Drubin Lecture 24 - Drubin - Lecture 24 Regulation of...

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Drubin - Lecture 24 Regulation of microtubule organization and motility Microtubule-binding proteins. Microtubule binding proteins fall into the same broad categories as actin-binding proteins, with a few exceptions. These classes include: I. Proteins that affect polymerization and depolymerization: Tubulin dimer binding proteins: Stathmin/Op18 binds tubulin heterodimers, promotes catastrophe and depolymerization. Nucleating proteins: The γ TURC nucleates MT assembly at the centrosome. End binding proteins: CLIP-170 binds to the (+) ends of MTs. It is thought to mediate the interaction of chromosomes and membranes with the (+) end. Severing proteins: A protein called katanin severs MTs and may promote depolymerization at (-) ends. Depolymerizing proteins: KinI kinesins , proteins that share sequence similarity with the MT motor protein kinesin, promotes MT depolymerization at the (+) end by binding to and inducing protofilament curling. This results in the destabilization of the MT end and susequent depolymerization. KinI kinesins can promote the depolymerization of GTP tubulin, so the mechanism does not involve hydrolysis of the GTP cap. II. Proteins that influence MT organization (Microtubule Associated Proteins or MAPS). General properties: (1) stabilize MTs by binding to their sides and inhibiting disassembly (2) enhance assembly by stabilizing nuclei and thus facilitating nucleation (3) organize MTs into bundles in various cellular structures (4) mediate MT interactions with other proteins in the cell, including intermediate filaments and actin. Organization of MAPS: Maps have two major domains. The microtubule binding domain binds several tubulin dimers at once and thus helps to stabilize the polymer. The projection domain can interact with MTs or other structures such as intermediate filaments. MAP2 and Tau : MAP2 and Tau are responsible for organizing microtubules in neuronal axons and dendrites. When MAP2 and tau are expressed in cells that do not normally form axons, long axon like structures are induced. The organization of MTs in the MAP2 and Tau induced structures are similar to one another with one exception. Since the projection domain of MAP2 is longer, the spacing between microtubules is greater in the MAP2 expressing cells. III. Microtubule motor proteins. Advances in video microscopy in the 1970s and 1980s facilitated the observation of organelles and particles moving along single MTs. This phenomenon could also be observed when cell cytoplasm (from the axons of giant neurons in squid) was added to MTs in the presence of ATP. These in vitro motility assays were used to isolate two
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Drubin Lecture 24 - Drubin - Lecture 24 Regulation of...

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