Role of SCD1 in Plant Cytokinesis and Polarized Cell Expansion

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Cell growth and division are crucial for normal growth and development of multicellular plants and animals. The long-term objective of this work is to understand the molecular events and mechanisms involved in intracellular vesicle trafficking that take place during plant cell division (cytokinesis) and plant cell expansion. During cytokinesis, one or more exocytic secretory pathways are polarized toward the plane of division - i.e., secretory vesicles move to the division site -- resulting in the de novo formation of the cytokinetic organelle known as the cell plate (a structure that is the precursor to the cell wall that will eventually divide the two halves into separate cells). Similarly, plant cell expansion is mediated through the delivery and fusion of secretory vesicles carrying membrane and cell wall components to specific sites on the plasma membrane, resulting in intercalation of new membrane and wall components to the existing cell surface and thereby cell expansion. At the present time, relatively little is known about the organization of the exocytic and endocytic pathways, their interaction with the cytoskeleton, and the molecular machinery involved in these complex processes in plants. Dr. Bednarek has recently demonstrated that, in the model plant Arabidopsis thaliana, a plant-specific protein, SCD1, is critical for cell plate formation and polarized cell expansion. SCD1 is a 130 kDa peripheral membrane protein containing an N-terminal DENN domain (named for the human DENN protein which it resembles in terms of amino acid sequence: Differentially Expressed in Normal and Neoplastic cells) and eight C-terminal WD-40 repeats, which are commonly associated with protein-protein interactions. Although the precise function of DENN domains is not known, they have been identified in animal proteins that interact with Rabs (small GTPases that regulate secretory vesicle trafficking) and other signaling proteins including mitogen-activated protein kinases. Dr. Bednarek hypothesizes that SCD1 may regulate vesicle trafficking and/or cytoskeletal dynamics during cell division and expansion. This project has been designed to: 1) study the localization and targeting of SCD1 within the plant endomembrane system; 2) learn the identity of SCD1-associated endomembranes; and 3) identify SCD1-interacting proteins required for polarized membrane trafficking in plants. The project will use experimental approaches that integrate complementary forward and reverse molecular genetic, cytological, and biochemical approaches. The outcome of this project is expected to contribute significantly to both an understanding of the function and organization of the plant secretory pathway and its role in cell plate biogenesis and polarized cell expansion. Upon completion of the project, it is expected that the function of SCD1 in plant morphogenesis will be understood. These studies will also contribute to a general understanding of the function of DENN-domain containing proteins in membrane trafficking and signaling in other eukaryotic cells. Broader Impact: This project will provide valuable research training in modern cell biology for young scientists at the undergraduate, graduate and postgraduate level. Graduate students and postdoctoral research associates will also learn valuable teaching and mentoring skills by assisting undergraduate classroom instruction and by supervising undergraduate independent study projects related to the funded project. All reagents and research tools generated by this project will be distributed to other laboratories to assist in the further characterization of membrane trafficking and cytoskeletal regulation of plant growth and development.