Mechanism of Depolymerization and Severing of Actin Filaments and Its Significance in Cytoskeletal Dynamics

Shoichiro Ono    Department of Pathology, Emory University, Atlanta, Georgia 30322

I Introduction

Actin is one of the major cytoskeletal components in most eukaryotic cells and supports not only the structural integrity of the cell, but also dynamic cellular events such as cell movement, cytokinesis, and even gene expression. Actin is a conserved 42‐kDa protein that can be spontaneously polymerized into a polar filament in vitro under physiological conditions. Actin polymerizes only when a concentration of monomer (or globular [G‐] actin) is higher than the critical concentration (Oosawa, 2001; Sheterline et al., 1998). However, the critical concentration at the minus (or pointed) end (∼0.6 μM) is higher than that at the plus (or barbed) end (∼0.1 μM). As a result, at a steady state, actin subunits are constantly depolymerized from the minus ends and added to the plus ends. This phenomenon is called actin treadmilling (Cleveland, 1982; Neuhaus et al., 1983). Actin treadmilling occurs in the actin filaments in living cells (Wang, 1985) and is considered to be one of the major mechanisms of actin filament turnover in vivo (Carlier et al., 2003; Pantaloni et al., 2001; Pollard and Borisy, 2003). However, the rates of actin turnover in living cells are 20–100 times faster than those of purified actin in vitro (Theriot and Mitchison, 1991; Wang, 1985). This is primarily because the off rate of actin subunits from the minus end is slow and becomes the rate‐limiting step in treadmilling, whereas association of actin monomers with exposed plus ends or de novo nucleation sites is relatively fast (Pollard, 1986; Wegner and Isenberg, 1983). To accelerate actin turnover, depolymerization of actin from the minus ends needs to be enhanced, or the number of filament ends needs to be increased by filament severing. Thus, depolymerization and severing of actin filaments are critical for regulating actin cytoskeletal dynamics, as well as actin nucleation, filament capping, nucleotide exchange, and monomer sequestration (Carlier et al., 2003; Pantaloni et al., 2001; Pollard and Borisy, 2003; Pollard et al., 2001). This review covers the mechanism of depolymerization and severing of actin filaments, which is currently known to be mediated by two major classes of actin‐binding proteins: gelsolin (Fig. 1A–C) and actin‐depolymerizing factor (ADF)/cofilin (Fig. 1D). In addition, actin‐interacting protein 1 (AIP1) is a protein that promotes actin filament disassembly in cooperation with ADF/cofilin (Fig. 1F). Actin‐severing or ‐depolymerizing activity has also been reported for twinfilin (Fig. 1E), coronin (Fig. 1G), a formin‐related protein (Fig. 1H), cyclase‐associated protein (Fig. 1I), and DNase I (Fig. 1J).

II Proteins That Depolymerize and/or Sever Actin Filaments

A Gelsolin and Gelsolin‐Related Proteins

1 Gelsolin Family

The gelsolin family of actin‐binding proteins is one of the major classes of actin‐severing proteins (Kwiatkowski, 1999; McGough et al., 2003; Silacci et al., 2004; Sun et al., 1999). Gelsolin was originally discovered in macrophages as a factor inducing the gel–sol transformation of actin filaments in a calcium‐dependent manner (Yin and Stossel, 1979). Gelsolin and gelsolin‐related proteins are widely present in metazoan species but are not found in yeast. Further biochemical characterization of gelsolin revealed that gelsolin binds to calcium, severs actin filaments, and caps the plus ends (Yin and Stossel, 1980; Yin et al., 1980, 1981b).

A number of proteins with similar calcium‐dependent actin‐severing activity were discovered in the early to mid‐1980s in various sources and designated by different names (Maruyama, 1986; Pollard and Cooper, 1986; Stossel et al., 1985). These include fragmin from Physarum (slime mold) (Hasegawa et al., 1980) (also known as actin‐modulatory protein [Hinssen, 1981a,b]), villin from intestinal brush border (Bretscher and Weber, 1980; Craig and Powell, 1980; Glenney et al., 1981b), severin from Dictyostelium (Brown et al., 1982; Yamamoto et al., 1982), brevin from rabbit serum (Harris and Schwartz, 1981) (also known as plasma F‐actin‐depolymerizing factor [Thorstensson et al., 1982]), a 45‐kDa protein (Coluccio et al., 1986; Hosoya and Mabuchi, 1984; Ohnuma and Mabuchi, 1986; Wang and Spudich, 1984) and a 100‐kDa protein from sea urchin egg (Hosoya et al., 1986), and 88‐ to 93‐kDa proteins from various vertebrate tissues (Ebisawa and Nonomura, 1985; Nishida et al., 1983; Petrucci et al., 1983; Wang and Bryan, 1981). Plants also have gelsolin‐like actin‐severing proteins (Fan et al., 2004; Huang et al., 2004; Yamashiro et al., 2001). However, a villin‐like protein in Arabidopsis does not sever actin filaments (Huang et al., 2005).

As discussed in greater detail later (Section II.A.2), determination of the primary structure of gelsolin revealed that it has six homologous domains of 100–120 amino acids, which are termed gelsolin‐like (G) domains 1–6 (Fig. 1B) (Kwiatkowski et al., 1986). Interestingly, cytoplasmic and secreted plasma isoforms of gelsolin (brevin or plasma F‐actin‐depolymerizing factor) are encoded by a single gene (Kwiatkowski et al., 1986, 1988). Subsequent sequence analyses of other calcium‐dependent actin‐severing proteins showed that the 40‐ to 45‐kDa proteins including fragmin (Ampe and Vandekerckhove, 1987) and severin (Andre et al., 1988; Schleicher et al., 1988) have three G domains, whereas other 85‐ to 100‐kDa proteins have six G domains (Fig. 1A–C) (Arpin et al., 1988; Bazari et al., 1988; Way and Weeds, 1988). Villin has an 8.5‐kDa C‐terminal headpiece domain in addition to six G domains (Fig. 1C) (Arpin et al., 1988; Bazari et al., 1988). The villin headpiece domain has calcium‐independent F‐actin‐binding activity and confers to villin its unique actin‐bundling activity (Glenney et al., 1981a). Vertebrates also have two additional gelsolin isoforms: adseverin (Ashino et al., 1987; Maekawa et al., 1989; Nakamura et al., 1994) (also known as scinderin [Rodriguez Del Castillo et al., 1990; Trifaro et al., 2000]) and gelsolin‐3 (Vouyiouklis and Brophy, 1997) and a villin isoform, advillin (Marks et al., 1998). A splice variant of adseverin that has only five G domains is expressed in blood cells (Robbens et al., 1998).

Other unconventional gelsolin‐related proteins have also been reported. CapG (also known as macrophage‐capping protein [Dabiri et al., 1992; Southwick and DiNubile, 1986], Mbh1 [Prendergast and Ziff, 1991], or gCap39 [Yu et al., 1990]) has three G domains and caps the plus ends of actin...