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Functional and Structural Characterization of TET/JANUS Signaling Complexes in A. Thaliana Sperm Cells
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Plants are used as a primary food source by humans. Some plants produce edible roots or leaves, but most crops used today are grown to harvest their nutrient-rich seeds which are a product of double fertilization in flowering plants.
Cell-cell recognition, adhesion, and fusion are widespread phenomena in many biological processes, where fertilization is an exemplary process. Many players have been identified to mediate sperm-egg fusion in both animals and plants. Interestingly several of these components were shown to be structurally and functionally conserved across kingdoms. In animals Tetraspanins act as facilitators of sperm-egg fusion. Tetraspanins are known to associate in clusters in the plasma membrane of cells, where they recruit diverse signaling proteins, forming the so called Tetraspanin-enriched microdomains (TEMs). TEMs are therefore recognized as major signaling platforms mediating specific cellular processes in the plasma membrane of cells. Two Arabidopsis-expressed tetraspanins, TET11 and TET12, are highly expressed in the sperm cells (SCs), however their function in fertilization are unknown. Using fluorescence microscopy, we quantified the expression of TETs in SCs and found evidence for the existence of a Tetraspanin-enriched microdomain (TEM) at the SC-SC adhesion interface. Sperm cell factors which are necessary for fertilization were found to accumulate at the TEM, suggesting that plant SC TEMs may function as protective platforms for fertilization factors. Sperm-expressed TETs directly interact with members of a novel, plant-specific family of unknown proteins, DMP8/9. DMP8/9 function as negative regulators of SC-SC adhesion and are required for double fertilization. Structural and functional analysis suggest that these two proteins may perform unique functions as membrane remodelers in SCs. In addition, we provide evidence of a new GEX2 function as a SC-SC adhesion factor and potential partner of TET-DMP complexes at the SC-SC interface.