Importance of spatial activation of Cdc42 and Rac small G proteins by frabin for microspike formation in MDCK cells

Background Frabin is an actin filament (F-actin)-binding protein that shows GDP/GTP exchange activity for Cdc42 small G protein (Cdc42). Frabin furthermore induces indirect activation of Rac small G protein (Rac) in intact cells. We have recently shown that in nonepithelial cells, frabin induces the formation of both filopodia- and lamellipodia-like processes through the activation of Cdc42 and Rac, respectively. In epithelial cells such as MDCK cells, Cdc42 and Rac regulate cell-cell adherens junctions (AJs) via the accumulation of F-actin and E-cadherin, although neither Cdc42 nor Rac induces the formation of filopodia or lamellipodia. In this study, we have examined the effects of frabin on the reorganization of the actin cytoskeleton in MDCK cells. Results Frabin induces the formation of microspikes at the basal area of the lateral membranes through the activation of Cdc42 and Rac in MDCK cells, although a dominant active mutant of Cdc42 or Rac alone, or both, did not induce the formation of microspikes. Furthermore, frabin weakly increased the accumulation of F-actin and E-cadherin at cell-cell AJs and the formation of stress fibres through the activation of Cdc42 and Rac, under conditions where the dominant active mutant of Cdc42 or Rac markedly showed these effects. The Cdc42- and Rac-induced formation of stress fibres was dependent on the activation of Rho small G protein. Conclusion These results indicate that the frabin-dependent spatial activation of Cdc42 and Rac is important for the formation of microspikes.