The Role of Protein-Protein Interactions in Sucrose Transport

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One defining feature of plant organisms is the photosynthetic production of sucrose. Essentially, carbon from the atmosphere is assimilated into sucrose which is then transported throughout the plant where it is used as an energy source. The productivity, or yield, of plants directly depends on the net assimilation of carbon into sucrose and ultimately how much of this sucrose is transported to the harvestable components of plants, such as the seed. Understanding how sucrose is transported through the plant and how this transport is regulated is critical for both for understanding fundamental plant biology and optimization of plant productivity.The sucrose transport protein from soybean cotyledons appears to interact with another protein in the cytosol of plant cells. This interacting protein contains at least three ankyrin repeat motifs which often are involved in linking membrane transport proteins to the cellular cytoskeleton. Since the sucrose transporter interacts with other proteins, it may be that this interaction is important in the assembly of sucrose transporters on the cell surface or in the formation/disassembly of protein complexes that both import sucrose and metabolize it further. Thus, sucrose transport may be regulated in part by the nature of interactions between the transporter and this interacting partner.This research will attempt to elucidate the functional significance of this protein-protein interaction in sucrose transport. The three primary objectives are as follows.Objective #1. Identify proteins that interact with the ankyrin-like protein. To understand the nature of this interaction, it is important to determine if other proteins might also be brought into contact with the plant sucrose transporter. For instance, if sucrose metabolizing enzymes are part of a complex with the sucrose transporter, then sucrose import may be directly linked to its subsequent metabolism. These experiments will address this possibility.Objective #2. Assess the role of protein-protein interaction on sucrose uptake. If the interaction of the sucrose transporter with the ankyrin-like protein has an impact on sucrose uptake in the plant, then disruption of the ankyrin-like protein function should disrupt sucrose transport in distinct ways. The ankyrin-like protein function will be disrupted using gene-silencing technology and sucrose uptake parameters assessed.Objective #3. Examine the subcellular localization of the sucrose transporter and the ankyrin-like protein. Several important questions are addressed by characterizing the nature of the sucrose transporter:ankyrin-like protein (ST:ALP) interaction in the cell. Does the ST:ALP complex change during cotyledon development? When does the ST:ALP complex form relative to synthesis of the sucrose transporter and the ankryin-like protein? Our mRNA analysis suggests that the ankyrin-like protein accumulates earlier in cotyledon development than does the sucrose transporter and this raises the question as to whether sucrose transporter is bound to the ankyrin-like protein as it is translated. Immunocytochemical analysis of the sucrose transporter, the ankyrin-like protein, and other potential interacting proteins confocal and electron microscopy will address these issues.