Dynamics of Fascin in FilopodiaPublic Deposited
Filopodia play a central role in proper cell navigation; they extend from the cell surface and scan the local environment to guide the cell to its proper destination. These dynamic events require precise mechanisms for the turnover of key molecules which determine the morphology of filopodia. As of yet, such mechanisms are undefined and compose a major gap in our understanding of filopodia function. Therefore, we investigated the properties and interaction of actin and the actin cross-linking protein, fascin, which are essential for filopodia formation. Using expression of phospho-mimetic mutants, we determined that actin bundling in filopodia is dependent upon fascin dephosphorylation at serine 39 while phospho-fascin is predominantly unbound and freely diffusing in filopodia. Fluorescence recovery after photobleaching (FRAP) analysis revealed that fascin exchanges rapidly in filopodia and that this exchange was recapitulated in vitro, indicating that its dynamic behavior is intrinsic to the fascin cross-linker. A computational model indicated that simple diffusion from a fascin pool in the cell body was not sufficient to keep up with observed rates of filopodial elongation. Rather, fascin exchange along the length of the filopodium enables the ready supply of fascin to growing filopodial tips. Altogether, these results suggest that filopodia have selected for dynamic cross-linking to form adequately stiff bundles and allow for remodeling.