The Role of START Lipid/Sterol Binding Domains in Homeodomain Transcription Factors of Plants

Keck Graduate Institute, Claremont CA

Investigators

Abstract

Sterols are small water-insoluble molecules that play critical roles in growth and development of both animals and plants. Ongoing research in the field of plant molecular biology has revealed the importance of the sterol-derived brassinosteroids, which are the only steroid hormones identified from plants thus far. In addition to brassinosteroids, plants contain a wide variety of sterol compounds whose biological functions are not understood. The long-term goal of the Schrick lab is to identify and characterize the components of putative sterol signaling pathways in plants. Candidate sterol-binding proteins include a family of homeodomain (HD) transcription factors having Steroidogenic Acute Regulatory (StAR)-related lipid transfer (START) lipid/sterol-binding domains. START domains, named after the mammalian cholesterol-binding StAR protein, are found in a variety of proteins across the eukaryotes, but have been studied in only a few cases from animals. Strikingly, START domains in combination with the HD DNA-binding motif are unique to plants. This project addresses the hypothesis that the START domain found in HD transcription factors binds to lipid/sterol ligands and that this interaction controls gene expression during development. Several members of the HD-START family from Arabidopsis, maize, and rice, have crucial roles in cell differentiation and/or exhibit layer-specific expression patterns. Using the Arabidopsis thaliana Meristem Layer 1 (ATML1) and GLABRA2 (GL2) genes as models, the aim of this project will be to examine the putative role of START domains as lipid/sterol binding motifs. Potential ligands will be experimentally identified in a novel in vivo lipid/sterol-binding assay utilizing the yeast Saccharomyces cerevisiae. In the assay the START domain is expressed as part of a synthetic transcription factor that activates a reporter gene in response to ligand binding. This assay will be implemented to test a chemical library of lipid/sterol compounds for binding to candidate START domains from Arabidopsis HD-START transcription factors, and additionally will be used for mutational analysis of selected START domains. Completion of this project will provide a conceptual framework for the presumed function of plant START domains in ligand binding, potentially revealing a link between lipid/sterol metabolism and cell differentiation in plants. Moreover, this work will result in significant advances in understanding of the role of HD-START transcription factors in plant development, and will lead to important insights underlying lipid/sterol signaling mechanisms. This research will have broader impacts through multidisciplinary education of graduate students as well as undergraduates, and the information gained from this research will facilitate improvements in plant-derived substances for practical applications in biotechnology.