Microtubules

• Wall of each is composed of globular proteins arranged in longitudinal rows (protofilaments) that are aligned paraleel to the long axis of the tubule
• -          actually contains 13 of these protofilaments side by side in a circular pattern held by noncovalent interactions

Protofilament: assembled from alpha and beta dimers, which together are a dimer and are arranged in a linear array along length of protofilament

• -          each dimer has polarity and each protofilament has the same polarity
• o   one end is plus, which is terminated by a row of beta-tubulin subunits
• o   the other is minus and is terminated by a row of alpha-tubulin subunits

• -          some can be labile, like those of the mitotic spindle, or highly stable
• disassembly can be induced by cold temp, hydrostatic pressure, elevated calcium ion concentrations, and chemicals

• Structural support and organizer:
• -          Can resist forces that might compress or bend the fiber; mechanical support; can determine shape of the cell
• -          In plants: maintain cell shape through influence of formation of cell wall
• In animals: maintain internal organization of cells

• Agents of Intracellular Motility
• -          serve as tracks for a variety of motor proteins that generate the forces required to move objects within a cell
• ex: axonal transport and neurotransmitter travel down the axon and RNA transport down stretch of extended cytoplasm

• Heterogeneous class of proteins
• Structure: one domain attaches to side of microtubule and other projects outward as a tail from the microtubule surface; can be seen as cross-bridges connecting microtubule to each other, maintaining their parallel alignment

• Function: increase stability of microtubules and promote their assembly
• How are they controlled? phosphorylation and dephosphorylation 

• Involved in nucleation (slow) and elongation
• -          nucleation takes place rapidly inside a cell, where it occurs in association with the MTOC
• Centrosome: site of microtubule nucleation
• -          contains two centrioles surrounded by PCM
• o   microtubules don’t actually penetrate into this region
• o   it is the PCM that initiates formation of microtubules

• §  centrioles are not directly involved in microtubule nucleation, but recruit the PCM during assembly and in centrosome duplication

• o   contains nine evenly spaced fibrils, each which appears as a band of 3 microtubules, and it is connected to the center of the centrioles by a radial spoke; arranged in a pattern that gives a centrioles a pinwheel appearance
• o   always found in pairs at right angles

• -          polarity of microtubules: minus end associated with the centrosome and the plus end at the opposite tip
• o   microtubules are nucleated at the MTOC and elongated at the opposite end 

• -          the centrosome of a non-polarized cell is situated near the center of the cell and tends to remain associated with the minus ends of microtubuels
• -          in polarized cells, the microtubules are anchored by their minus ends at dispersed sites near the apical end of the cell as their plus ends extend toward the basal surface

-          outer microtubules in a cilium or flagellum are generated as outgrowths from the microtubules in the basal body, which are identical in structure to centrioles and can give rise to one another

• -          function: control the number of microtubules, their polarity, the number of protofilaments that make up their walls, and the time and location of their assembly
• all contain gamma tubulin