The eps8 Family of Proteins Links Growth Factor Stimulation to Actin Reorganization Generating Functional Redundancy in the Ras/Rac Pathway

Abstract

Sos-1, a guanine nucleotide exchange factor (GEF), eps8 and Abi1, two signaling proteins, and the lipid kinase phosphoinositide 3-kinase (PI3-K), assemble in a multimolecular complex required for Rac activation leading to actin cytoskeletal remodeling. Consistently, eps8 ؊/؊ fibroblasts fail to form membrane ruffles in response to growth factor stimulation. Surprisingly, eps8 null mice are healthy, fertile, and display no overt phenotype, suggesting the existence of functional redundancy within this pathway. Here, we describe the identification and characterization of a family of eps8-related proteins, comprising three novel gene products, named eps8L1, eps8L2, and eps8L3. Eps8Ls display collinear topology and 27-42% identity to eps8. Similarly to eps8, eps8Ls interact with Abi1 and Sos-1; however, only eps8L1 and eps8L2 activate the Rac-GEF activity of Sos-1, and bind to actin in vivo. Consistently, eps8L1 and eps8L2, but not eps8L3, localize to PDGF-induced, F-actin-rich ruffles and restore receptor tyrosine kinase (RTK)-mediated actin remodeling when expressed in eps8 ؊/؊ fibroblasts. Thus, the eps8Ls define a novel family of proteins responsible for functional redundancy in the RTK-activated signaling pathway leading to actin remodeling. Finally, the patterns of expression of eps8 and eps8L2 in mice are remarkably overlapping, thus providing a likely explanation for the lack of overt phenotype in eps8 null mice.

Progressive hearing loss and gradual deterioration of sensory hair bundles in the ears of mice lacking the actin-binding protein Eps8L2

“…S2). The distribution of Eps8L2 overlapped with that of Eps8 (15,16) as previously suggested in other systems (17). However, the spatiotemporal expression patterns in cochlear hair cells were different.…”

“…Eps8L2 is structurally the most closely related to Eps8 and shares very similar expression patterns, suggesting some level of redundancy in most tissues (17). We have shown that Eps8L2 is expressed at the apical ends of hair cells and at the tips of mechanosensory stereocilia in mammalian inner ear hair cells.…”

“…1E), which are short and show a relatively similar height among rows (15,16). Despite the similar expression profile and high homology of the two genes (17), there is very little functional redundancy between the two proteins in cochlear hair cells. Although the localization of Eps8 requires the motor protein myosin XVa (15,16), we found that in shaker-2 mice, which are deficient in myosin XVa (18), Eps8L2 was still present at the stereocilia tip (Fig.…”

“…In mammals, the Eps8 family includes three Eps8-related proteins named Eps8L1-L3. Eps8L1 and L2 also show actin binding activity, displaying similar domain organization and 27-42% identity to Eps8 (17). Eps8L2 shares a common modular protein structure to Eps8, indicating a possible overlap in function (17).…”

“…Eps8L1 and L2 also show actin binding activity, displaying similar domain organization and 27-42% identity to Eps8 (17). Eps8L2 shares a common modular protein structure to Eps8, indicating a possible overlap in function (17). However, despite the identification of Eps8L2 several years ago, its function remains unknown.…”