MCB 252 Topic 24 Microtubule Organizing CentersProf David Rivier MCB 252 Spring 2015 MCB 252 Topic 24 Microtubule Organizing Centers Reading Lodish 18 5 and the section of 18 6 entitled Centrosomes duplicate early in the cell cycle in preparation for mitosis Microtubule Organizing Centers MTOCs Spontaneous polymerization of MTs is highly unfavorable So instead of spontaneous nucleation cellular MTs arise from nucleation sites called microtubule organizing centers Centrosomes basal bodies Microtubule organizing centers MTOCs Centrioles and Basal Bodies Cilia and Flagella Lodish Fig 18 5 Specialized MT Structures found in MTOCs Cilia and Flagella 3 types of Microtubules MTOCs Centriole and Basal Body Structure Array of 9 triplets Microtubule organizing centers MTOCs Basal body Axoneme Basal body Basal bodies at base of cilia and flagella Pollard and Earnshaw Fig 41 12 Basal Bodies are Nucleation Centers for Axoneme Assembly Basal Bodies Axoneme Axoneme MTs are extensions Basal Bodies MTs Doublet extends from 2 3 MTs in Triplet Basal body caps and anchors minus ends of axoneme MTs Structure of Cilia and Flagella Relation to Basal Body Centrosomes Lodish 18 5 1875 Flemming and Van Benedon First visualized cellular components nucleus centrosome Ironic in that it is still one of the least understood components Centrosome body in the center of the cell Nucleate cap and anchor minus ends of MTs Centrosomes Spindle Poles Centrosome and Centriole Structure See also Lodish 18 6 Centrioles are surrounded by pericentriolar material PCM Centrosome centrioles PCM Gamma tubulin Genetic studies identified a new tubulin called gamma tubulin Shown to localize to centrosomes gamma tub tubulin Component of pericentriolar material Antibodies against gammatubulin block MT assembly DAPI J Cell Sci 109 875 867 1998 Gamma tubulin exists in complex with other proteins gamma Tubulin Ring Complex TuRC immuno gold EM tubulin gamma tubulin Lodish Fig 18 7 Gamma tubulin exists in complex with other proteins gamma Tubulin Ring Complex TuRC purified gamma turc viewed by EM Alberts Figs 16 22b Lodish Fig 18 7 TuRC is in PCM not part of the centrioles MTs nucleated in the PCM not from the centriole Centrosomes Nucleation and Anchoring Microtubule organizing centers MTOCs Centrosome pericentriolar material PCM Centrioles Lodish 18 6 Centrosome Duplication Precise control of number of centrosomes Cell cycle regulated Lodish 18 35 Centrosome Duplication Key points 1 cell cycle to form a fully matured centriole Centrioles nucleate formation of daughter centrioles Centrioles can form de novo if destroyed Centrosome Formation and Maturation Green mature mother Yellow Semi mature daughter Red immature newborn M S G1 G1 S Logic 1 active Centriole in Interphase 2 in Mitosis one in each Mitotic Spindle M Mature and Immature Centrioles within the Centrosome Alternate pathway of MTOC formation Organization of severed MTs and formation of an MTOC even in the absence of centrioles Centrosomes and MTOCs Mysteries and Unusual Features Number of centrosomes is regulated how Centrosome assembly requires maturation over at least 2 cell cycles Centriole duplication vs de novo formation MTOCs can form without centrioles Centrosomes organize the PCM but the structure and organization of the PCM is unclear not known Centering activity Centering is an intrinsic property of centrosomes and microtubules Purified centrosomes tubulin GTP centrosome moves to the center No motors here centering must be a property of microtubule dynamics or physical properties What is the Role of the Microtubule Cytoskeleton Does the organization and properties of the microtubule cytoskeleton give you any ideas Centrosomes Centrosomes and MTs A map or grid that provides positional information within a cell Search and Capture Making a map by trial and error Regulation of Centrosome Activity Cell cycle dependent phosphorylation parallel to cell cycle phosphorylation of MAPs Roles of MTs in Interphase and Mitosis In Mitosis Lodish 18 5 1 Cease Interphase Roles Organelle transport and positioning 2 New Role Attach to Position and Move Chromosomes