MICROTUBULE ASSOCIATED PROTEINS (MAPS) FUNCTION

Microtubule associated proteins (MAPs) are tissue and cell type specific. They are high molecular weight proteins (200-300 K) or the tau (20-60 k) proteins. One domain binds to tubulin polymers or unpolymerized tubulin. This speeds up polymerization, facilitates assembly and stabilizes the microtubules. The other end will bind to vesicles or granules. MAPs vary with the cell type. The best examples are found in neurons. 

Furthermore, it is believed that some of these MAPs may bind to special sites on the alpha tubulin that form after it is in the microtubule. These are sites where a specific molecule is acetylated or the tyrosine residue is removed from the carboxy terminal. These sites are important marker sites for stabilized microtubules, because they disappear when microtubules are depolymerized. 

This figure shows a 3-D view of a neuron with its processes containing microtubules. At higher magnifications, the vesicles are seen attached to MAPs and moving along the microtubule conveyer belt. The MAPs include kinesins and dynein which "walk" along the microtubules in opposite directions.The kinesins move the vesicle along towards the plus end and dynein walks towards the minus end. In neurons, as the microtubules grow from the cell body through the processes, the plus end is more peripheral. These proteins have head regions that bind to microtubules and also bind ATP. The head domains are thus ATPase motors. The tail domain binds to the organelle to be moved. It is not known how the energy from ATP breakdown is converted into vectorial transport.