Clathrin and TOM1L1 regulate epidermal growth factor receptor signaling at the plasma membrane

Abstract

Epidermal growth factor (EGF) receptor (EGFR) controls many aspects of cell physiology via the activation of intracellular signaling pathways. Aberrant EGFR signaling and overexpression of EGFR is a characteristic of various cancer types. Ligandbound EGFR elicits phosphorylation of Gab1, which activates phosphatidylinositol-3-kinase (PI3K), which in turn leads to Akt phosphorylation and activation. Akt is a central regulator of cell survival and metabolism. Rapidly upon binding EGF, the receptor is recruited to clathrin-coated pits (CCPs), eventually leading to EGFR endocytosis. As such, receptor-proximal EGFR signaling occurs while the receptor is enriched with CCPs at the plasma membrane. We have recently found that clathrin, but not receptor endocytosis, is required for EGF-stimulated phosphorylation of Akt, yet how clathrin controls EGFR signaling at the plasma membrane remains poorly understood. Clathrin interacts with and recruits numerous proteins to the plasma membrane. To examine how specific clathrin-interacting proteins may underlie the requirement for clathrin in EGFR signaling, I have examined the role of TOM1L1, which interacts with clathrin, EGFR and Src-family kinases (SFKs) through distinct domains. SiRNA gene silencing of TOM1L1 impairs EGF-stimulated Akt phosphorylation, as did expression of a dominant-interfering mutant of TOM1L1 deficient in clathrin binding. Using fluorescent labeling of clathrin, TOM1L1 and other signaling intermediates coupled to total internal reflection fluorescence microscopy; I found that CCPs are enriched in TOM1L1 and phosphorylated Gab1 upon EGF stimulation. These findings indicate that the clathrin-binding adaptor TOM1L1 regulates EGF-stimulated Akt phosphorylation, thus revealing a novel molecular mechanism of regulation of EGFR signaling at the plasma membrane.