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Lecture 15

Lecture 15: "Intermediate Filaments"

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Biology 2382B
Robert Cumming

Cell Biology Lecture No. 15: Intermediate Filaments th Wednesday March 6 , 2013 LECTURE 14 (CONT’D) Myosin & Cytoplasmic Streaming: -The cytoplasmic movement of algal Nitella cells can be readily observed with a simple microscope. The center of a Nitella cell is filled with a single large water-filled vacuole, which is surrounded by a layer of moving cytoplasm and a non-moving layer of cortical cytoplasm filled with chloroplasts lies just under the plasma membrane. On the inner side of this layer are bundles of stationary actin filaments, all oriented with the same polarity. A plant myosin V motor protein carries parts of the endoplasmic reticulum along the actin filaments (that comprise the inner layers encircling the moving cytoplasm). The movement of the ER network propels the entire viscous cytoplasm, including organelles that are enmeshed in the ER network. Cell Migration & Chemotaxis: -Cell migration is crucial to the development of the immune system of organisms especially during embryonic stages. Although leucocytes engage in cell migration to a large extent, adult eukaryotes have cells that typically show very little migration because too much of this process as an adult could result in severe conditions such as cancer. Contractile stress fibres associated with cell adhesion are long actin fibres which are interacting with myosin and other regulatory molecules; they are involved in cell movement as well. The meshwork of actin filaments in the leading edge advances the cell forward. Contractile fibres in the cell cortex squeeze the cell body forward, and stress fibres terminating in focal adhesions also pull the bulk of the cell body up as the rear adhesions are released. The structure of focal adhesions involves the attachment of the ends of stress fibres through integrins to the underlying extracellular matrix. Focal adhesions also contain many signalling molecules important for cell locomotion. The dynamic actin meshwork in the leading edge is nucleated by the Arp2/3 complex and employs the same set of factors that control assembly and disassembly of actin filaments in the tail of Listeria. Cell surface receptors are needed to respond to chemical signals (cytokines), which (like GTPases/GTP proteins) must be on all parts of the membrane. For example, a leucocyte would have membrane receptors specific to cytokines originating from an infection site in the body. When it receives this chemical signal, this membrane receptor is going to trigger migration in a specific direction through the polymerization of actin (dependent upon GTPases that become activate upon reception of the signal). The Four Steps Of Cell Movement: -Everywhere a focal adhesion is present, there are contractile stress fibres connected that hold the cell in place during adhesions. When a particular signal is received by the cell to move in a given direction, this activates certain GTP proteins that assist in actin polymerization. Throughout each step of cell movement, the lamellipodiae and filopdiae are constantly searching for a place to adhere. The four steps of cell movement begins with the extension of one or more lamellipodia from the leading edge of the cell. Some lamellipodiae adhere to the substratum by focal adhesions. Then the bulk of the cytoplasm in the cell
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