A pair of orthogonal disks, the centrioles, are the source, the microtubule organizing center (MTOC), from which microtubules radiate. The microtubule protein, tubulin, is added in pairs like bricks stacked spirally up from a base, the MTOC, to make a cylindrical, hollow tower. Other proteins, molecular motors, carry cargo organelles or macromolecules from the nucleus to the cytoplasmic membrane, or in the reverse direction.
Dynamic Instability Means Continuous Elongation or Disassembly
The MTOC holds what is considered the minus end (-) of the microtubule. Tubulin proteins are activated by GTP and are stacked into the growing microtubule. The GTP remains bound to the newly incorporated tubulin, but then spontaneously hydrolyzes to GDP. As long as the tubulin exposed at the growing, plus (+) end is in the GTP form and continues to grow, then the microtubule is stable. If growth slows long enough for the end-exposed tubule to change to the GDP form, then the microtubule becomes unstable and disassembles back toward the MTOC.
This growth-requirement is called dynamic instability. As a consequence of dynamic instability, microtubules continuously grow from the MTOC and then shrink again. Microtubules that extend all the way to the region of cytoplasm underlying the cytoplasmic membrane, the cortex, can bind capping proteins to the plus ends to inhibit disassembly.
Microtubules are Polar and Motor Proteins Move In or Out
There are two types of motor proteins that use ATP to move either from the MTOC to the cytoplasmic membrane, kinesins, or toward the nucleus, dyneins. These proteins have legs and feet that change conformations by binding and hydrolyzing ATP to walk along the microtubules.
Other accessory proteins bind to molecular cargos and to one or the other type of motor proteins. Thus dyneins attach to the nucleus and to importin proteins to move the nucleus to the cell center or to carry proteins for transport into the nucleus. All of the proteins that are involved in binding to DNA for replication or gene expression are transported to the nucleus on dynein/importin complexes. Alternatively, enzymes involved in glucose metabolism, glycolysis, are translated on ribosomes in the cell cortex from mRNA that is carried by kinesins to the cortex.
ER and Golgi are Extensions of the Nuclear Membrane
The MTOC defines the center of the cell and is pushed into that location by the uniform extension of microtubules to the cytoplasmic membrane. The nucleus is thrust toward the MTOC by dynein motors attached to the intermediate filament mesh that lines the surface of the inner nuclear membrane. The outer nuclear membrane has integral membrane proteins that bind to kinesins that attach to the microtubules and extend the membrane in long fingers toward the cell surface. These fingers, or tubes of membrane are the origin of the endoplasmic reticulum.
Vesicles Travel Between the Layers of the Golgi on Microtubules
Vesicles bud from the ER and kinesin-binding proteins embedded in the vesicle membranes carry the vesicles to the Golgi Apparatus. Vesicles bud and fuse to carry proteins and polysaccharides modified or polymerized in the Golgi from one layer to the next and ultimately to the cell surface. The movement of all of these vesicles and those that return membrane from the cytoplasmic membrane all the way back to the nucleus is controlled by kinesin and dynein motor proteins moving on microtubules that radiate from the MTOC.
Microtubules are Part of the Cytoskeleton
Microtubules are the largest fibers of the cytoskeleton. Microtubules guide movement of cell components within the cell. The other cytoskeletal fibers, intermediate filaments and actin microfilaments, provide tensile strength and dynamic control of the cytoplasmic membrane, respectively.
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