Fig. 1. The domain structure of twinfilin. (A) Twinfilin is composed of two domains homologous to the ADF/cofilin proteins (ADF-H domain), separated by a short, 30 amino acid, linker and followed by a C-terminal tail region. The ADF-H domains are approximately 20% identical to each other, and they both appear to be involved in interactions with actin monomers. (B) A phylogenetic tree of twinfilin ADF-H domains from different species as produced by Clustal-X software. The N-terminal and C-terminal ADF-H domains of twinfilins from different species form two independent branches in the tree, demonstrating that each one of the ADF-H domains between different species are more homologous than the N- and C-terminal ADF-H domains within any particular twinfilin. This suggests that the two ADF-H domains of twinfilin were already present before the divergence of fungal and animal lineages. Databases and accession numbers for twinfilin sequences are: S. pombe twinfilin (EMBL, AL034490); S. cerevisiae twinfilin (EMBL, Z72865); C. elegans twinfilin (EMBL, U46668); D. melanogaster twinfilin (EMBL, AE003703); H. sapiens twinfilin-1 (PIR, A55922); H. sapiens twinfilin-2 (EMBL, Y17169); M. musculus twinfilin-1 (GenBank, U82324); and M. musculus twinfilin-2 (EMBL, Y17808).