Dynamics of the actin-binding protein drebrin in motile cells and definition of a juxtanuclear drebrin-enriched zone.

The actin-binding protein (ABP) drebrin, isoform E2, is involved in remodelling of the actin cytoskeleton and in formation of cell processes, but its role in cell migration has not yet been investigated. Therefore, we have studied the organization of drebrin in motile cultured cells such as murine B16F1 melanoma and human SV80 fibroblast cells, using live cell confocal microscopy. In cells overexpressing DNA constructs encoding drebrin linked to EGFP, numerous long, branched cell processes were formed which slowly retracted and extended, whereas forward movement was halted. In contrast, stably transfected B16F1 cells containing drebrin-EGFP at physiological levels displayed lamellipodia and were able to migrate on laminin. Surprisingly, in such cells, drebrin was absent from anterior lamellipodia but was enriched in a specific juxtanuclear zone, the "drebrin-enriched zone" (DZ), and in the tail. In leading edges of SV80 cells, characterized by pronounced actin microspikes, drebrin was specifically enriched along posterior portions of the microspikes, together with tropomyosin. Drebrin knock-down by small interfering RNAs did not impair movements of SV80 cells. Our results confirm the role of drebrin E2 in the formation of branching processes and further indicate that during cell migration, the protein contributes to retraction of the cell body and the tail but not to lamellipodia formation. In particular, the novel, sizable juxtanuclear DZ structure will have to be characterized in future experiments with respect to its molecular assembly and cell biological functions.