Dynamics and Structure of Arabinogalactan-Proteins at the Plasma Membrane-Cell Wall Interface in Plants

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The abundance of arabinogalactan-proteins (AGPs) and other glycoconjugates at the interface between the plasma membrane and the cell wall of rose cells suggests that these molecules serve as mediating agents between the surfaces of these two distinct structures. This hypothesis will be tested on whole cells and model systems by analyzing the effect of perturbations of AGPs on both lateral diffusion of plasma membrane components and on linear motion between the plasma membrane and cell wall. Characteristics of these types of motions will be determined as a function of turgor or applied pressure. Perturbations of AGPs will include aggregation, enzymatic cleavage, and altered synthesis. Such characterization will provide further information on the effects of both turgor and AGPs on membrane dynamics. Enzymes capable of cleaving plasma membrane AGPs on live protoplasts will be further purified and their mode of action elucidated. Analysis of the biochemical structure of plasma membrane AGPs and molecules that copurify with plasma membrane AGPs during isolation will also be conducted. Structural analysis of plasma membrane AGPs will include determination of glycosyl linkage composition, predominant anomeric configurations, and amino terminal sequence. The biochemical studies will provide important information towards understanding the role of AGPs as mediating agents or/and as specific recognition molecules during development. The cell walls of plants are complex extracellular matrix structures containing protein and polysaccharide components, the detailed organization of which is largely unknown. As a result of the cellular high water pressure state known as turgor, the plasma membrane must be tightly appressed to the wall by about 7 atmospheres of pressure. However, the components of the membrane must be free to undergo rapid lateral diffusion, even while appressed against the wall. This is particularly true during cell growth. A number of protein-linked glycoconjugates have been found in the plasma membrane, the carbohydrate portion of which is external to the cell and extends between the membrane and the wall. Some of these are analogous to proteoglycans of animal cell membranes. The functions of these are totally unknown. This project will explore functional and structural aspects of one group of these glycoconjugates, the arabinogalactans, with the hypothesis in mind that the arabinogalactans may serve as physical mediators or cushions between the membrane and the wall of plant cells.