Analysis of the myosinII-responsive focal adhesion proteome reveals a role for β-Pix in negative regulation of focal adhesion maturation

Focal adhesions (FAs) undergo myosinII-mediated maturation wherein they grow and change composition to modulate integrin signaling for cell migration, growth and differentiation. To determine how FA composition is modulated by myosinII activity, we performed proteomic analysis of isolated FAs and compared protein abundance in FAs from cells with and without myosinII inhibition. We identified FA 905 proteins, 459 of which changed in FA abundance with myosinII inhibition, defining the myosinII-responsive FA proteome. FA abundance of 73% of proteins was enhanced by contractility, including those involved in Rho-mediated FA maturation and endocytosis- and calpain-dependent FA disassembly. 27% of proteins, including those involved in Rac-mediated lamellipodial protrusion, were enriched in FA by myosinII inhibition, establishing for the first time negative regulation of FA protein recruitment by contractility. We focused on the Rac guanine nucleotide exchange factor, β-PIX, documenting its role in negative regulation of FA maturation and promotion of lamellipodial protrusion, FA turnover to drive cell migration.