Figure 2.

Actin-binding proteins modulate PLC activities with or without actin filaments. (A) A model based on in vitro reaction mixture containing phospholipid bilayer with PIP2, purified EGFR, PLC-γ1, ATP, and profilin. Profilin binds to PIP2, and thereby prevents PLC-γ1 from accessing its substrate PIP2. When PLC-γ1 is phosphorylated by the activation of EGFR, the inhibition by profilin is overcome and PIP2 is hydrolyzed to produce InsP₃ and DAG (not shown). (B) Likewise, villin masks PIP2 from PLC-γ, but phosphorylated villin (by Src) loses its binding affinity to PIP2. Then, PLC has access to PIP2 to produce InsP₃. (C) The focal adhesion complex in human mammary epithelial (HME) cells comprises PLC-γ, integrin β1, α-actinin, F-actin, and EGFR. Binding of EGF to EGFR induces EGF receptor dimerization, phosphorylation of PLC-γ, and increased α-actinin loading on PIP2. Here, α-actinin’s binding to PIP2 facilitates its hydrolysis by PLC-γ, and consequently the production of InsP₃ doubles. Disruption of the actin cytoskeleton with Cytochalasin-D (CYT-D) dissociates α-actinin from PIP2, and the EGFR-associated PLC-γ is unable to cleave the PIP2 not bound to α-actinin. (D) Actin filament-associated protein (AFAP) in rat fetal lung cells presents Src to PLC-γ1 via actin filaments so as to activate the enzyme. As a result, InsP₃ production is increased, but the effect is inhibited by CYT-D.