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Section 4

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Biology 2382B
Sashko Damjanovski

SECTION 4 Intermediate Filaments • Not globular, form from large proteins having head and tail domains • Don’t needATP or GTP to polymerize; when they do polymerize, there is no polarity (no (+) or (-) ends) • No known motor proteins, though there are proteins that bind to IFs and function • Have great tensile strength, great at holding tissues together Main Types of Intermediate Filaments Memorize 5 types in the chart • Most intermediate filaments are cell type specific Intermediate Filaments: Structure • IF proteins have head, rod and tail domains; first form a dimer then form a tetramer (two dimers together) o When forming a dimer, that dimer has polarity o But, basic fundamental unit is not a dimer, it is a tetramer • When polymerizing and depolymerizing IFs, the tetramer is the basic subunit • Because of the way the dimers form the tetramer, the tetramer has no polarity • Tetramer has C- and N-terminal domain on both sides • Tetramer comes together and polymerize to form a polymer (protofilament); polymerization does not requireATP or GTP • When polymerized, the tetramer makes an IF • Though no energy is required, IFs in a cell are dynamic – constantly remodeled IFs are in dynamic state, proteins within filament are exchanged • IFs are not static, they are dynamic – constantly changing, but don’t do it as quickly since they don’t require energy • Need to disassemble during mitosis – cyclin dependent kinase • N-terminal domain of laminA phosphorylated at serine residue • Induces disassembly, and prevents reassembly • If serine residue is mutated, no disassembly • Balance of opposing action of kinases and phosphotases is crucial • Intermediate filaments are not as dynamic as actin and tubulin • If this cell had to undergo mitosis, it will have to rearrange things • Actin and IFs also have to be depolymerized and rearranged • Regulated by cyclins • Some of this is responsible for depolymerizing the IF network • Lamins are the IFs found in all cells, and they support the nuclear membrane • Lamin network must be depolymerized, occurs through phosphorylation • Has a specific serin residue, which when is phosphorylated induces disassembly and prevents assembly • Becomes less stable and the nuclear membrane falls apart, the lamin cannot reassemble • If the serin residue is removed, mitosis is blocked as the nuclear membrane cannot be broken Intermediate FilamentAssociated Proteins • Only organizational proteins identified (no motors, caps, severing) • IFAPs are proteins that bind to IFs but we don’t know what they do • Just like MAPs, IFAPs tell you nothing about function – generally link filaments to other components in the cell • Plectin – links IFs to microtubules o Involved in maintaining stability between different cytoskeletal components IF Functions – Membranes • Lamins support nuclear membrane • Lamin supports the nuclear envelope, must be bound to it in order to support it • LaminsA&B form network- linked to nuclear lamina through lamin associated polypeptide (LAP2) – an intermediate filament associated protein • IFs provide structural support necessary for cell shape – eg. vimentin links to ankyrin • Provides structure and strength • Ankyrin is a plasma membrane-associated protein; it binds the PM, can be used to support it by linking it to the cytoskeleton • Ankyrin can be linked to actin, but also to vimentin – thereby, can support PM by actin cytoskeleton or IF cytoskeleton Keratin Mutations • Transgenic mice carrying a mutant keratin gene exhibit skin blistering – weak skin integrity • IFs anchored to desmosomes and hemidesmosomes • Found in epithelial cells, protein found in nails, hair, skin, etc. • In your epidermis, keratin’s job is to hold things together • Epidermis has to both stick to itself and underlying dermis, resiliency of skin is because of keratin • Epidermal cells are constantly dividing and epidermal cells basically lose their cytoplasm, and what you end up with is dead epidermal cells that contain nothing but keratin • Top layer of cells are empty cells that have keratin in them – provides a barrier • Underneath is a living layer of epidermis, where keratin provides strength • Keratin in epidermal cells links living cells together, does this by providing desmosomes and hemidesmosomes IF Functions – Junctions • IFs are anchoring parts of the cell to structures desmosomes and hemidesmosomes • Layer of epidermal cells – provide a barrier, thus have to be stuck together well and stuck to the underlying extracellular matrix • Plasma membrane is weak, need something strong in epithelial and epidermal cells (such as keratin) • Keratin links membranes and cells together, then links them to the ECM below them – forms a network, through linkage and holds epidermal cells together • Desmosome – adhesion junction found between cells that are linked to IFs • If it is linking the cell to extracellular matrix – hemidesmosome • Desmosomes (cell to cell) and hemidesmosomes (cell to ECM) = IF • Keratin = epidermis Cell Junctions and CellAdhesion • 4 types – tight junctions, gap junctions, cell-cell adhesions, cell-ECM adhesions • IFs and actin cytoskeletal components are involved in cell adhesion • For cell-cell adhesion, linking PM to cytoskeleton o Actin is cortical, at PM o IFs can also interact at PM – involved in cell adhesion o MTs never really get to PM – do not play a role in cell adhesion • Epithelial cells have to provide a barrier and provide strength, various cell junctions and adhesion structures do this • Gap junctions and tight junctions – hold cell together but do not provide adhesion or strength • Tensile strength is provided by other cell adhesion complexes such as desmosomes, hemidesmosomes, and adhesion junctions o These structures are all connected to the cytoskeleton Gap Junctions • In between adjacent cells • Form a channel between cytoplasm of two cells, allows small molecules to pass between both cells • Made up of protein called connexin • 6 connexin molecules on one cell form half of a channel, this channel is called a connexon, and 6 connexin on another cell forms the other half of the connexon • There are many different types of connexons, size varies and what passes through the channel varies • If you imagine an epithelial layer of cell, all these cells are linked together through gap junctions • Gap juncti
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