The actin filament severing protein actophorin promotes the formation of rigid bundles of actin filaments crosslinked with alpha-actinin.

Abstract

Abstract. The actin filament severing protein, Acanthamoeba actophorin, decreases the viscosity of actin filaments, but increases the stiffness and viscosity of mixtures of actin filaments and the crosslinking protein a-actinin . The explanation of this paradox is that in the presence of both the severing protein and crosslinker the actin filaments aggregate into an interlocking meshwork of bundles large enough to be visualized by light microscopy. The size of these bundles depends on the size of the containing vessel . The actin filaments in these bundles are tightly packed in some areas while in others they are more disperse. The bundles form a continuous reticulum that fills the container, since the filaments from a particular bundle S NCE the first descriptions of ameboid locomotion a century and a half ago, the concept of a reversible transformation between "sol" and "gel" has been central to hypothesis attempting to explain the phenomenon. Although it was proposed that the protoplasm is a contractile three-dimensional reticulum as early as 1873 (De Bruyn, 1947), only in comparatively recent times has this reticulum been shown to be mainly an actin-based system (Pollard and Ito, 1970) . Subsequent research has revealed that cytoplasmic actin filaments are associated with myosin (reviewed by Korn and Hammer, 1988) and a number of other proteins that regulate actin filament assembly and crosslinking (reviewed by Stossel et al., 1985, andPollard and . Because there are multiple crosslinking proteins in these cytoplasmic actin gels, the physiological function of the individual crosslinking proteins by mutation or gene disruption is difficult to demonstrate (Wallraff et al ., 1986;Schleicher et al., 1988). Consequently most of our knowledge about cytoplasmic actin gels has come from in vitro reconstitution with actin and purified individual crosslinkers such as a-actinin.Alpha-actinin is a major actin crosslinking protein in skeletal muscle and nonmuscle cells . Skeletal muscle a-actinins are calcium-insensitive crosslinking proteins. Some, but not all, a-actinins from smooth muscle and nonmuscle cells are inhibited by calcium . Acanthamoeba a-actinin is calcium insensitive but is otherwise a typical a-actinin (Pollard, 1981;

Hyperosmotic stress‐induced reorganization of actin bundles in Dictyostelium cells over‐expressing cofilin

“…These results shed light on the mechanism by which actin bundles are induced in cells. As was originally demonstrated by Maciver et al (1991b), and followed by us , networks of actin filaments with an excess of crosslinking protein are transformed into bundles in vitro by actophorin or cofilin. This transformation is explained by the severing activity of these proteins (Maciver et al 1991b).…”

“…Cofilin binds in vitro to both filamentous and monomeric actin in a 1:1 molar ratio (Nishida et al 1984b), and severs actin filaments (Mabuchi 1983;Nishida et al 1984aNishida et al , 1985Maciver et al 1991a). Cofilin transforms a meshwork, which was reconstituted from actin filaments and ␣-actinin, into bundles in vitro, presumably by severing the filaments (Maciver et al 1991b;Aizawa et al 1996). Furthermore, over-expression of cofilin induces actin bundles and stimulates cell motility in D. discoideum , suggesting that cofilin plays a pivotal role in the formation of actin bundles for cell movement.…”

Actophorin preferentially binds monomeric ADP‐Actin over ATP‐bound actin: consequences for cell locomotion

“…These proteins would be ideally suited for severing filaments into measured lengths if they were unable to interact with ATP or ADPPi-monomers also present in cells. An unexpected result of the severing activity of actophorin has been shown to influence the bundling activity of cc-actinin [30]. This would potentially give the cell enormous scope in controlling the exact type of gel that is being created, which in turn would be expected to inlluence the physical properties of that particular part of the cell.…”

“…This would potentially give the cell enormous scope in controlling the exact type of gel that is being created, which in turn would be expected to inlluence the physical properties of that particular part of the cell. Not only does the geometry of cytoplasmic gels strongly influence the rheological properties of gels [30], but the geometry is important in determining interactions with motor proteins such as the myosin I family, located at the cells leading edge [3 1,321. …”

ADF/cofilin family proteins control formation of oriented actin-filament bundles in the cell body to trigger fibroblast polarization

“…AC severing activity can also form actin bundles (Aizawa et al, 1996;Maciver et al, 1991;Moriyama et al, 1996) and severing would generate shorter actin filaments that could be manoeuvred into a bundled organization by proteins such as a-actinin and myosin II. In support of this idea, we observed both movement of bundles in live cells (data not shown) and association of polymerized actin with myosin II during formation of oriented actinfilament bundles (T.M.…”