Flotillin-1 is essential for the activation of EGF receptor

Membrane microdomains have been shown to be involved in the activation and endocytosis of transmembrane receptor tyrosine kinases, such as the epidermal growth factor receptor (EGFR). Proteins of the flotillin family (flotillin-1 and -2) have been demonstrated to participate in the formation of those microdomains. We could verify that both flotillins colocalize with the EGFR at the plasma membrane as well as in late endosomal compartments. In addition, we found that flotillin-1 is essential for an efficient activation of the EGFR and its downstream mitogen activated protein kinase (MAPK) signalling cascade. Once flotillin-1 is depleted in mammalian cells by a siRNA-mediated knockdown, the phosphorylation of EGFR at specific tyrosine residues, as well as the activation of the downstream MAPK cascade significantly decreases. While flotillin-1 appears to influence the clustering behavior of EGFR at the plasma membrane prior to its internalization, the early trafficking of EGFR is not influenced by the absence of flotillin-1. Additionally we could show that EGFR and both flotillins form a constitutive complex that seems to be independent of EGFR kinase activity. Taken together, these results suggest a novel role for flotillin-1 in the membrane microdomain-mediated activation of receptor tyrosine kinases as well as their downstream signalling pathways.