MICROFILAMENTS

Microfilaments - Microfilaments are solid rods made of globular proteins called actin. These filaments are primarily structural in function and are an important component of the cytoskeleton.



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Common to all eukaryotic cells, these filaments are primarily structural in function and are an important component of the cytoskeleton, along with microtubules and often the intermediate filaments. Microfilaments range from 5 to 9 nanometers in diameter and are designed to bear large amounts of tension. In association with myosin, microfilaments help to generate the forces used in cellular contraction and basic cell movements. The filaments also enable a dividing cell to pinch off into two cells and are involved in amoeboid movements of certain types of cells.
Microfilament Structural Organization
Microfilament Structural Organization
Microfilaments are solid rods made of a protein known as actin. When it is first produced by the cell, actin appears in a globular form (G-actin; see Figure 1). In microfilaments, however, which are also often referred to as actin filaments, long polymerized chains of the molecules are intertwined in a helix, creating a filamentous form of the protein (F-actin). All of the subunits that compose a microfilament are connected in such a way that they have the same orientation. Due to this fact, each microfilament exhibits polarity, the two ends of the filament being distinctly different. This polarity affects the growth rate of microfilaments, one end (termed the plus end) typically assembling and disassembling faster than the other (the minus end).
microfilament structure
microfilament structure

Unlike microtubules, which typically extend out from the centrosome of a cell, microfilaments are typically nucleated at the plasma membrane. Therefore, the periphery (edges) of a cell generally contains the highest concentration of microfilaments. A number of external factors and a group of special proteins influence microfilament characteristics, however, and enable them to make rapid changes if needed, even if the filaments must be completely disassembled in one region of the cell and reassembled somewhere else. When found directly beneath the plasma membrane, microfilaments are considered part of the cell cortex, which regulates the shape and movement of the cell's surface. Consequently, microfilaments play a key role in development of various cell surface projections (as illustrated in Figure 2), including filopodia, lamellipodia, and stereocilia.
Animal Cell Microfilament Network
Animal Cell Microfilament Network
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Illustrated in Figure 2 is a fluorescence digital image of an Indian Muntjac deer skin fibroblast cell stained with fluorescent probes targeting the nucleus (blue) and the actin cytoskeletal network (green). Individually, microfilaments are relatively flexible. In the cells of living organisms, however, the actin filaments are usually organized into larger, much stronger structures by various accessory proteins. The exact structural form that a group of microfilaments assumes depends on their primary function and the particular proteins that bind them together. For instance, in the core of surface protrusions called microspikes, microfilaments are organized into tight parallel bundles by the bundling protein fimbrin. Bundles of the filaments are less tightly packed together, however, when they are bound by alpha-actinin or are associated with fibroblast stress fibers (the parallel green fibers in Figure 2). Notably, the microfilament connections created by some cross-linking proteins result in a web-like network or gel form rather than filament bundles.
Over the course of evolutionary history of the cell, actin has remained relatively unchanged. This, along with the fact that all eukaryotic cells heavily depend upon the integrity of their actin filaments in order to be able to survive the many stresses they are faced with in their environment, makes actin an excellent target for organisms seeking to injure cells. Accordingly, many plants, which are unable to physically avoid predators that might want to eat them or harm them in some other way, produce toxins that affect cellular actin and microfilaments as a defensive mechanism. The death cap mushroom, for example, produces a substance called phalloidin that binds to and stabilizes actin filaments, which can be fatal to cells.

Animal cells are typical of the eukaryotic cell, enclosed by a plasma membrane and containing a membrane-bound nucleus and organelles. Unlike the eukaryotic cells of plants and fungi, animal cells do not have a cell wall. This feature was lost in the distant past by the single-celled organisms that gave rise to the kingdom Animalia. Most cells, both animal and plant, range in size between 1 and 100 micrometers and are thus visible only with the aid of a microscope.
Anatomy of the Animal Cell
Anatomy of the Animal Cell


external image microfilament.gifYou will find microfilaments in most cells. They are the partner of microtubules. They are long, thin, and stringy proteins (mainly actin) compared to the rounder, tube-shaped microtubules. We'd like to say you can find them here or there, but they are everywhere in a cell. They work with microtubules to form the structure that allows a cell to hold its shape, move itself, and move its organelles. All of the microfilaments and microtubules combine to form the cytoskeleton of the cell.

Microfilaments are fine, thread-like protein fibers, 3-6 nm in diameter. They are composed predominantly of a contractile protein called actin, which is the most abundant cellular protein. Microfilaments' association with the protein myosin is responsible for muscle contraction. Microfilaments can also carry out cellular movements including gliding, contraction, and cytokinesis.


A linear assemblage of the protein actin; microfilaments, also called actin filaments, are one of three main components of the cytoskeleton. Microfilaments serve a number of functions. They form a band just beneath the cell membrane that
    • provides mechanical strength to the cell
    • links transmembrane proteins (e.g., cell surface receptors) to cytoplasmic proteins
    • anchors the centrosomes at opposite poles of the cell during mitosis
    • pinches dividing animal cells apart during cytokinesis;
  • generate cytoplasmic streaming in some cells;
  • generate locomotion in cells such as some leukocytes (white blood cells) and the amoeba;
  • interact with myosin ("thick") filaments in skeletal muscle fibers to provide the force of muscular contraction.

    Because actin subunits have polarity, so also do the microfilaments from which they are built. Traditionally, the ends of a microfilament have been referred to as "pointed" and "barbed," a nomenclature that arises from the resemblance of microfilaments decorated with fragments of myosin parallel to arrowheads in the electron microscope.

Microfilaments are solid rods made of globular proteins called actin and are common to all eukaryotic cells. Long chains of the molecules are intertwined in a helix to form individual microfilaments. These filaments are primarily structural in function and are an important component of the cytoskeleton, along with microtubules.

Microfilaments
Microfilaments


In association with myosin, microfilaments help to generate the forces used in cellular contraction and basic cell movements. They enable a dividing cell to pinch off into two cells and are involved in amoeboid movements of certain types of cells. They also enable the contractions of muscle cells.

Definition - narrow tubelike cell structure composed of a protein similar to actin, occurring singly and in bundles, involved in cytoplasmic movement and changes in cell shape.
external image 250px-MEF_microfilaments.jpg Actin cytoskeleton of mouse embryo fibroblasts, stained with FITC-phalloidin
Microfilaments (or actin filaments) are the thinnest filaments of the cytoskeleton found in the cytoplasm of all eukaryotic cells. These linear polymers of actin subunits are flexible and relatively strong, resisting buckling by multi-piconewton compressive forces and filament fracture by nanonewton tensile forces. Microfilaments are highly versatile, functioning in (a) actoclampin-driven expansile molecular motors, where each elongating filament harnesses the hydrolysis energy of its "on-board" ATP to drive actoclampin end-tracking motors to propel cell crawling, ameboid movement, and changes in cell shape, and (b) actomyosin-driven contractile molecular motors, where the thin filaments serve as tensile platforms for myosin's ATP hydrolysis-dependent pulling action in muscle contraction and uropod advancement.


Motion due to microfilaments-
Cells display an amazing variety of movement. They swim, crawl, contract, extend and move things around inside. These movements perform an equal variety of functions such as to capture prey or invading organisms, to feed, or communicate with other cells. All this is accomplished by the microfilament and/or microtubule systems in association with various “motor proteins” which are capable of transforming chemical energy in the form of nucleotides into kinetic energy.

Microfilaments and their functions/structures-


Microfilament structure ----
A linear assemblage of the protein actin; microfilaments, also called actin filaments, are one of three main components of the cytoskeleton. Microfilaments serve a number of functions. They:
  • form a band just beneath the cell membrane that
    • provides mechanical strength to the cell
    • links transmembrane proteins (e.g., cell surface receptors) to cytoplasmic proteins
    • anchors the centrosomes at opposite poles of the cell during mitosis
    • pinches dividing animal cells apart during cytokinesis;
  • generate cytoplasmic streaming in some cells;
  • generate locomotion in cells such as some leukocytes (white blood cells) and the amoeba;
  • interact with myosin ("thick") filaments in skeletal muscle fibers to provide the force of muscular contraction.

external image microfilaments2.jpg
external image microfilaments.gif

external image coverfig.gifAll microfilaments are not created equal! Actin filaments are involved in many cellular activities at sites throughout the cell, including on the Golgi complex. This functional diversity involves differences in actin filament organization, dynamics, and mechanochemical properties, which are generated by molecules such as tropomyosin.

Microfilaments (or actin filaments) are the thinnest filaments of the cytoskeleton found in the cytoplasm of all eukaryotic cells. These linear polymers of actin subunits are flexible and relatively strong, resisting buckling by multi-piconewton compressive forces and filament fracture by nanonewton tensile forces. Microfilaments are highly versatile, functioning in (a) actoclampin-driven expansile molecular motors, where each elongating filament harnesses the hydrolysis energy of its "on-board" ATP to drive actoclampin end-tracking motors to propel cell crawling, ameboid movement, and changes in cell shape, and (b) actomyosin-driven contractile molecular motors, where the thin filaments serve as tensile platforms for myosin's ATP hydrolysis-dependent pulling action in muscle contraction and uropod advancement.