Insights into the Role of the Membrane on Phospholipase C Beta and G Alpha Q-Mediated Activation

Phospholipase Cβ (PLCβ) cleaves phosphatidylinositol-4,5-bisphosphate (PIP₂) into the second messengers inositol-1,4,5-triphosphate (IP₃) and diacylglycerol (DAG). IP₃ increases intracellular Ca²⁺, while DAG remains in the membrane, and together with increased Ca²⁺, activates protein kinase C (PKC). PLCβ has low basal activity but is activated following stimulation of G_i- and G_q-coupled receptors through direct interactions with Gα_q and Gβγ. PLCβ is essential for normal cardiomyocyte and vascular smooth muscle function and regulates cell proliferation, survival, migration, and differentiation. However, increased PLCβ activity and expression results in arrhythmias, hypertrophy, and heart failure. PLCβ must interact with the cell membrane for its activity. While heterotrimeric G proteins stimulate PLCβ, they are insufficient for full activation, suggesting the membrane itself contributes to increased lipid hydrolysis, potentially via interfacial activation. However, how the composition of the membrane and its resulting properties, such as surface charge, contribute to adsorption and interfacial activation is not well-established. Furthermore, whether or how interfacial activation also impacts other regulatory elements in PLCβ and Gα_q-dependent activation is unknown. Using an innovative combination of atomic force microscopy on compressed lipid monolayers and biochemical assays, we are beginning to understand how the membrane itself, PLCβ autoinhibitory elements and Gα_q regulate PLCβ activation. These studies provide the first structure-based approach to understanding how the cell membrane regulates the activity of this essential effector enzyme.