Spatial Coordination of Actin Polymerization and ILK-Akt2 Activity during Endothelial Cell Migration

Eukaryotic cell migration proceeds by cycles of protrusion, adhesion, and contraction, regulated by actin polymerization, focal adhesion assembly, and matrix degradation. However, mechanisms coordinating these processes remain largely unknown. Here, we show that local regulation of thymosin-beta 4 (T beta 4) binding to actin monomer (G-actin) coordinates actin polymerization with metalloproteinase synthesis to promote endothelial cell motility. In particular and quite unexpectedly, FRET analysis reveals diminished interaction between T beta 4 and G-actin at the cell leading edge despite their colocalization there. Profilin-dependent dissociation of G-actin-T beta 4 complexes simultaneously liberates actin for filament assembly and facilitates T beta 4 binding to integrin-linked kinase (ILK) in the lamellipodia. T beta 4-ILK complexes then recruit and activate Akt2, resulting in matrix metalloproteinase-2 production. Thus, the actin-T beta 4 complex constitutes a latent coordinating center for cell migratory behavior, allowing profilin to initiate a cascade of events at the leading edge that couples actin polymerization to matrix degradation.