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Endocytosis and exocytosis are key processes in the control of protein trafficking and cell polarity. The membrane lipid PtdIns(4,5)P₂ displays polarized distribution in plant membranes and controls both endocytosis and exocytosis. Therefore, PtdIns(4,5)P₂ likely represents an important regulatory factor for the distribution of proteins in plant cells. Nonetheless, the exact molecular targets of PtdIns(4,5)P₂ in plants are not known to date. A number of hypotheses contended to explain the effects of PtdIns(4,5)P₂ draw immediate analogies to mammalian systems or yeast. However, there are clear differences between phosphoinositide metabolism in these systems and in plants, and in particular plant PI4P 5-kinases contain plant-specific protein domains. In order to move beyond mere speculation and to provide a solid basis for the study of PtdIns(4,5)P₂-effects on endocytosis and exocytosis in plants, we propose to systematically test the regulatory influence of PtdIns(4,5)P₂ on a selection of known elements of the endocytotic and exocytotic machineries. Experiments in this project are designed to elucidate the influence of PtdIns(4,5)P₂ on key players in plant endocytosis and exocytosis, and in vesicle-to-membrane targeting and fusion.