MAPs bind to the tubulin subunits that make up microtubules to regulate their stability. A large variety of MAPs have been identified in many different cell types, and they have been found to carry out a wide range of functions. These include both stabilizing and destabilizing microtubules, guiding microtubules towards specific cellular locations, cross-linking microtubules and mediating the interactions of microtubules with other proteins in the cell .
Within the cell, MAPs bind directly to the tubulin monomers of microtubules. This binding can occur with either polymerized or depolymerized tubulin, and in most case leads to the stabilization of microtubule structure, further encouraging polymerization. Usually, it is the C-terminal domain of the MAP that interacts with tubulin, while the N-terminal domain can bind with cellular vesicles, intermediate filaments or other microtubules. MAP-microtubule binding is regulated through MAP phosphorylation. This is accomplished through the function of the microtubule-affinity-regulating-kinase (MARK) protein. Phosphorylation of the MAP by the MARK causes the MAP to detach from any bound microtubules . This detachment is usually associated with a destabilization of the microtubule causing it to fall apart. In this way the stabilization of microtubules by MAPs is regulated within the cell through phosphorylation.
The numerous identified MAPs have been largely divided into two categories: Type I including MAP1 proteins and type II including MAP2, MAP4 and tau proteins.
Type I: MAP1
MAP1a (MAP1A) and MAP1b (MAP1B), which make up the MAP1 family, bind to microtubules differently than other MAPs, utilizing charged interactions . While the C-terminals of these MAPs bind the microtubules, the N-terminals bind other parts of the cytoskeleton or the plasma membrane to control spacing of the microtubule within the cell. Members of the MAP1 family are found in the axons and dendrites of nerve cells .
Type II: MAP2, MAP4 and tau
Also found exclusively in nerve cells are the most well studied MAPs, MAP2 (MAP2) and tau (MAPT), which participate in determining the structure of different parts of nerve cells - MAP2 being found mostly in dendrites and tau in the axon. These proteins have a conserved C-terminal microtubule-binding domain and variable N-terminal domains projecting outwards probably interacting with other proteins. MAP2 and tau stabilize microtubules, and thus shift the reaction kinetics in favor of addition of new subunits, accelerating microtubule growth. Both MAP2 and tau have been shown to stabilize microtubules by binding to the outer surface of the microtubule protofilaments ,. A single study has been suggested that MAP2 and tau bind on the inner microtubule surface on same site in tubulin monomers as the drug Taxol® which is used in treating cancer . However, the evidence is in favor of MAP2 and tau binding to the outer microtubule surface and this study has not been confirmed. MAP2 binds in a cooperative manner with many MAP2 proteins binding a single microtubule to promote stabilization. Tau as well helps to stabilize microtubules, however it forms the additional, important function of facilitating bundling of microtubules within the nerve cell .
The function of tau has been linked to the neurological condition known as Alzheimer's Disease. In the nervous tissue of Alzheimer's patients tau forms abnormal aggregates. This aggregated tau is often severely modified, most commonly through hyperphosphorylation. As described above, phosphorylation of MAPs causes them to detach from microtubules. Thus, the hyperphosphorylation of tau leads to massive detachment which in turn greatly reduces the stability of microtubules in nerve cells . This increase in microtubule instability may be one of the main causes of the symptoms of Alzheimer's Disease.
In contrast to the MAPs described above, MAP4 (MAP4) is not confined to just nerve cells, but rather can be found in nearly all types of cells. Like MAP2 and tau, MAP4 is responsible for stabilization of microtubules . MAP4 has also been linked to the process of cell division .
Other MAPs, and naming issues
Another MAP whose function has been investigated during cell division is known as XMAP215 (the "X" stands for Xenopus). XMAP215 has generally been linked to microtubule stabilization. During mitosis the dynamic instability of microtubules has been observed to rise approximately tenfold. This is partly due to phosphorylation of XMAP215, which makes catastrophes (rapid depolymerization of microtubules) more likely . In this way the phosphorylation of MAPs plays a role in mitosis.
There are many other proteins which affect microtubule behavior, such as catastrophin, which destabilizes microtubules, katanin, which severs them, and a number of motor proteins that transport vesicles along them. Certain motor proteins were originally designated as MAPs before it was found that they untilized ATP hydrolysis to transport cargo. In general, all these proteins are not considered "MAPs" because they do not bind directly to tubulin monomers, a defining characteristic of MAPs . MAPs bind directly to microtubules to stabilize or destabilize them and link them to various cellular components including other microtubules.
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