The novel lipid raft adaptor p18 controls endosome dynamics by anchoring the MEK-ERK pathway to late endosomes.

The regulation of endosome dynamics is crucial for fundamental cellular functions, such as nutrient intake/digestion, membrane protein cycling, cell migration and intracellular signalling. Here, we show that a novel lipid raft adaptor protein, p18, is involved in controlling endosome dynamics by anchoring the MEK1-ERK pathway to late endosomes. p18 is anchored to lipid rafts of late endosomes through its N-terminal unique region. p18(-/-) mice are embryonic lethal and have severe defects in endosome/lysosome organization and membrane protein transport in the visceral endoderm. p18(-/-) cells exhibit apparent defects in endosome dynamics through perinuclear compartment, such as aberrant distribution and/or processing of lysosomes and impaired cycling of Rab11-positive recycling endosomes. p18 specifically binds to the p14-MP1 complex, a scaffold for MEK1. Loss of p18 function excludes the p14-MP1 complex from late endosomes, resulting in a downregulation of the MEK-ERK activity. These results indicate that the lipid raft adaptor p18 is essential for anchoring the MEK-ERK pathway to late endosomes, and shed new light on a role of endosomal MEK-ERK pathway in controlling endosome dynamics.

Role of Src family tyrosine kinases in the down-regulation of epidermal growth factor signaling in PC12 cells.

Src family tyrosine kinases (SFKs) play pivotal roles as molecular switches for various intracellular signaling pathways. SFKs have been implicated in epidermal growth factor (EGF) signaling, although their precise mechanisms of action in this pathway remain elusive. To address this issue, we focused on a membrane microdomain, lipid rafts, where SFKs are enriched. In PC12 cells, the EGF receptor (EGFR) is constitutively concentrated in lipid rafts, and further accumulation takes place upon EGF stimulation, followed by activation of SFKs, especially Src and Yes. Inhibition of SFK or disruption of lipid raft function causes EGF-induced neurite extension of PC12 cells. These effects are accompanied by an extended duration of Erk1/2 activation and are suppressed by a MEK inhibitor. In Csk(-/-) fibroblasts, suppression of SFK results in prolonged EGF-induced activation of Erk1/2, with concomitant suppression of EGFR degradation. Furthermore, analysis of the behavior of labeled EGF in PC12 cells reveals that suppression of SFK activity attenuates the rate of clustering of activated EGFR on the membrane. These results suggest that SFK activity in lipid rafts is required to facilitate the down-regulation of EGF signaling, by regulating the clustering of activated EGFR on the membrane in PC12 cells.