Mechanism of control of F-actin cortex architecture by SWAP-70

Abstract

F-actin binding and bundling are crucial to a plethora of cell processes including morphogenesis, migration, adhesion and many others. SWAP-70 was recently described as an in vitro F-actin binding and bundling protein. Using FCCS measurements with purified recombinant SWAP-70 confirmed that it forms stable oligomers facilitating F-actin bundling. It remained unclear how SWAP-70's oligomerization and F-actin binding are controlled in living cells. We addressed this by biophysical approaches including seFRET, FACS-FRET and FLIM-FRET. PIP3-mediated association with the cytoplasmic membrane and non-phosphorylated Y426 are required for SWAP-70 to dimerize and to bind F-actin. The dimerization region was identified near the C-terminus where R546 is required for dimerization and thus F-actin bundling. The in vitro and in vivo data presented here reveal the functional relationship between SWAP-70's cytoplasm-to-membrane translocation, dimerization, F-actin binding and bundling, and illustrate SWAP-70 as a finely controlled modulator of membrane-proximal F-actin dynamics.