Structure-function analysis of the interaction of secretory peptides with their target proteins

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In higher plants sperm cells are immobile. For a successful fertilization they need to be transported to the female gametophyte via growth of the pollen tube. Upon arrival at the micropyle the pollen tube stops to grow and bursts, thereby delivering the sperm cells to the egg apparatus. In this context, we have recently shown that the defensin-like (DEFL) protein ZmES4 (Zea mays embryo sac 4) mediates pollen tube burst in maize via opening of the potassium channel KZM1. Since the exact molecular mechanism of this interaction and its relation to pollen tube burst is not known a main goal of the proposal relies on understanding the structure-function relationship of the interaction of ZmES4 with the potassium ion channel KZM1. As this interaction is very specific, we plan to identify domains or residues within the ion channel which are essential for ZmES4 binding. Furthermore we aim on elucidating how ZmES4-mediated channel modulation finally leads to pollen tube burst. We postulate that in addition to K+ flux, sucrose uptake and ensuing water influx represents an essential step for pollen tube rupture. Therefore we will investigate whether the corresponding sucrose transporters are also affected by ZmES4 application. Finally, if time permits we will elucidate whether the interaction of DEFL proteins with ion channels represents a general principle in higher plants to induce pollen tube rupture and sperm release. Therefore, we will test DEFL proteins and ion channels of other plant species regarding their ability to fulfill this function. In case of positively tested DEFL proteins and their respective targets we will model and compare their structures to clarify if the DEFL/target binding site possesses a species-specific structure.