stress responses and nitrogen fixing symbioses are fundamental topics of agricultural importance. The project will serve as a foundation for the training of undergraduate research scholars, as well as Ph.D. candidates and post doctoral associates at the University of Tennessee. In addition, this project will also serve as a foundation for a continuing collaboration with international scientists in plant membrane biology and signal transduction processes related to plant drought and osmotic stress signaling.The Molecular Biochemistry and Integrative Plant Biology Programs jointly fund this project.