To survive, an organism must constantly adjust its internal state to changes in the environment from which it receives signals. The signals set off a chain of events referred to signal transduction. Signal transduction systems are especially important in multicellular organisms, such as plants and animals, because of the need to coordinate the activities of hundreds to trillions of cells. Plants, in particular, have a special need for perceiving signals from their environment because of their static nature. As in the animal cell, the first steps in perception of a signal include signal interaction with a receptor, signal amplification through second messenger production, and signal termination through second messenger hydrolysis. Myo-inositol polyphosphate 5-phosphatases (5PTases) (EC 3.1.3.56) have unique signal terminating abilities toward the second messenger inositol trisphosphate (Ins (1,4,5)P3, InsP3). In Arabidopsis thaliana there are 15 members of the 5PTase family, the majority of which contain a single 5PTase catalytic domain. Four members of the Arabidopsis 5PTase family, however, have a unique protein domain structure, with additional N-terminal WD40 repeats that are implicated in protein-protein interactions. The research presented here focused on the identification of 5PTase interacting proteins and the characterization of their functional role in Arabidopsis. To accomplish this goal, I examined a 5PTase13-interacting protein, the sucrose (Suc) nonfermenting-1-related kinase, SnRK1.1, an important energy sensor that is highly conserved among eukaryotes. My identification of a 5PTase13:SnRK1.1 complex points to the novel interaction of this metabolic modulator and inositol signaling/metabolism. 5PTase13 , however, plays a regulatory role in other plant specific processes as well, since I also identified the Arabidopsis homolog (Atp80) of the human WDR48 (HsWDR48, Hsp80) as a novel protein interactor of 5PTase13. My results indicate that Atp80 is important for leaf emergence, development and senescence likely via a regulatory interaction with 5PTase13 and PINOID â binding protein (PBP1). / Ph. D.