The Cytoskeleton

Chapter 13 The Cytoskeleton


As diffusion barriers, biological membranes form the boundary between the cell and its surroundings, and they form compartments within eukaryotic cells. However, they do not give the cell its shape. They do not provide structural strength, resistance to mechanical stress, or resilience to deformation. These properties require a network of cellular fibers known collectively as the cytoskeleton.


In addition to giving the cell its shape and mechanical strength, the cytoskeleton has two additional functions: intracellular transport and cell motility. Transport of proteins and organelles down the axons of neurons, amoeboid movement of phagocytic cells, beating of cilia and flagella, and muscle contraction all are specialized functions of the cytoskeleton.



The erythrocyte membrane is reinforced by a spectrin network


Erythrocytes travel about 300 miles during their 120-day life, part of this through tortuous capillaries in which they suffer mechanical deformation. They can survive this only because their membrane is reinforced by a meshwork of fibers formed by the proteins α-spectrin and β-spectrin. Each spectrin monomer consists of spectrin repeats, a domain of 106 amino acids that forms a coiled coil of three intertwined α-helices. It is repeated (with variations) 20 times in the α-chain and 17 times in the β-chain (Fig. 13.1).


image

Figure 13.1 A, The spectrin repeat consists of three α-helical coiled coils with a total of 106 amino acid residues. B, Structure of a spectrin dimer consisting of an α-chain and β-chain, which have 20 and 17 spectrin repeats, respectively.


Spectrin forms an antiparallel dimer, with an α-chain and a β-chain lying side by side. These α-β dimers condense head to head to form a tetramer, which is a long, wriggly, wormlike molecule with a contour length of 200 nm and a diameter of 5 nm. The ends of the spectrin tetramer are bound noncovalently to short (35-nm) actin filaments. This interaction is facilitated by two other proteins: band 4.1 protein (so named after its migration in gel electrophoresis) and adducin. By binding several spectrin tetramers, the actin filaments form the nodes of a two-dimensional network that can be likened to a fishing net or a piece of very thin, flexible chicken wire (Fig. 13.2, B).


image

Figure 13.2 Hypothetical model of the membrane skeleton in red blood cells. A, Transverse section. B, Tangential section.


The spectrin network is anchored to the membrane by the peripheral membrane protein ankyrin, which itself is bound to the integral membrane protein band 3 protein. This binding is stabilized by band 4.2 protein (pallidin). The actin microfilaments are attached to the membrane mainly through band 4.1 protein and the integral membrane protein glycophorin. The erythrocyte membrane skeleton is important because inherited defects in its components give rise to hemolytic anemias (Clinical Example 13.1).



CLINICAL EXAMPLE 13.1: Spherocytosis and Elliptocytosis


Hereditary spherocytosis is defined by the presence of erythrocytes that are spherical instead of biconcave. Mild anemia can result because the spherocytes are fragile and are easily trapped and destroyed in the spleen.


Most patients with hereditary spherocytosis have primary defects in ankyrin, β-spectrin, or band 3 protein. The amount of spectrin is always reduced because any spectrin that is not tied into the membrane skeleton falls prey to proteolytic enzymes during erythrocyte maturation.


In hereditary elliptocytosis, the erythrocytes are ellipsoidal rather than spherical. Mutations in the genes for band 4.1 protein or α-spectrin are the most common causes.


With a prevalence of 1 in 5000 each, spherocytosis and elliptocytosis are the most common inherited hemolytic anemias in many countries. Seventy-five percent of cases are inherited as autosomal dominant traits. Splenectomy cures the anemia in most patients.



Keratins are the most important structural proteins of epithelial tissues


Epithelial cells receive most of their structural support from keratin, which is one of several classes of intermediate filaments. In addition to its role in living epithelia, the keratin cytoskeleton of dead cells forms hair, fingernails, and the horny layer of the skin.


All keratins contain long stretches of α-helix interrupted by short nonhelical segments (Fig. 13.3, A). The two different types are the acidic (type I) and the basic (type II) keratins. Each comes in about 15 different variants. They form heterodimers, with a type I polypeptide forming a coiled coil with a type II polypeptide (Fig. 13.3, B). The α-helices of the two keratins make contact through hydrophobic amino acid side chains on one edge of each helix. Typical keratin fibrils contain between 12 and 24 of these heterodimers in a staggered array.


image

Figure 13.3 Structure of keratin, the major intermediate filament protein of epithelial tissues. A, Domain structure of a single polypeptide (type I keratin). The central, mostly α-helical part consists of approximately 310 amino acids. B, Parallel heterodimer formed from a type I and a type II keratin polypeptide.


Different keratins are expressed in different cell types. The basal layer of the epidermis forms K14 as the major type I keratin and K5 as the major type II keratin. In the more mature cells of the spinous and granular layers, keratins K10 and K1 are the major type I and type II keratins, respectively (Fig. 13.4). Single-layered epithelia express keratins 18, 19, and/or 20 (type I) and keratins 7 and 8 (type II). Various other keratin pairs are expressed in the cells that form hair and nails.


image

Figure 13.4 Layers of human skin. The epidermal cells are held together by numerous spot desmosomes. These spot desmosomes are attachment points for the intracellular keratin filaments.


Several intermediate filament proteins other than the keratins are expressed in various cell types (Table 13.1). All of them are dynamic structures that are assembled and disassembled continuously.


Table 13.1 Major Types of Intermediate Filament Proteins*






























Protein Tissue or Cell Type
Keratin Epithelial cells, hair, nails
Vimentin Embryonic tissues, mesenchymal cells, most cultured cells
Desmin Myocardium, at Z disk in skeletal muscle
Glial fibrillary acidic protein Astrocytes, Schwann cells
Peripherin Neurons of PNS
α-Internexin Neurons of CNS
Neurofilament proteins (NF-L, NF-M, NF-H) Neurons of CNS and PNS
Lamin Nucleus of all nucleated cells.

CNS, Central nervous system; PNS, peripheral nervous system.


* All of these proteins have the general structure depicted in Figure 13.3, for keratin.


The lamins are the only intermediate filament proteins that are found in the nucleus rather than the cytoplasm. They form a supporting fiber network under the nuclear envelope. During mitosis, the lamins become phosphorylated by the cell cycle–induced protein kinase Cdk1. This leads to the disassembly of the fibers and the collapse of the nuclear envelope (see Chapter 18).



CLINICAL EXAMPLE 13.2: Skin Blistering Diseases


A blister forms when the epidermis detaches from the dermis. Therefore any condition that weakens the boundary between dermis and epidermis leads to abnormal blistering.


Epidermolysis bullosa (EB) is a group of dominantly inherited skin blistering diseases in which even mild mechanical stress damages the dermal-epidermal junction. It comes in all degrees of severity, from mild forms with occasional blistering to severe forms that are fatal shortly after birth.


The classic forms of EB are caused by point mutations in the genes of keratin K14 or keratin K5, which are expressed in the basal cells of the epidermis. Therefore shear forces easily destroy the basal cell layer but leave the overlying cells intact.


Point mutations in the genes for K1 and K10, the major keratins of the spinous and granular cell layers, cause epidermolytic hyperkeratosis, a dominantly inherited type of skin disease with scaling, hyperkeratosis, and blistering.



CLINICAL EXAMPLE 13.3: Laminopathies


Mutations that affect the lamins of the nuclear lamina, especially the predominant lamin A, cause an astonishing spectrum of disease. Hutchinson-Gilford progeria is an extremely rare syndrome of premature aging, with an incidence of about 1 in 5 million live births. Although normal at birth, patients present with failure to thrive at 1 or 2 years, followed by signs of premature aging: hair loss, osteoporosis, loss of subcutaneous fat, atherosclerosis. Most patients die of myocardial infarction or stroke at age 12 to 14 years. The usual mutation in this disease is a point mutation that activates a cryptic splice site, creating a messenger RNA (mRNA) that is missing 150 nucleotides and a lamin A protein that is missing 50 amino acids.


Different mutations in the lamin A gene cause different diseases, including subtypes of limb girdle and Emery-Dreifuss muscular dystrophies, cardiomyopathies, lipodystrophies, skin disorders, and peripheral neuropathy.


The mechanisms by which lamin mutations cause so many seemingly unrelated syndromes is not known. The lamins interact not only with each other and with proteins of the inner nuclear membrane but also with core histones and many other components of chromatin. In addition to mechanical fragility of the nucleus, deranged gene expression is a possible mechanism.



Actin filaments are formed from globular subunits


All cells contain microfilaments that are formed by the polymerization of globular actin subunits. Collectively, the six isoforms of actin that occur in different tissues are among the most abundant types of protein in the human body. In most cells, the microfilaments are concentrated under the plasma membrane where they form the gel-like cortex of the cytoplasm. When actin monomers polymerize into microfilaments, the cytoplasm turns into a gel; when they disassemble, the cytoplasm turns into a viscous liquid.


The loose subunits are called G-actin (G for globular). They have a molecular weight (MW) of 42,000 and a nucleotide binding site that is occupied by ATP or ADP. These subunits can polymerize into a filament in which two strands are coiled gently around one another (Fig. 13.5). Microfilaments are dynamic structures that can be assembled and disassembled continuously.


image

Figure 13.5 Assembly and disassembly of an actin microfilament. The filament grows at the + end and is disassembled at the − end. image, Actin monomer with bound ADP; image, actin monomer with bound ATP.


The two ends of the actin filament are not equivalent. At the positive (+) end, addition and dissociation of actin monomers are fast. At the opposite end, the negative (−) end, both processes are slow. The bound nucleotide is also important. ATP-actin binds strongly to other actin monomers and tends to add to the microfilament, whereas ADP-actin binds weakly and tends to break away from the microfilament.


The large majority of free actin monomers in the cytoplasm contain a bound ATP. This form adds to the + end of the microfilament. In the microfilament, however, the ATP is hydrolyzed. When the concentration of G-actin is high, the addition of new actin monomers to the + end is faster than the hydrolysis of the bound ATP. As a result, the last subunits at the + end are in the ATP form, whereas the rest of the microfilament is in the ADP form. This filament tends to grow at the + end and frizzle away at the − end.


Cells have a bloated bureaucracy of proteins to regulate the formation, growth, and dissolution of microfilaments. Some initiate the formation of a new microfilament, some anchor the filaments to membranes or cytoskeletal structures, and others bundle them into networks or parallel arrays (Table 13.2).


Table 13.2 Proteins That Regulate Actin Microfilaments







































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Jun 18, 2016 | Posted by in BIOCHEMISTRY | Comments Off on The Cytoskeleton

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Protein Function
Thymosin Binds free actin monomers, making them unavailable for polymerization
Profilin Delivers actin monomers to growing microfilaments
ARP complex Nucleates microfilaments at the − end
Formin Binds to the + end of microfilaments, promotes elongation
Tropomyosin Strengthens microfilaments, regulates their length
image Prevents myosin from binding to actin/tropomyosin
image Link microfilaments into a gel
image Link microfilaments into parallel bundles
image Link microfilaments to the plasma membrane
Cap Z Caps and stabilizes the + end of microfilaments
Tropomodulin Caps and stabilizes the − end of microfilaments