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In our database searches, we have identified mammalian homologues of yeast
actin-binding protein, twinfilin. Previous studies suggested that these
mammalian proteins were tyrosine kinases, and therefore they were named A6
protein tyrosine kinase. In contrast to these earlier studies, we did not find
any tyrosine kinase activity in our recombinant protein. However, biochemical
analysis showed that mouse A6/twinfilin forms a complex with actin monomer and
prevents actin filament assembly in vitro. A6/twinfilin mRNA is expressed in
most adult tissues but not in skeletal muscle and spleen. In mouse cells,
A6/twinfilin protein is concentrated to the areas at the cell cortex which
overlap with G-actin-rich actin structures. A6/twinfilin also colocalizes with
the activated forms of small GTPases Rac1 and Cdc42 to membrane ruffles and to
cell-cell contacts, respectively. Furthermore, expression of the activated
Rac1(V12) in NIH 3T3 cells leads to an increased A6/twinfilin localization to
nucleus and cell cortex, whereas a dominant negative form of Rac1(V12,N17)
induces A6/twinfilin localization to cytoplasm. Taken together, these studies
show that mouse A6/twinfilin is an actin monomer-binding protein whose
localization to cortical G-actin-rich structures may be regulated by the small
GTPase Rac1.
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