Microtubules by Jessica Belliveau

Introduction

Figure 1. A. The microtubule is in dynamic instability with consistent growing and shrinking. B. The cycle of growth (polymerization) and shrinking (depolymerization)

Microtubules, along with actin and intermediate filaments, compose the cytoskeleton of eukaryotic cell. Microtubules are involved with mitosis, cell movement, intracellular transport, and cell structure. Microtubules (MT) are made up of the globular protein, tubulin. The two main forms of tubulin found in MT are α-tubulin and β-tubulin. α-tubulin and β-tubulin spontaneously bind to form a dimer and many tubulin dimers form together into protofilaments. The protofilaments assemble into MT. Microtubules are polar structures with a fast-growing positive end and a slow-growing negative end. At the site of MT attachment to a cellular unit at the slow-growing negative end are γ-tubulin rings, responsible for the nucleation of MT.

Mitosis

Mitosis is the process of nuclear division represented in four stages: prophase, metaphase, anaphase, telophase, followed by cytokinesis. MT, also called spindle fibers, are responsible for establishing cell polarity and pulling the sister chromatids apart into daughter chromatids on opposite sides of the cells. MT attach to the chromosomes at the centrosome and through depolymerazation, pull the daughter chromosome. The centrosome has been traditionally thought as the MT assembly initiation site, however, it is now know MT attach to organelles such as the endoplasmic reticulum (ER).

Through the transition from interphase to mitotic phases, the MT structure completely restructures. First, the rates of MT disassembly increase leading to a decrease in cell polarity. Second, the rate of MT growth starting from the centosomes increase, reestablishing cell polarity in the direction the MT will eventually pull the daughter chromatids.

Figure 2. The interphase cell has a cell polarity with the positive ends on the perimeter of the cell and the negative ends in the center of the cell attached to the centromere. During the mitotic phase, the same polarity does not exsist. Instead, MT form a circle around the centromeres allowing for some positive ends to be in the center of the cell

Intracellular Transport and Cell Movement

Intracellular Transport

MT are used as a transportation network to transport membrane vesicles through the cell. One example of using MT for transport is in nerve cell axons. The axons can be over a meter long, however, ribosomes are only located in the cell body and dendrites. Using the MT network, proteins, membrane vesicles, and mitochondria are transported from the cell body and dendrites to the axons. MT also transport cellular stress responses throughout the cell. The two main types of motor proteins are dyneins and kinesins. Dyneins consist of light and intermediate chains that are ATP-binding domain that provide the energy necessary to move the protein. Kinesins consist of