18.10: Actin Mircofilaments in NonMuscle Cells

Microfilaments are found throughout the Eukarya category. Electron microscopy revealed that thin (~10 nm) filaments permeate the cytoskeleton of eukaryotic cells (see Figure 18.3). They typically lie in the cortex of cells, just under the plasma membrane, where they support cell shape. These same microfilaments can also reorganize dynamically, allowing cells to change shape. A dramatic example of this occurs during cytokinesis in dividing cells, when the dividing (meiotic or mitotic) cell forms a cleavage furrow in the middle of the cell (discussed further in another chapter). The cortical microfilaments slide past each other with the help of nonmuscle myosin, progressively pinching the cell until it divides into two new cells. To test whether these 10 nm “microfilaments” were in fact actin, scientists placed myosin monomers or S1 myosin head fragments atop actins isolated from many different cell types. When seen in the electron microscope, such preparations always revealed that the 10 nm actin microfilaments were decorated with arrowheads, just like S1 fragments decorated muscle cell actin or Z-line-bound actin. See S1 arrowheads on cortical actin and on microvillar actin at S1 Fragment Decorates Cortical Actin and S1 Fragment Decorates Ciliary Actin Bundle (respectively) Clearly the cytoplasmic microfilaments are a form of F-actin. The role of cortical filaments in cell division is animated at Cortical Actin Filament Action in Cytokinesis. We now know that actin microfilaments are involved in all manner of cell motility, in addition to their role in cell division, enabling cell movement and cytoplasmic streaming within cells. They give intestinal microvilli strength; they even enable them to move independently of the passive pressures of peristalsis. Other examples of microfilaments in cell motility include the ability of amoeba and other phagocytic cells to extend pseudopodia to engulf food or foreign particles (e.g., bacteria). A well-studied example of microfilament-powered cell movement is the spread of fibroblast cells along surfaces, shown in Figure 18.29.

Figure 18.29: In the anti-actin immunofluorescence micrograph of fibroblasts, actin localizes with stress fibers, which help maintain cell shape (left). Actin also localizes in lamellipodia and retraction fibers in migrating fibroblasts (right), orienting in the direction of movement.

Migrating fibroblasts move forward by extend lamellipodia (and thin filipodia beyond them) by assembling actin bundles along the axis of movement. In the immunofluorescence micrograph (left, Figure 18.29), the actin stress fibers that maintain cell shape fluoresce green; the dual roles of actin in fibroblast shape and movement are also illustrated at the right.

The extension of filipodia at the moving front of a fibroblast is mainly based on actin assembly and disassembly (not unlike motility based on microtubules). A retraction fiber forms at the hind end of the cell as the fibroblast moves forward. The retraction fiber remains attached to the surface (substratum) on which it is migrating until actin-myosin interactions (in fact, sliding) cause retraction of most of this “fiber” back into the body of the cell.

Studies of nonmuscle cell motility suggest the structure and interacting molecular components of stress fibers modeled in Figure 18.30.

Figure 18.30: The molecular structure of stress fibers: Myosin as well as other actin-binding proteins interact with actin in nonmuscle cell motility.

The illustration suggests roles for actin-binding proteins in the sliding of overlapping myosin and actin filaments during movement. This model may also explain the cytoplasmic streaming that distributes cellular components and nutrients throughout a cell. In fact, both movements involve actin-myosin interactions. Filamin in this drawing is shown holding actin filaments together at an angle, while \(\alpha\)−actinin also helps to bundle the actin filaments. Titin (not shown) also seems to be associated with stress fibers.

Unlike the highly organized skeletal muscle sarcomeres, the proteins and filaments in stress fibers are not part of Z- or M-line superstructures. Were these less-organized nonmuscle stress-fiber actin bundles the evolutionary ancestors of muscle cell sarcomeres? Or vice-versa?