9. Basic Functions of Microtubules
Figure 9-1. A microtubule segment from cryo-electron microscopy.
Structure of Microtubules
Most microtubules consist of 13 protofilaments, which form a hollow tube with about 25 nanometers in diameter. Each protofilament is made up of tubulin dimers: α and β. The α subunit of one dimer is attached to the β subunit of the next dimer. Thus, in a protofilament, one end (called "minus end") has the α subunit exposed while another end (called "plus" end) has the β subunit exposed. Note that the definition of "+" and "-" on both ends does not imply that the protofilament is an electric dipole. Under physiological conditions, protofilaments and the entire microtubule are highly negatively charged because the C-termini of both α and β subunits are rich in acidic residues (aspartate and glutamate) (Minoura and Muto, 2006 ).
Polymerization of a microtubule is usually from the plus end. In the absence of microtubule-associated proteins (MAPs), a microtubule may shrink. MAPs also regulate the connection between a microtubule and neighboring microtubules or other cellular structures. In general, binding of MAPs to a microtubule makes it more stable and rigid. Their binding is determined by MAP phosphorylation, which in turn is regulated by calcium ions.
In a neuron, the lengths of microtubules vary widely, ranging from less than 1 micron to more than 100 microns. Their polarities are unidirectional in the axon, with plus ends distal to the cell body, but are mixed in dendrites (Bass and Lin, 2011). The mixed polarities give the transportation system more flexibility, allowing motor proteins to move in either anterograde or retrograde direction.
Transport by Motor Proteins
The microtubule network is a major component of cytoskeleton responsible for intracellular trafficking of various cargos such as mRNA and proteins. In this transportation system, kinesin and dynein serve as the motor protein which can carry the cargo and "walk" along the microtubule. Kinesin walks toward the plus end while dynein walks toward the minus end (Schlager and Hoogenraad, 2009). At least 14 kinesins have been identified (Lawrence et al., 2004).
The motor protein can carry mRNA, proteins or organelles. Transport of mRNA is mediated by ribonucleoprotein particles (RNPs) containing the mRNA and RNA-binding proteins (RBPs), which recognize a spatial code in mRNA for dendritic or axonal targeting. At the destination, some mRNAs may be translated by the action of RBPs such as fragile X mental retardation protein (FMRP), whereas others, such as PKMζ mRNA, require different proteins. FMRP is the RBP for CaMKIIα, MAP1b and PSD-95 mRNA (Doyle and Kiebler, 2011).
Transport of proteins is typically mediated by vesicles which can contain many proteins, thus more efficient than transporting a single one. The vesicular transport requires protein palmitoylation, which may play an important role in long-term memory. Further details are given in the next chapter.