The Three Mouse Actin-depolymerizing Factor/Cofilins Evolved to Fulfill Cell-Type–specific Requirements for Actin Dynamics

Abstract

Actin-depolymerizing factor (ADF)/cofilins are essential regulators of actin filament turnover. Several ADF/cofilin isoforms are found in multicellular organisms, but their biological differences have remained unclear. Herein, we show that three ADF/cofilins exist in mouse and most likely in all other mammalian species. Northern blot and in situ hybridization analyses demonstrate that cofilin-1 is expressed in most cell types of embryos and adult mice. Cofilin-2 is expressed in muscle cells and ADF is restricted to epithelia and endothelia. Although the three mouse ADF/cofilins do not show actin isoform specificity, they all depolymerize platelet actin filaments more efficiently than muscle actin. Furthermore, these ADF/cofilins are biochemically different. The epithelial-specific ADF is the most efficient in turning over actin filaments and promotes a stronger pH-dependent actin filament disassembly than the two other isoforms. The musclespecific cofilin-2 has a weaker actin filament depolymerization activity and displays a 5-10-fold higher affinity for ATP-actin monomers than cofilin-1 and ADF. In steady-state assays, cofilin-2 also promotes filament assembly rather than disassembly. Taken together, these data suggest that the three biochemically distinct mammalian ADF/cofilin isoforms evolved to fulfill specific requirements for actin filament dynamics in different cell types. INTRODUCTIONActin is a highly conserved and ubiquitous protein found in probably all eukaryotic cells. In muscle cells actin filaments assemble into highly ordered, relatively stable structures that together with myosin form muscle cells' basic contractile apparatus. In nonmuscle cells actin filaments are highly dynamic and participate in a range of processes such as cell polarization and movement, cytokinesis, and endocytosis. The dynamics of actin filaments are tightly regulated, both spatially and temporally, by a large number of actin-binding proteins (Ayscough, 1998;Sheterline, 1998).ADF/cofilins form a family of actin monomer-and filament-binding proteins (reviewed in Bamburg, 1999), whose activities are fundamental to cells because ADF/cofilin-inactivating mutations are lethal (Moon et al., 1993;McKim et al., 1994;Gunsalus et al., 1995). ADF/cofilins localize to regions of rapid actin dynamics, such as yeast cortical actin patches, neuronal growth cones, and the leading edge and ruffling membranes of motile cells (Bamburg and Bray, 1987;Yonezawa et al., 1987;Moon et al., 1993;Nagaoka et al., 1995). They are central in the dynamics of yeast's cortical actin cytoskeleton and in Listeria actin tails (Carlier et al., 1997;Rosenblatt et al., 1997), where ADF/cofilin is among the minimal set of proteins required for motility of this intracellular pathogen (Loisel et al., 1999).ADF/cofilins' most important physiological function is to depolymerize filaments from their pointed ends, thereby increasing actin dynamics (Carlier et al., 1997). Under physiological conditions ADF/cofilins bind ADP-actin monomers and filaments with higher affi...

The Two ADF-H Domains of Twinfilin Play Functionally Distinct Roles in Interactions with Actin Monomers

“…Our results further suggest that the affinity of twinfilin for ADP-G-actin is somewhat higher than that of ADF/cofilins for ADP-G-actin, because the mean value of measurements for a variety of isoforms of ADF and cofilin is ϳ0.15 M (Blanchoin and Pollard, 1998;Ressad et al, 1998;Vartiainen et al, 2002;Yeoh et al, 2002). This, together with a relatively high cellular concentration of twinfilin, suggests that twinfilin might displace ADF/cofilin from ADP-actin (Palmgren et al, 2001).…”

The cofilin pathway in breast cancer invasion and metastasis

“…However, ADF is more sensitive to changes in pH than cofilin 61 . In addition, in the mouse and C. elegans, the non-muscle isoforms cofilin 1 and ADF are more efficient at depolymerizing F-actin in vitro than the muscle isoform of cofilin 62,63 . These findings suggest that specific cofilin isoforms may be crucial to the changes in cell shape and migration that occur during different stages of embryonic development and morphogenesis.…”

“…In general, mammals have many isoforms classified as ADFs and cofilins based on sequence homology and biological activity. All mammals are predicted to have a single ADF isoform expressed most strongly in epithelia and neurons, and two cofilin isoforms, a non-muscle type cofilin 1 expressed in most embryonic and adult cells, and a muscle-type cofilin 2 expressed almost exclusively in skeletal muscle 62 . Based on studies in various mouse cells, cofilin 1 is the predominant isoform in non-muscle cells, expressed at 6-11 times greater molar amounts than ADF 40 .…”

Active maintenance of nuclear actin by importin 9 supports transcription

“…3 A and B). To test the role of cofilin in mammalian cells, we codepleted Exp6 and cofilin-1, which is the major actin-depolymerization factor/cofilin family member in the NIH 3T3 cells used here (37,38). Cofilin-1 depletion effectively abolished the nuclear accumulation of actin caused by Exp6 silencing (Fig.…”