Signaling from plant endosomes: compartments with something to say!

It is well established in animal systems that compartmentalization of signaling receptors within the endocytic pathway contributes to signaling specificity and regulation (Miaczynska et al. 2004; Fischer et al. 2006). Multiple examples have been reported in vertebrates that demonstrate this concept. Epidermal growth factor receptor (EGFR) is found at the plasma membrane, but when clathrin-mediated endocytosis is impaired, downstream signaling components such as mitogen-activated protein kinases (MAPKs) have reduced activity (Viera et al. 1996). In endosomes, EGFR has been shown to interact with other signaling components (Sorkin et al. 2000). In fact, treatment with a chemical that causes the internalization of inactive (dephosphorylated) ligand-bound receptors, followed by chemical washout, reveals that the endosomal pool of EGFRs is able to promote signaling and a biological response (Pennock and Wang 2003). As important, the plasma membrane and endosomal pools of EGFR appear to be functionally distinct in that the pools show selectivity in their association with other signaling components (Burke et al. 2001). Other examples of active receptor association with endosomes include nerve growth factor association with its receptor TrkA and phospholipase C (Grimes et al. 1996) and the G-protein-coupled β₂-adrenergic receptor (Daaka et al. 1998). The TGF-β receptor forms heteromeric complexes that undergo endocytosis and phosphorylation. This endosomal complex in turn phosphorylates and activates the transcription factor R-Smad2, which is targeted to the nucleus. A central modulator of endosomal signaling appears to be SARA (Smad anchor for receptor activation) (Tsukazaki et al. 1998), which serves as an adaptor between the TGF-β complex and R-Smad2 (Fig. 1). Endocytosis and endosomal signaling pathways have also been characterized in Drosophila, where they are critical for development via the establishment of morphogen gradients and signaling involving endosomal complexes. In Drosphila, the TGF-β-like morphogen Decapentaplegic (Dpp) is involved in wing disc …