Cytoskeleton

cytoskeleton

The system of protein filaments that enable the cell to ensure its structural integrity and rigidity, regulate its shape and the morphology, exert the forces and produce motion is known as the eukaryotic cytoskeleton. As a framework that ensures the structural integrity, the cytoskeleton is mainly constituted of a cohesive meshwork of the protein filaments that extend throughout the cytoplasm of the cell. But it being the essential structure that produces movement at the cellular level, and thereby needing to highly adaptable to the extracellular stimuli or rapid environmental changes, the cytoskeleton has evolved into the highly dynamic structure. The cytoskeletal filaments constantly grow and shrink, associate and the dissociate via multiple linkages, the organize on the large scales into a dynamic network, and serve as the intricated set of the tracks to motor proteins that transport the cargos from one part of the cell to the other, or slide filaments with the respect to one another to produce the contractile forces. This section is devoted to the biochemical description of very complex structure.

Biopolymers

With a diameter of 10 microns or more, be spatially organized by cytoskeletal proteins that are typically 2000 times smaller in the linear dimensions. The answer lies in polymerization, this ability of the elementary protein subunits to assemble via physical interactions into extended linear structures that are made of a large number of them, typically the thousands (called thereby polymers, or here more precisely biopolymers). There are three types of the biopolymers in a given eukaryotic cell, namely actin filaments, microtubules and intermediate filaments. Although they can be classified according to their respective thickness, more interesting for cellular structures and the functions are their rigidity, which at the thermodynamic equilibrium is characterized by their persistence length Lp. Actin is the most abundant protein in the eukaryotic cell and has been highly conserved throughout evolution. It organizes into the variety of structures, namely linear bundles, two-dimensional networks or three-dimensional gels, and is mainly concentrated in the layer located just beneath the plasma membrane and called the actin cortex.

Molecular motors

Molecular motors the constitute the subset of proteins and the macromolecular complexes that convert a given source of energy into mechanical work. The energy they need is generally stored into the either of two forms by the cell: high-energy chemical bonds, such as the phosphor- anhydride bonds found in ATP and GTP, and asymmetric ion gradients across membranes. Known molecular motors can be classified into the roughly five categories, namely rotary motors, linear stepper motors, assembly-disassembly motors, extrusion nozzles, and prestressed springs. A nice table of the major different cell-movements’ categories with the different cellular structures and the molecular motors they rely on, can be found in ref. All known the biological rotary motors use ion-gradient based sources of energy, and most of them use electrochemical forces based on hydrogen ion (or proton) gradients, also known as the proton-motive forces. This is the case for example for the propulsion motor of bacteria that is responsible for flagella to rotate, as well as for the surprising rotary motor F0F1-ATPase that is responsible for ATP synthase in mitochondria and bacteria. This rotary machine usually converts the electrochemical energy stored in the proton concentration gradients, first into mechanical motion, and then back into chemical energy under the form of ATP.

Motor families

Eukaryotic cytoskeletal motor proteins are divided into three superfamilies, namely the myosins, the kinesins, and the dyneins. The motor proteins known longest belong to the myosin superfamily , because of their high concentration in the skeletal muscles. All myosin motors walk on actin filaments through a general four-step process: binding, power-stroke, unbinding, and recovery stroke. Today, they are classified into 18 different classes, with possible dozens of the different members in each class, even in a single organism. The skeletal muscle myosins belong to the Myosin II family; they have the long tails that form dimeric α-helices and associate into the so-called “thick filaments”

Other cytoskeleton-associated proteins

Coordination of the numerous different processes that happen during amoeboid motility relies on the tight regulation of the activity of the cell cytoskeleton, as well as its anchoring to the substrate. In particular, as it shall see below, the protrusion of the leading edge of the cell - the first step of amoeboid motility - relies on the formation of the highly cross-linked and the dynamic network of actin filaments. Its formation and dynamical regulation are carried out with the help of the numerous accessory proteins. Following Pollard’s presentation, we can focus on the main proteins that are involved in the formation, structure, and dynamics of the actin network. Nucleation of the network starts after biochemical signals have been integrated via G-protein linked membrane receptors, namely small GTPases.

The prokaryotic cytoskeleton

These sections are completely independent of the rest of the article. Readers not interested in the biochemical composition of the prokaryotic cytoskeleton might want to skip this section. As cytoskeletal protein’s structures are highly conserved throughout the three domains of the life (archaea, bacteria, and eukarya), prokaryotic cytoskeletal proteins differ strongly in their sequences from eukaryotic counterparts. For this reason, and the fact that prokaryotes have a relatively simple organization as compared to the eukaryotic cells, it was long thought that they were lacking a cytoskeleton. It is only in the 1990s that the prokaryotic homologs of tubulin, actin, and intermediate filaments started to be discovered.

References

Dhami, P S, and J K Dhami. A textbook of zoology Vol. II and Vol.III. Latest edition. New Delhi: Pradeep publication, n.d.

Kotpal, R L. Modern textbook of Zoology. Meerut, India: Rastogi Publication, n.d.

Rastogi, S C. Cell, and Molecular biology. New Delhi: New Age International (P) Limited, 2001.

Verma, P S, and V K Agrawal. cell biology,Genetics,Molecular Biology,Evolution, and Ecology. New Dehli, India: S. Chand and company Ltd., 2012.