L11 - We are on chapter 16: the cytoskeleton Today we will...

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We are on chapter 16: the cytoskeleton Today we will fnish microtubules: binding proteins and Function and then move onto actin Exam next week 11/10/09 Class survey For the break
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Signaling networks are complicated….but I expect you to know the basic 4 and only 4 GPCR and RTK pathways
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HOW ARE THESE TUBULIN HETERODIMERS USED TO MAKE A MICROTUBULE? 1. Head to tail interactions form a protofilament. 2. Lateral interactions between protofilaments form a hollow tube - a microtubule There are 13 protofilaments/microtubule 25nM across - hollow inside is 14 nM across. No known function for hollow inside. MICROTUBULE STRUCTURE Microtubules are polymers of tubulin. There are two tubulin proteins - alpha and beta. Basic unit of the microtubule is the tubulin heterodimer - contains one alpha and one beta tubulin. Alpha and beta tubulin are highly related - 450 amino acids long Very abundant and highly conserved proteins.
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DYNAMIC INSTABILITY In situations where microtubules are nucleated at one end, assembly and disassembly of heterodimers occurs only at the + end Current model for this is the dynamic instability model 1- Minus end of microtubules lies in the MTOC - microtubule organizing center (see later) 2-growth occurs only at the plus end 3-loss occurs only from the plus end 4- two growth states - slow growth or rapid depolymerization 5-microtubule is stable only if the plus end reaches a cap
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TREADMILLING and under this circumstance elongation is still faster than hydrolysis at the + end (because adding a T form subunit is faster), but elongation becomes slower than hydrolysis at the – end so the terminal units go to the D form. As the filament grows, the subunit concentration falls. Usually in a cell, [GTP] > [GDP] so most free subunits are in the T form (GTP-form) and the filament grows from both ends. But the cell is dynamic and the rate of subunit addition changes with fluctuating subunit concentration.
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Treadmilling occurs at intermediate subunit concentrations, higher that Cc of the T form (the filament still adds subunits to the + end), but lower than the Cc of the D form (the filament simultaneously loses subunits from the – end) Critical concentration is the concentration at which the rate of subunit addition equals to that of subunit loss TREADMILLING
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MICROTUBULE STRUCTURE, ASSEMBY and DISASSEMBY
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The number and location of cytoskeletal filaments are tightly controlled within the cell •Tubulin subunits are present in the cell at concentrations above the critical concentration. •Yet most tubulin is not assembled into polymer. •How do cells ensure that microtubules and actin filaments form at the correct time and place?
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MAPS: Microtubule Associated Proteins
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Cells use multiple mechanisms to regulate microtubule assembly, disassembly and organization in the cell: 1) Nucleating sites determine where polymers form. 2) Proteins that bind free subunits and prevent assembly
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L11 - We are on chapter 16: the cytoskeleton Today we will...

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