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Molecular and Cellular Biology, March 2000, p. 1772-1783, Vol. 20, No. 5
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

Mouse A6/Twinfilin Is an Actin Monomer-Binding Protein That Localizes to the Regions of Rapid Actin Dynamics

Maria Vartiainen, Pauli J. Ojala, Petri Auvinen, Johan Peränen, and Pekka Lappalainen*

Institute of Biotechnology, University of Helsinki, 00014 Helsinki, Finland

Received 4 August 1999/Returned for modification 17 October 1999/Accepted 14 November 1999

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.

* Corresponding author. Mailing address: Institute of Biotechnology, University of Helsinki, P.O. Box 56, 00014 Helsinki, Finland. Phone: 358-9-19159499. Fax: 358-9-19159366. E-mail: pekka.lappalainen{at}helsinki.fi.

Molecular and Cellular Biology, March 2000, p. 1772-1783, Vol. 20, No. 5
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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