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6.3: Microfilaments - Biology


Of the three types of protein fibers in the cytoskeleton, microfilaments are the narrowest. For this reason, microfilaments are also known as actin filaments.

Actin is powered by ATP to assemble its filamentous form, which serves as a track for the movement of a motor protein called myosin. This enables actin to engage in cellular events requiring motion, such as cell division in animal cells and cytoplasmic streaming, which is the circular movement of the cell cytoplasm in plant cells. Actin and myosin are plentiful in muscle cells. When your actin and myosin filaments slide past each other, your muscles contract.

Microfilaments also provide some rigidity and shape to the cell. They can depolymerize (disassemble) and reform quickly, thus enabling a cell to change its shape and move. White blood cells (your body’s infection-fighting cells) make good use of this ability. They can move to the site of an infection and phagocytize the pathogen.

Look below see an example of a white blood cell in action. Watch this short time-lapse video of the cell capturing two bacteria. It engulfs one and then moves on to the other. Note that this video has no audio.

A link to an interactive elements can be found at the bottom of this page.

You can view the audio description text for “White Blood Cell Chases Bacteria” here (link opens in new window).


What is Microfilaments in biology?

Click to read further detail. Regarding this, what is the main function of Microfilaments?

Microfilaments assist with cell movement and are made of a protein called actin. Actin works with another protein called myosin to produce muscle movements, cell division, and cytoplasmic streaming. Microfilaments keep organelles in place within the cell.

Subsequently, question is, what are the 4 functions of Microfilaments? Microfilament functions include cytokinesis, amoeboid movement, cell motility, changes in cell shape, endocytosis and exocytosis, cell contractility, and mechanical stability.

In respect to this, what is microtubule in biology?

Microtubules are fibrous, hollow rods that function primarily to help support and shape the cell. Microtubules are typically found in all eukaryotic cells and are a component of the cytoskeleton, as well as cilia and flagella. Microtubules are composed of the protein tubulin.

What are Microfilaments composed of?

The primary types of fibers comprising the cytoskeleton are microfilaments, microtubules, and intermediate filaments. 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.


Difference in Microtubule and Microfilament Reorgnization

The process of adding on or removing monomers from microfilaments and microtubules differs. For microfilaments, actin monomers can be added to either end of the filament, including the barbed end (positive end) and the pointed end (negative end). For microtubules, unlike microfilaments, tubulin monomers are only added and removed on the positive end. In microtubules, a tubulin heterodimer consisting of beta-tubulin and alpha-tubulin is added or removed each time. Microtubules have gamma-tubulin proteins at the minus ends of the complex that prevent removal or addition of tubulin on that minus end. In fact, during mitosis and meiosis, gamma tubulin complexes are loosely organized around centrioles.

Microfilaments: Added on either end (plus or minus)

Microtubules: Added only on the plus end

Microtubules: have gamma-tubulin proteins on the minus end, prevents addition or removal of tubulin

Mitosis requires 2 centrosomes whose spindles are made up of microtubules, not microfilaments. These microtubules in the spindles help position chromosomes in the middle of the cell during metaphase and pull them apart during anaphase.

Microtubules grow and shrink only on the plus end. When the plus end is capped with GTP bound tubulin, the microtubule is stable and no longer grows. When the plus end is capped with GDP bound tubulin at the end, microtubule is unstable and can shrink.

Another difference between microtubules and microfilaments is that for microfilaments, nucleating complexes are involved in initiating filament formation. There are 3 proteins involved in filament formation: actin, Arp2, and Arp3. They make a nucleating complex, bind to either minus or plus end on the microfilament.

Microfilament Regulating Actin Assembly Rates

Actin-profilin complex causes rapid plus-end growth. Actin-thymosin complex inhibits binding and plus-end growth. Profilin and thymosin compete with each other for the binding of actin monomers and the promotion of microfilament assembly.

Microfilament: Arp2 + Arp3 + Actin = Nucleating complex to start formation

Microfilament: Actin-thymosin = Inhibit vs Actin-profilin = Proliferate/Grow


Contents

Actin and microfilament-mediated processes have long been a subject of research. American-German botanist George Engelmann (1879) suggested that many kinds of movement observed in plants and protozoa like cytoplasmic streaming and amoeboid movement were in fact a primitive version of the movements of muscle contraction.

In the 1930s, Szent-Györgyi and collaborators, violating one of the canons of biochemistry, started to "study the residue instead of the extract", that is, structural proteins and not enzymes, leading to the many discoveries related to microfilaments. [3]

Actin filaments are assembled in two general types of structures: bundles and networks. Bundles can be composed of polar filament arrays, in which all barbed ends point to the same end of the bundle, or non-polar arrays, where the barbed ends point towards both ends. A class of actin-binding proteins, called cross-linking proteins, dictate the formation of these structures. Cross-linking proteins determine filament orientation and spacing in the bundles and networks. These structures are regulated by many other classes of actin-binding proteins, including motor proteins, branching proteins, severing proteins, polymerization promoters, and capping proteins.

Measuring approximately 6 nm in diameter, [4] microfilaments are the thinnest fibers of the cytoskeleton. They are polymers of actin subunits (globular actin, or G-actin), which as part of the fiber are referred to as filamentous actin, or F-actin. Each microfilament is made up of two helical, interlaced strands of subunits. Much like microtubules, actin filaments are polarized. Electron micrographs have provided evidence of their fast-growing barbed-ends and their slow-growing pointed-end. This polarity has been determined by the pattern created by the binding of myosin S1 fragments: they themselves are subunits of the larger myosin II protein complex. The pointed end is commonly referred to as the minus (−) end and the barbed end is referred to as the plus (+) end.

In vitro actin polymerization, or nucleation, starts with the self-association of three G-actin monomers to form a trimer. ATP-bound actin then itself binds the barbed end, and the ATP is subsequently hydrolyzed. ATP hydrolysis occurs with a half time of about 2 seconds, [5] while the half time for the dissociation of the inorganic phosphate is about 6 minutes. [5] This autocatalyzed event reduces the binding strength between neighboring subunits, and thus generally destabilizes the filament. In vivo actin polymerization is catalyzed by a class of filament end-tracking molecular motors known as actoclampins. Recent evidence suggests that the rate of ATP hydrolysis and the rate of monomer incorporation are strongly coupled.

Subsequently, ADP-actin dissociates slowly from the pointed end, a process significantly accelerated by the actin-binding protein, cofilin. ADP bound cofilin severs ADP-rich regions nearest the (−)-ends. Upon release, the free actin monomer slowly dissociates from ADP, which in turn rapidly binds to the free ATP diffusing in the cytosol, thereby forming the ATP-actin monomeric units needed for further barbed-end filament elongation. This rapid turnover is important for the cell's movement. End-capping proteins such as CapZ prevent the addition or loss of monomers at the filament end where actin turnover is unfavorable, such as in the muscle apparatus.

Actin polymerization together with capping proteins were recently used to control the 3-dimensional growth of protein filament so as to perform 3D topologies useful in technology and the making of electrical interconnect. Electrical conductivity is obtained by metallisation of the protein 3D structure. [6] [7]

As a result of ATP hydrolysis, filaments elongate approximately 10 times faster at their barbed ends than their pointed ends. At steady-state, the polymerization rate at the barbed end matches the depolymerization rate at the pointed end, and microfilaments are said to be treadmilling. Treadmilling results in elongation in the barbed end and shortening in the pointed-end, so that the filament in total moves. Since both processes are energetically favorable, this means force is generated, the energy ultimately coming from ATP. [1]

Intracellular actin cytoskeletal assembly and disassembly are tightly regulated by cell signaling mechanisms. Many signal transduction systems use the actin cytoskeleton as a scaffold, holding them at or near the inner face of the peripheral membrane. This subcellular location allows immediate responsiveness to transmembrane receptor action and the resulting cascade of signal-processing enzymes.

Because actin monomers must be recycled to sustain high rates of actin-based motility during chemotaxis, cell signalling is believed to activate cofilin, the actin-filament depolymerizing protein which binds to ADP-rich actin subunits nearest the filament's pointed-end and promotes filament fragmentation, with concomitant depolymerization in order to liberate actin monomers. In most animal cells, monomeric actin is bound to profilin and thymosin beta-4, both of which preferentially bind with one-to-one stoichiometry to ATP-containing monomers. Although thymosin beta-4 is strictly a monomer-sequestering protein, the behavior of profilin is far more complex. Profilin enhances the ability of monomers to assemble by stimulating the exchange of actin-bound ADP for solution-phase ATP to yield actin-ATP and ADP. Profilin is transferred to the leading edge by virtue of its PIP2 binding site, and it employs its poly-L-proline binding site to dock onto end-tracking proteins. Once bound, profilin-actin-ATP is loaded into the monomer-insertion site of actoclampin motors.

Another important component in filament formation is the Arp2/3 complex, which binds to the side of an already existing filament (or "mother filament"), where it nucleates the formation of a new daughter filament at a 70 degree angle relative to the mother filament, effecting a fan-like branched filament network. [8]

Specialized unique actin cytoskeletal structures are found adjacent to the plasma membrane. Four remarkable examples include red blood cells, human embryonic kidney cells, neurons, and sperm cells. In red blood cells, a spectrin-actin hexagonal lattice is formed by interconnected short actin filaments. [9] In human embryonic kidney cells, the cortical actin forms a scale-free fractal structure. [10] In neuronal axons, actin forms periodic rings that are stabilized by spectrin and adducin. [11] [12] And in mammalian sperm, actin forms a helical structure in the midpiece, i.e., the first segment of the flagellum. [13]

In non-muscle cells, actin filaments are formed proximal to membrane surfaces. Their formation and turnover are regulated by many proteins, including:

  • Filament end-tracking protein (e.g., formins, VASP, N-WASP)
  • Filament-nucleator known as the Actin-Related Protein-2/3 (or Arp2/3) complex
  • Filament cross-linkers (e.g., α-actinin, fascin, and fimbrin)
  • Actin monomer-binding proteins profilin and thymosin β4
  • Filament barbed-end cappers such as Capping Protein and CapG, etc.
  • Filament-severing proteins like gelsolin.
  • Actin depolymerizing proteins such as ADF/cofilin.

The actin filament network in non-muscle cells is highly dynamic. The actin filament network is arranged with the barbed-end of each filament attached to the cell's peripheral membrane by means of clamped-filament elongation motors, the above-mentioned "actoclampins", formed from a filament barbed-end and a clamping protein (formins, VASP, Mena, WASP, and N-WASP). [14] The primary substrate for these elongation motors is profilin-actin-ATP complex which is directly transferred to elongating filament ends. [15] The pointed-end of each filament is oriented toward the cell's interior. In the case of lamellipodial growth, the Arp2/3 complex generates a branched network, and in filopodia a parallel array of filaments is formed.

Myosin motors are intracellular ATP-dependent enzymes that bind to and move along actin filaments. Various classes of myosin motors have very different behaviors, including exerting tension in the cell and transporting cargo vesicles.

One proposed model suggests the existence of actin filament barbed-end-tracking molecular motors termed "actoclampin". [16] The proposed actoclampins generate the propulsive forces needed for actin-based motility of lamellipodia, filopodia, invadipodia, dendritic spines, intracellular vesicles, and motile processes in endocytosis, exocytosis, podosome formation, and phagocytosis. Actoclampin motors also propel such intracellular pathogens as Listeria monocytogenes, Shigella flexneri, Vaccinia and Rickettsia. When assembled under suitable conditions, these end-tracking molecular motors can also propel biomimetic particles.

The term actoclampin is derived from acto- to indicate the involvement of an actin filament, as in actomyosin, and clamp to indicate a clasping device used for strengthening flexible/moving objects and for securely fastening two or more components, followed by the suffix -in to indicate its protein origin. An actin filament end-tracking protein may thus be termed a clampin.

Dickinson and Purich recognized that prompt ATP hydrolysis could explain the forces achieved during actin-based motility. [14] They proposed a simple mechanoenzymatic sequence known as the Lock, Load & Fire Model, in which an end-tracking protein remains tightly bound ("locked" or clamped) onto the end of one sub-filament of the double-stranded actin filament. After binding to Glycyl-Prolyl-Prolyl-Prolyl-Prolyl-Prolyl-registers on tracker proteins, Profilin-ATP-actin is delivered ("loaded") to the unclamped end of the other sub-filament, whereupon ATP within the already clamped terminal subunit of the other subfragment is hydrolyzed ("fired"), providing the energy needed to release that arm of the end-tracker, which then can bind another Profilin-ATP-actin to begin a new monomer-addition round.

Steps involved Edit

The following steps describe one force-generating cycle of an actoclampin molecular motor:

  1. The polymerization cofactor profilin and the ATP·actin combine to form a profilin-ATP-actin complex that then binds to the end-tracking unit
  2. The cofactor and monomer are transferred to the barbed-end of an actin already clamped filament
  3. The tracking unit and cofactor dissociate from the adjacent protofilament, in a step that can be facilitated by ATP hydrolysis energy to modulate the affinity of the cofactor and/or the tracking unit for the filament and this mechanoenzymatic cycle is then repeated, starting this time on the other sub-filament growth site.

When operating with the benefit of ATP hydrolysis, AC motors generate per-filament forces of 8–9 pN, which is far greater than the per-filament limit of 1–2 pN for motors operating without ATP hydrolysis. [14] [16] [17] The term actoclampin is generic and applies to all actin filament end-tracking molecular motors, irrespective of whether they are driven actively by an ATP-activated mechanism or passively.


Intermediate Filaments and Microtubules

Microtubules are part of the cell’s cytoskeleton, helping the cell resist compression, move vesicles, and separate chromosomes at mitosis.

Learning Objectives

Describe the roles of microtubules as part of the cell’s cytoskeleton

Key Takeaways

Key Points

  • Microtubules help the cell resist compression, provide a track along which vesicles can move throughout the cell, and are the components of cilia and flagella.
  • Cilia and flagella are hair-like structures that assist with locomotion in some cells, as well as line various structures to trap particles.
  • The structures of cilia and flagella are a 𔄡+2 array,” meaning that a ring of nine microtubules is surrounded by two more microtubules.
  • Microtubules attach to replicated chromosomes during cell division and pull them apart to opposite ends of the pole, allowing the cell to divide with a complete set of chromosomes in each daughter cell.

Key Terms

  • microtubule: Small tubes made of protein and found in cells part of the cytoskeleton
  • flagellum: a flagellum is a lash-like appendage that protrudes from the cell body of certain prokaryotic and eukaryotic cells
  • cytoskeleton: A cellular structure like a skeleton, contained within the cytoplasm.

Microtubules

As their name implies, microtubules are small hollow tubes. Microtubules, along with microfilaments and intermediate filaments, come under the class of organelles known as the cytoskeleton. The cytoskeleton is the framework of the cell which forms the structural supporting component. Microtubules are the largest element of the cytoskeleton. The walls of the microtubule are made of polymerized dimers of α-tubulin and β-tubulin, two globular proteins. With a diameter of about 25 nm, microtubules are the widest components of the cytoskeleton. They help the cell resist compression, provide a track along which vesicles move through the cell, and pull replicated chromosomes to opposite ends of a dividing cell. Like microfilaments, microtubules can dissolve and reform quickly.

Micrtubule Structure: Microtubules are hollow, with walls consisting of 13 polymerized dimers of α-tubulin and β-tubulin (right image). The left image shows the molecular structure of the tube.

Microtubules are also the structural elements of flagella, cilia, and centrioles (the latter are the two perpendicular bodies of the centrosome ). In animal cells, the centrosome is the microtubule-organizing center. In eukaryotic cells, flagella and cilia are quite different structurally from their counterparts in prokaryotes.

Stained Keratin Intermediate filaments: Keratin cytoskeletal intermediate filaments are concentrated around the edge of the cells and merge into the surface membrane. This network of intermediate filaments from cell to cell holds together tissues like skin.

Intermediate Filaments

Intermediate filaments (IFs) are cytoskeletal components found in animal cells. They are composed of a family of related proteins sharing common structural and sequence features. Intermediate filaments have an average diameter of 10 nanometers, which is between that of 7 nm actin (microfilaments), and that of 25 nm microtubules, although they were initially designated ‘intermediate’ because their average diameter is between those of narrower microfilaments (actin) and wider myosin filaments found in muscle cells. Intermediate filaments contribute to cellular structural elements and are often crucial in holding together tissues like skin.

Flagella and Cilia

Flagella (singular = flagellum ) are long, hair-like structures that extend from the plasma membrane and are used to move an entire cell (for example, sperm, Euglena). When present, the cell has just one flagellum or a few flagella. When cilia (singular = cilium) are present, however, many of them extend along the entire surface of the plasma membrane. They are short, hair-like structures that are used to move entire cells (such as paramecia) or substances along the outer surface of the cell (for example, the cilia of cells lining the Fallopian tubes that move the ovum toward the uterus, or cilia lining the cells of the respiratory tract that trap particulate matter and move it toward your nostrils).

Despite their differences in length and number, flagella and cilia share a common structural arrangement of microtubules called a 𔄡 + 2 array.” This is an appropriate name because a single flagellum or cilium is made of a ring of nine microtubule doublets surrounding a single microtubule doublet in the center.

Microtubules are the structural component of flagella: This transmission electron micrograph of two flagella shows the 9 + 2 array of microtubules: nine microtubule doublets surround a single microtubule doublet.


Hepatitis and cholestasis in infancy: intrahepatic disorders

Progressive intrahepatic cholestasis

Progressive intrahepatic cholestasis is a heterogeneous assortment of familial disorders of unknown cause which progress to cirrhosis and death, usually in the first or second decades. Most reported cases are without distinguishing pathological or biochemical criteria. The presenting feature may be jaundice in the neonatal period or jaundice, pruritus or malabsorption appearing later in infancy. Some families present with a syndrome which initially seems similar to benign recurrent cholestasis. Pruritus is frequently severe. Diarrhoea, steatorrhoea and failure to thrive are often prominent features. Remission is never complete. Rarely clinical features clear but liver function tests remain abnormal. Exacerbations lasting a few days to 20 months occur, often provoked by infection. Families with high sweat sodium concentrations but without other features of cystic fibrosis have been identified. Kayser-Fleischer rings have been noted in those with long-standing cholestasis. Inspissated bile, gall-stones, large gall bladder, pancreatitis and myocarditis have all been described. Hepatomegaly persists with increasing intrahepatic fibrosis proceeding usually to biliary-type cirrhosis but in some patients, particularly of Arab descent, with features of chronic aggressive hepatitis. Splenomegaly, portal hypertension and bleeding from varices ensues. Hepatoma may develop ( Ugarte and Gonzalez-Crussi, 1981 ).

A number of distinct subgroups have been described within specific families or racial groups.

Cholestasis with actin and microfilament accumulation in North American Indian children

Prominent pericanalicular microfilament accumulation with much actin surrounding dilated bile canaliculi and well preserved microvilli, are characteristic features of a disorder described in 14 North American Indian children. Jaundice in the neonatal period progressing to cirrhosis by 2 years of age was the usual presentation, but some presented with features of cirrhosis without prior jaundice. Pruritus, heart murmurs, recurrent skin and ear infections, severe epistaxis and a characteristic leash of small blood vessels on the cheeks, are prominent clinical features ( Weber et al., 1981 ).

Fatal familial cholestatic syndrome in Greenland Eskimo children

Persistent jaundice starting in the first 3 months of life, pruritus, malabsorption and its complications, thrombocytosis and low serum cholesterol concentrations were distinctive features of this autosomal recessively inherited syndrome. Eight of 16 died of infection and bleeding between 6 weeks and 3 years with the oldest survivor being 30 months. The ultrastructural features were unremarkable but even at postmortem examination there was no cirrhosis although there was portal-portal fibrosis.

Byler disease

Severe familial idiopathic cholestasis proceeding to cirrhosis with early death, often in the first decade was described in eight members of the Byler family and in other Amish families ( Clayton et al., 1969 ). The longest life span recorded is 18 years ( Jones et al., 1976 ).

Three further categories have been identified recently, two on the basis of unusual biochemical findings of uncertain pathogenic significance and one by distinctive changes in the major intrahepatic bile ducts.

Maggiore and coworkers (1987) reported a subgroup characterized by normal gamma-glutamyl transpeptidase concentrations in serum which is most unusual in infants with any other form of hepatobiliary disease. These infants had clear clinical and pathological evidence of progressive hepatic fibrosis with the emergence of complications of cirrhosis in the first decade. Other biochemical test of liver damage were abnormal.

Strumm et al (1990) more recently have demonstrated impaired apolipoprotein A-1 synthesis in hepatocytes with very low serum levels as a distinctive feature in 18 patients with this disorder.

Sclerosing cholangitis of neonatal onset

Amedee-Manesme et al. (1989) described eight children, three of whom had consanguineous parents with jaundice in the first 36 months of life with the cholangiographic features of sclerosing cholangitis. Although jaundice cleared other liver function tests remained abnormal and cirrhosis with portal hypertension was established by 9 years of age. The same findings were observed in two siblings of consanguineous patents, with the elder being performed successfully at 6 years of age ( Baker et al., 1993 ).

Iron storage disorders

These usually lethal disorders are characterized by a four- and sevenfold increase in iron in the liver, pancreas, heart, endocrine and exocrine glands and in skin (see Chapter 15 ). Haemosiderosis, both hepatocellular and reticulo-endothelial, is found in leprechaunism. In this disorder the liver also contains multiple small nodules composed of large, pale foamy hepatocytes with large quantities of glycogen and little fat. Some cases show intrahepatic cholestasis and bile duct proliferation. Others have no hepatic abnormality ( Ordway and Stout, 1973 ).

Hepatosteatosis

A number of families have been described with conjugated hyperbilirubinaemia, sometimes kernicterus, a bleeding disorder and marked fatty infiltration in the liver as well as other organs and viscera, leading to death in the neonatal period. In some families males only are affected. In one case serum lipids and fatty acids were markedly increased. With modern investigations these should be found to have specific inborn errors of metabolism (see Chapter 15 ).

Rare associations

Liver disease presenting in infancy has been associated with mild de Jeune's syndrome and also with renal tubular insufficiency and multiple congenital anomalies (see Chapter 16 ).


Watch the video: Notes for IB Biology Chapter (January 2022).