An alpaca single-domain antibody blocks filopodia formation by obstructing L-plastin-mediated F-actin bundling

L-plastin, a conserved modular F-actin bundling protein, is ectopically expressed in tumor cells and contributes to cell malignancy and invasion. The underlying molecular mechanisms involved remain unclear, in part, because specific inhibitors of L-plastin are lacking. We used recombinant alpaca-derived L-plastin single-domain antibodies (nanobodies) as effector of L-plastin function in cells. Key findings were compared with L-plastin down-regulation by RNAi. We show that nanobodies strongly interact with L-plastin by targeting discrete conformational epitopes with nanomolar affinity. A nanobody that selectively interacts with the tandem ABDs in L-plastin completely inhibits F-actin bundling at equimolar ratios, in contrast to a control green fluorescent protein (GFP) nanobody. This knockout nanobody inhibits filopodia formation, motility, and invasion when expressed in PC-3 cells. L-plastin RNA interference showed no significant effect on filopodial integrity and only marginally restrained the motile properties of cells. L-plastin nanobodies uniquely expose a fundamental role for this protein in filopodia formation and cell migration. Therefore, these molecules represent a potent instrument to ablate functions of structural proteins without manipulating gene expression. In addition, we show that they can be instrumental in uncovering new functions of proteins that remain obscured by RNAi.-Delanote, V., Vanloo, B., Catillon, M., Friederich, E., Vandekerckhove, J., Gettemans, J. An alpaca singledomain antibody blocks filopodia formation by obstructing L-plastin-mediated F-actin bundling. FASEB J. 24, 105-118(2010). www.fasebj.org