Thesis advisor: Mary F. Roberts / Thesis advisor: Steven D. Bruner / The bacterial phosphatidylinositol-specific phospholipase C (PI-PLC) from <italic>Bacillus thuringiensis</italic> is specifically activated by low concentrations of a non-substrate lipid, phosphatidylcholine (PC), presented as an interface. However, if the PC concentration in the interface is too high relative to substrate, the enzyme exhibits surface dilution inhibition. Understanding this bacterial enzyme, which shares many kinetic features with the larger and more complex mammalian PI-PLC enzymes, requires elucidating the mechanism for PC activation and inhibition. Various techniques were applied to study the interaction of the protein with vesicles composed of both the activator lipid PC and the substrate lipid (or a nonhydrolyzable analogue). Fluorescence correlation spectroscopy (FCS), used to monitor bulk partitioning of the enzyme on vesicles, revealed that both the PC and the substrate analogue are required for the tightest binding of the PI-PLC to vesicles. Furthermore, the tightest binding occurred at low mole fractions of substrate-like phospholipids. Field cycling <super>31</super>P NMR (fc-P-NMR) spin-lattice relaxation studies provided information on how bound protein affects the lipid dynamics in mixed substrate analogue/PC vesicles. The combination of the two techniques could explain the enzyme kinetic profile for the PC activation and surface dilution inhibition: small amounts of PC in an interface enhanced PI-PLC binding to substrate-rich vesicles while high fractions of PC tended to sequester the enzyme from the bulk of its substrate leading to reduced specific activity. FCS binding profiles of mutant proteins were particularly useful in determining if a specific mutation affected a single or both phospholipid binding modes. In addition, an allosteric PC binding site was identified by fc-P-NMR and site directed spin labeling. A proposed model for PC activation suggested surface-induced dimerization of the protein. Experiments in support of the model used cysteine mutations to create covalent dimers of this PI-PLC. Two of these disulfide linked dimers, formed from W242C or S250C, exhibited higher specific activities and tighter binding to PC surfaces. In addition, single molecule total internal reflection fluorescence microscopy was used to monitor the off-rate of PI-PLC from surface tethered vesicles, providing us with a direct measure of off-rates of the protein from different composition vesicles. / Thesis (PhD) — Boston College, 2009. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.