REGULATION OF THE CORTICAL ACTIN CYTOSKELETON IN BUDDING YEAST BY TWINFILIN, A UBIQUITOUS ACTIN MONOMER-SEQUESTERING PROTEIN

Here we describe the identification of a novel 37-kD actin monomer bin ding protein in budding yeast. This protein, which we named twinfilin, is composed of two cofilin-like regions. In our sequence database sea rches we also identified human, mouse, and Caenorhabditis elegans homo logues of yeast twinfilin, suggesting that twinfilins form an evolutio narily conserved family of actin-binding proteins. Purified recombinan t twinfilin prevents actin filament assembly by forming a 1:1 complex with actin monomers, and inhibits the nucleotide exchange reaction of actin monomers, Despite the sequence homology with the actin filament depolymerizing cofilin/actin-depolymerizing factor (ADF) proteins, our data suggests that twinfilin does not induce actin filament depolymer ization. In yeast cells, a green fluorescent protein (GFP)-twinfilin f usion protein localizes primarily to cytoplasm, but also to cortical a ctin patches. Overexpression of the twinfilin gene (TWF1) results in d epolarization of the cortical actin patches. A twf1 null mutation appe ars to result in increased assembly of cortical actin structures and i s synthetically lethal with the yeast cofilin mutant cof1-22, shown pr eviously to cause pronounced reduction in turnover of cortical actin f ilaments. Taken together, these results demonstrate that twinfilin is a novel, highly conserved actin monomer-sequestering protein involved in regulation of the cortical actin cytoskeleton.