Targeting to the Vacuoles in Plant Cells

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Comparison of the amino acid sequences of plant vacuolar proteins indicates that there is no sequence in common among these proteins which can be solely responsible for targeting to vacuoles. Therefore, the study of sorting signals from different proteins will be required to understand the structural determinants involved in this process. Gramineae lectins, which are vacuolar proteins, are among the best characterized plant proteins. Amino acid sequences, 3-D protein structures, and cDNAs for these lectins are available. Transgenic tobacco plants and suspension cultures have been obtained which correctly express and process barley lectin. The goals of this research for the next five years include the analysis and characterization of protein sequences responsible for sorting to the vacuole and the identification of a putative receptor (or receptors) which recognizes these sorting sequences and subsequently mediates protein transport to vacuoles. The vacuolar sorting signal of the barley lectin proprotein will be identified and analyzed. Once the sorting signal is identified, the following questions will be addressed: 1) What is the shortest sequence capable of redirecting a reporter protein to the vacuole? 2) What are the specific amino acids within the sorting domain that are responsible for correct targeting? 3) Is secondary structure of the sorting signal important? The characteristics of binding of probarley lectin to its putative receptor(s) will also be determined. Maintenance of separate subcellular compartments in eukaryotic cells depends on the correct sorting and targeting of newly synthesized proteins. Thus, mechanisms must exist in the cell to assure that these proteins are targeted to the correct intracellular membranes. In plants, many proteins are deposited in multifunctional organelles called vacuoles. These proteins contribute to the lysosomal, storage, and plant defense functions of the vacuole. The goal of this research is to understand the mechanisms by which proteins are targeted to the plant cell vacuole. The results will contribute important new information on this important problem in plant cell biology.