Allosteric activation of phosphatidylinositol-specific phospholipase C: Specific phospholipid binding anchors the enzyme to the interface

Phosphatidylinositol-specific phospholipase C (PI-PLC) from Bacillus thuringiensis exhibits 'interfacial activation' toward the water-soluble substrate myo-inositol 1,2-(cyclic)phosphate [Zhou et al, (1997) Biochemistry 36, 347-355]. The activation of PI-PLC enzyme is optimal with PC or PE interfaces. NMR experiments (TRNOE and P-31 line width analyses) were carried out to investigate the interaction of PI-PLC with activator amphiphiles. These studies showed that the enzyme had high affinity for phosphatidylcholine (or PE) molecules with dissociation constants of 0.5 and 0.3 mM for diC(6)PC and diC(7)PC, respectively. TRNOE cross-peaks of bound PC were confirmed to represent intramolecular relaxation pathways using partially perdeuterated PC molecules consistent with a single molecule binding tightly. The large activation by a PC interface can be explained by a single PC molecule binding specifically to PI-PLC and anchoring the enzyme-lipid complex to the interface. Other interfaces, such as micellar diC(8)PS, can activate PI-PLC about 2-3-fold; however, the monomers of these detergents showed little affinity for the enzyme as measured by TRNOE or P-31 NMR Line widths. The 3.6-fold activation produced by polymerized vesicles of 1,2-bis[12-(lipoyloxy)dodecanoyl]-sn-glycero-3-phosphocholine (compared to the 15-fold activation generated by nonpolymerized PC vesicles) was comparable to the nonspecific activation of other detergents, This confirmed that single-PC molecule binding was allosteric and anchored the enzyme in the interface. The conformation of interfacially activated enzyme is discussed in term of the stabilization of a critical surface loop and helix B observed with weak intensity in the X-ray crystal structure.