Molecular Mechanisms of Phospholipase Cβ Regulation

Abstract

Phospholipase C β (PLCβ) enzymes hydrolyze phosphatidylinositol‐4,5‐bisphosphate to produce the second messengers inositol‐1,4,5‐triphosphate and diacylglycerol. PLCβ enzymes are autoinhibited by an X—Y linker region within catalytic core that occludes the active site and a helix from within the C‐terminal extension. Protease protection assays, site‐directed mutagenesis, biochemical assays, and X‐ray crystallography were used to investigate the molecular basis of autoinhibition by these regulatory elements and define their respective roles. Distinct regions within the X—Y linker were shown to have specific roles in regulating access to the PLCβ active site, with the conserved acidic stretch within the linker being the most autoinhibitory element. Crystal structures of PLCβ3 variants lacking the acidic stretch have disordered X—Y linkers and bind IP3. The C‐terminal inhibitory helix acts independently of the linker and prevents full activation of the enzyme in the absence of Gαq. Our results demonstrate that PLCβ activation is a multi‐step process requiring independent molecular events induced by membrane association and the binding of activating proteins.