Drubin.Lecture 17.18_3-3-08_ - Drubin Lecture 17 and 18...

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Drubin – Lecture 17 and 18 Actin filaments, structural and dynamic properties, actin-binding proteins and cell migration I. The actin cytoskeleton functions in cell motility and in control of cell shape. Cell locomotion involves the ability of cells to crawl over a surface. It is important for embryonic development, defense against infection, wound healing and metastasis of cancer cells. Cells that do not move may change shape. These morphological changes including the contraction of muscle cells, elongation of nerve axons, formation of surface protrusions (microvilli, filopodia and lamellipodia), and cytokinesis. Cellular movements are a result of mechanical work that is carried out by proteins like actin and myosin, which convert the energy stored in ATP into motion. II. The properties of actin Actin is one of the most abundant proteins in eukaryotic cells . It comprises from 1- 20% of the total cell protein. Because they form structures that span the cell, cytoskeletal subunit proteins are generally abundant. The sequence of actin has been highly conserved through evolution. Actins contain 375 amino acid residues with a molecular weight of 42 kD. Actins from amoeba and humans are >80% identical to each other in amino acid sequence. Some simple eukaryotes like yeast have one actin gene. Mammals have several genes that produce multiple types of actin: α -actin in muscle, β and γ -actin in nonmuscle cells. The actin molecule is separated into two lobes by a deep cleft that binds ATP or ADP complexed with Mg 2+ . Actin exists in two forms in the cell – as a monomer called G-actin (globular actin) and as a filamentous polymer called F-actin (filamentous actin). F- actin is a polymer of G- actin subunits held together by non-covalent (hydrophobic and ionic) interactions. F-actin is a polar polymer. All of the subunits within an actin filament have the same polarity - they are all oriented in the same direction. Thus the two ends of a filament are different. At one end the ATP cleft is exposed, and at the other end it contacts a neighboring subunit. The polarity of actin filaments can be determined by decorating the filaments with the actin-binding “head” domain of the myosin II molecule (called subfragment-1 or S1). When myosin S1 is mixed with actin filaments, it attaches to the sides at an angle, creating an arrowhead like pattern. One end of the filament is called the pointed end (- end) and the other the barbed end (+ end). One important consequence of filament polarity is that the two ends of the filament have different functional properties. Figure 1: 3D structure of an actin monomer, from Alberts “Molecular Biology of the Cell” text.
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Drubin – Lecture 17 and 18 Actin filaments, structural and dynamic properties, actin-binding proteins and cell migration III. Actin polymerization dynamics.
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