Figure 1.

Structure and signaling machinery of the Met receptor. Met is an α/β heterodimer formed by a completely extracellular α subunit and a transmembrane β subunit that contains the tyrosine kinase activity. The extracellular region of Met encompasses a large Sema domain - which spans the α subunit and part of the β subunit, folding into a β-propeller structure - a cysteine-rich domain and four repeats of an unusual type of immunoglobulin-like domain. The intracellular portion of Met includes the kinase domain - with two catalytic tyrosines (Tyr1234 and Tyr1235) that enhance the receptor enzymatic activity following transphosphorylation - and key tyrosine residues in the carboxy-terminal tail (Tyr1349 and Tyr1356). Phosphorylation of these distal tyrosines creates docking sites for several interactors, many of which are schematized here. Recruitment of these signaling effectors activates downstream pathways that together enable biological execution of the invasive growth process. The Ras-Erk/mitogen-activated protein kinase (MAPK) cascade launches a program of transcriptional modulation that involves changes in the expression of cell-cycle regulators and extracellular matrix proteinases. Ras also stimulates the Rac1/Cdc42-PAK pathway, which, together with the Gab1-Crk-C3G-Rap1 axis, regulates the activity of cytoskeletal and adhesion molecules such as cadherins, Arp, N-WASP, paxillin, integrins and focal adhesion kinase. The Gab1-phosphoinositide 3-kinase (PI3K)-Akt pathway encourages cell survival by inhibiting the proapoptotic molecule Bad and the apopototic effector caspase 9.