STRUCTURE OF INTERMEDIATE FILAMENTS
8.2.1. Structure :
All intermediate filaments share a common structural organization. The intermediate filament proteins appear to have a control -helical rod domain. Which is composed of four, a-helical segments (1A, 1B, 2B, 2A) and separated by three linker regions.
The NH2 (amino) and carboxy terminal of intermediate filament, are not a helical structure. They show wide variation in their lengths and sequences across the intermediate filament families. The basic building block of intermediate filaments is regid and parallel dimer. Two dimers then line up side by side to form a antiparallel tetramer of four polypeptide chains. This tetramer further show organisation to form higher level of arrangement. This tetramer is analogous to the tubulin heterodimer or G-Action filament. Intermediate filaments (IFs) lack polarity and can not participate in cell motility and intra cellular transportation.
Functions of intermediate filaments
Keratin filaments constitute the primary structure proteins of epithelial cells. e.g., linear hepatocytes, epidermal cells, pancreatic aciner cells etc. Intermediate filaments network serve as scaffold for organizing and maintaining cellular architecture and absorbs mechanical stresses, which are applied by the extracellular environment. Intermediate filaments provide mechanical strength to cells, situated in epithelial layers. Desmin plays role in maintaining the alignment of the myofibril of muscle cells. Intermediate filaments have tissue specific functions also which are more important in some cells than in others.
8.3. Micro filament
Micro filaments are the thinnest filaments of the cytoskeleton found in the cytoplasm of eukaryotic cells. These are linear polymers of actin subunits.
Micro filaments are highly versatile, show amoeboid movement functioning in cytokinesis and it changes the cell shape. They function as part of actomyosin driven contractile molecular motors and including cell motility.
Actin filaments show assembly in two general types of structures : bundles and networks. Bundles are composed of polar filament arrays or non polar arrays. Cross linking proteins, determine filament orientation and spacing in the networks and bundles. These structures are regulated by many other classes of proteins.
8.3.1. Actin filament
Actin is the major cytoskeletal protein of most cells. Which is polymerized to form the actin filaments. Thus filaments are thin and flexible which are approximately 7 nm in diameter and may be up to several mm in length. Action filaments are particularly abundant beneath the plasma membrane, where they form a network, which provides mechanical support and allow movement of the cell surface.
Assembly and Disassembly of Actin Filaments
Actin filament was first isolated from muscle cells. Each actin filament is made up of G-actions (globular actin), which is of 43 kDa (375 amino acids) and has ATPase power.
These actin monomers polymerize and form F actin filament (filamentous). Two parallel F-actins, twist around each others (in a right handed helix) and form an actin filament. An actin filament is a polar structure, which has a slow growing negative end and a faster growing (barbed end) positive end. The rate of their polymerization into filaments equals to the rate of dissociation. So the monomers and filaments show apparent equilibrium.
The actin monomers also bind with ATP, which is hydrolyzed to ADP for assembles of filament. Actin monomers, which are ATP bound, polymerize more rigidity, than those which are ADP bond. Actin polymerization is reversible. The filaments show depolymerization by dissociation of G-actin subunits. All this process occur in 3 steps : Nucleation, elongation and steady state phase. There is no changes in the total length of filaments.
There are some drugs, which affect polymerization of actin. They bind with positive ends of acting filaments and block their elongation. eg. cytochalasin D. Another drug is Phalloidin, which binds tightly with actin filament and stabilize them against depolymerization.
Myosin was first isolated from mammalian skeletal muscle tissue. Myosin is motor protein and actin filaments are tracks, along which myosin show movement. Myosis have different major classes. Myosins have two classes. Each class has its own specialization functions. Interaction of myosin with actin show a special form of movement called as “contraction”.
Myosin I and Myosin II, are most abundant proteins, present in nearly all eukaryotic cells. All myosins are composed of one or two identical heavy chains and a variety of low molecular weight (light) Chains. The each heavy chain has three structurally and functionally differs domains. A globular head domain is present at N-termines, contains actin and ATP binding sites. The head hydrolyzes ATP. By the energy of hydrolyzed ATP myosin filaments walk towards the end of an actin filament. The head is followed by a -helical reck region. Which associates with light chains and show regulation of the activity of the head domain. The tail domain has the binding sites which allow the molecules to polymerize into bipolar filaments and determine the specific activities of a particular myosin. The tail-tail interactions forms large bipolar filaments, that contains several myosin heads.
Many other types of myosin I, III, IV, XIV are also found in non-muscle cells. Myosins may be involved in a variety of other kinds of cell movements and vesicles transport, Phagocytosis, extension of pseudopodia in amoeba cell locomotion etc.