Mouse A6/twinfilin is an actin monomer-binding protein that localizes to the regions of rapid actin dynamics

In our database searches, we have identified mammalian homologues of yeast actin-binding protein, twin-filin, Previous studies suggested that these ma mmalian proteins were tyrosine kinases, and therefore they were named A6 pr otein tyrosine kinase, In contrast to these earlier studies, we did not fin d any tyrosine kinase activity in our recombinant protein, However, biochem ical analysis showed that mouse A6/twinfilin forms a complex with actin mon omer and prevents actin filament assembly in vitro. A6/twinfilin mRNA is ex pressed in most adult tissues but not in skeletal muscle and spleen. In mou se cells, A6/twinfilin protein is concentrated to the areas at the cell cor tex which overlap with G-actin-rich actin structures. A6/twinfilin also col ocalizes with the activated forms of small GTPases Rad and Cdc42 to membran e ruffles and to cell-cell contacts, respectively. Furthermore, expression of the activated Rac1(V12) in NIH 3T3 cells leads to an increased A6/twinfi lin localization to nucleus and cell cortex, whereas a dominant negative fo rm of Rac1(V12,N17) induces A6/ twinfilin localization to cytoplasm, Taken together, these studies show that mouse A6/twinfilin is an actin monomer-bi nding protein whose localization to cortical G-actin-rich structures may be regulated by the small GTPase Rac1.