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Anatomy & Cell Biology
ANAT 262
John Presley

Cytoskeleton: Cytoskeleton is made of three components: 1) microtubules 2) actin 3) intermediate filaments Main functions of the cytoskeleton are: 1) structure and support - determination of cell shape - establishing intracellular contacts - actin contraction allows the cell to move away from a repulsive cue 2) intracellular transport (highway to move vesicles) - intracellular transport nerve cell sends out growth cone to sense the environ- ment 3) contraction and motility (ex: muscle cells) - cell division - centrioles organize microtubules (negative at centrioles and positive end out) 4) spatial organization - signal transduction - tensile strength - cell is squishy, it has structure and resistance (tensile strength) The term cytoskeleton may in one sense have an unfortunate connotation since it im- plies that the various structural elements making up the cytoskeleton form a static struc- tural framework similar to the bone skeleton that frames the body - well this only partly is true because certain types of cytoskeleton elements are more or less permanent features of the cell - examples of such cytoskeleton structures include the actin and myosin filament bun- dles in muscle cells and the microtubules arrays in cilia and flagella Other cytoskeleton structures are very dynamic, continuously assembling and disas- sembling as part of their functional cycle for use in various cellular processes - perhaps the most dramatic example of such a dynamic process is the complete re- modeling of the microtubule array of a cell during mitosis - when it changes from a net- work radiating throughout the cell to a compact, bipolar, mitotic spindle - at interphase, the cell needs bipolar mitotic spindle in order to divide F-actin: 8 nm in diameter and is highly concentrated at the cell cortex and is associated with the plasma membrane - looks like 2 chains twisted together, but this is just the way actin polymerizes Microtubules: 25 nm in diameter and is a hollow tube made of tubulin - there are 13 laterally associated protofilaments each being a linear polymer - one end of the microtubule is attached to the centrosome Intermediate filament: 10 nm in diameter, a complex assembly of tetramers made from subunits - various types of IF subunits - over 50 cytoplasmic IFs and nuclear lamins - ex: desmosomes in epithelial cells The basic structural elements of the cytoskeleton do not act alone - there is a multitude of cytoskeleton associated proteins that anchor, cross-link and oth- erwise regulate these major elements - the ability of actin, tubulin, and IF proteins to form polymers, to depolarize, to interact, elongate and contract in conjugation with subtle cell movements and shape changes gives the cytoplasm its unique properties of life The organized arrays of fibrous elements interact to form a highly integrated structural network called the cytoskeleton - this cytoskeleton gives the cytoplasm its unique properties Actin and microtubules are found in all eukaryotic cells - tubulin is generally present in smaller amounts than actin, except in neurons where it is very abundant - prokaryotes have analogous proteins (FtsZ protein is bacteria tubulin and MreB protein is the bacteria’s F-actin) - cytoplasmic intermediate filaments are only in multi-cellular organisms - IF have no motors because there is no difference (polarity) between the two sides (so it would not know where to go) Properties shared by actin and microtubules: 1) AFs and MTs are polar structures - the ends of the linear polymers are different (IFs are non-polar) - AFs and MTs have an inherent polarity because the subunits in each chain are ar- ranged in a specific head to tail orientation - the two ends of AF or MT have different growth properties - polymerization at one end is more rapid than at the opposite end - the fast growing end is called the plus end - the slow growing end is called the minus end - MTs and AFs grow and shrink only from their ends (besides during severing activity when they split into two halves), whereas IFs can undergo subunit exchange along their length - the reason for the difference in growth rate at the plus and minus end is due to changes in conformation of the subunit as it is incorporated into the polymer The polarity of an AF is determined by decorating it with a
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