For the first time, scientists have watched a protein called tubulin acetyltransferase (TAT) label the inside of microtubules. Microtubules are hollow cylinders within brain cells which function as skeletons and internal highways.
The resulting data answers long-standing issues about how TAT tagging works and gives clues as to why it is important for brain health.
Microtubules are continuously tagged by proteins in the cell. The purpose is to allocate them for specialized functions, in the same way that roads are labeled for fast or slow traffic or for maintenance.
Tubulin Acetyltransferase Tagging Problems
TAT coats explicit locations inside the microtubules with a chemical called an acetyl group. How the various labels are added to the cellular microtubule network is still unknown.
Recent studies suggest that problems with tagging microtubules may lead to some forms of cancer and nervous system disorders, including Alzheimer’s disease, and have been linked to Joubert Syndrome, an unusual brain development disorder that affects many parts of the body.
“This is the first time anyone has been able to peer inside microtubules and catch TAT in action,” said the National Institutes of Heath’s Antonina Roll-Mecak, Ph.D., leader of the study.
Found in all of the body’s cells, microtubules are put together like building blocks, using the protein called tubulin. Microtubules are assembled first by aligning tubulin building blocks into long strings.
These strings then align themselves side by side to form a sheet. Eventually the sheet becomes wide enough that it closes up into a cylinder. TAT then bonds an acetyl group to alpha tubulin, a subunit of the tubulin protein.
Recognizing Stable Microtubules
Certain microtubules are shorter lived and are able to quickly change lengths by adding or removing tubulin pieces along one end. Others stay unchanged for longer times. Recognizing the difference may help cells function properly.
For instance, cells may send cargo along stable microtubules and avoid ones that are being rebuilt. Cells appear to use a variety of chemical labels to describe the stability of microtubules.
“Our study uncovers how TAT may help cells distinguish between stable microtubules and ones that are under construction,” said Dr. Roll-Mecak.
High levels of microtubule tagging appear to be unique to nerve cells and may be the reason that they have complex shapes allowing them to make elaborate connections in the brain.
Surfing through Microtubules
Scientists have known for decades that the insides of long-lived microtubules were often tagged with acetyl groups by TAT. Changes in acetylation, it is now thought, may influence the health of nerve cells.
Some studies have shown that blocking this form of microtubule tagging leads to nerve defects, brain abnormalities or degeneration of nerve fibers. Since the discovery of microtubule acetylation, scientists have been puzzled about how TAT accesses the inside of the microtubules and how the tagging reaction happens.
The researchers took real time high resolution movies of individual TAT molecules interacting with microtubules. They observed that TAT surfs through the inside of microtubules and although it can find acetylation sites quickly, the process of adding the tag occurs quite slowly.
On the whole, tagging reactions work like keys fitting into locks: the better the key fits, the faster the lock can open. Likewise, the rate of the reactions is determined by how well TAT molecules fit around tagging sites.
X-ray crystallography was the tool used to look at how atoms on TAT molecules interact with acetylation sites on tubulin molecules. Their results suggested that TAT fit poorly around the sites.
“It looks as though TAT can easily journey through microtubules spotting acetylation sites but may only label those that are stable for longer periods of time,” said Dr. Roll-Mecak.