Scientists discovered a new “letter” in the tubulin code, important (not only) in neuron development

The tubulin code is an alphabet that cells use to control the behavior of their skeleton – the microtubules. This code influences neuronal development and brain plasticity (the ability to change the strength of connections between neurons), and its disruption is linked to neurodegenerative diseases such as Parkinson’s and Alzheimer’s disease. Scientists from the Laboratory of Structural Biology at the Institute of Biotechnology of the Czech Academy of Sciences, BIOCEV, together with international colleagues, have now described a previously unknown modification that expands this alphabet with a new “letter.”

The tubulin code is an alphabet that cells use to control the behavior of their skeleton – the microtubules. This code influences neuronal development and brain plasticity (the ability to change the strength of connections between neurons), and its disruption is linked to neurodegenerative diseases such as Parkinson’s and Alzheimer’s disease. Scientists from the Laboratory of Structural Biology at the Institute of Biotechnology of the Czech Academy of Sciences, BIOCEV, together with international colleagues, have now described a previously unknown modification that expands this alphabet with a new “letter.”

Every cell has an internal skeleton. Its components include microtubules – long “tracks” built from α- and β-tubulin subunits. These tracks are essential in intracellular transport, cell division, and the formation of cilia. Instructions for microtubule functions and communication with their environment are written on their surface as the so-called tubulin code, which is a set of chemical marks at the ends of α- and β-tubulins.

How TTLL11 writes the tubulin code
One of the proteins involved in writing the tubulin code is the enzyme TTLL11. In the journal Science Advances, scientists showed how TTLL11 writes the tubulin code. Using cryo-electron microscopy, they revealed that TTLL11 binds to the microtubule in an unusual way: its binding domain attaches to one “rail” (protofilament), while its catalytic domain writes the code on the neighboring one. This unique mode of substrate recognition explains why TTLL11 prefers to modify tubulin directly in the assembled microtubule. TTLL11 also expands the alphabet of the code by direct elongation of their terminal chains by addition of up to dozens of glutamate molecules instead of branching, known “standard” way of glutamylation. This creates a new type of modification, distinct from traditional branching, which can fundamentally change microtubule behavior.

Mass spectrometry and cell experiments further showed that TTLL11 can also restore the ends of shortened variants of tubulin that were previously thought to be irreversibly lost. In this way, TTLL11 can extend the lifespan of tubulin molecules and return them into coupled enzymatic cycles, such as the tyrosination/detyrosination cycle, which are essential for the proper functioning of (not only) nerve cells.

Figure: On the left, there is an illustration of a microtubule composed of proteins called α- (light green) and β-tubulin (dark green). Upper zoom-in shows C-terminal chains of the tubulin units with possible glutamylations (E) – side branches and direct C-terminal extensions (light blue). Lower zoom-in illustrates how TTLL11 binds the microtubule to modify either α- or β-tubulin C-terminus (top view). When tilted by 90°, we can see how it sits on the microtubule structure, binding one protofilament (pink) and modifying the neighbouring one (purple).

Broader relevance
The findings add another piece to the puzzle of research on post-translational modifications of microtubules, helping to describe and understand fundamental physiological processes in cells and organisms. In the future, this research may inspire new therapeutic strategies for diseases associated with impaired microtubule function.

Publication: https://www.science.org/doi/10.1126/sciadv.adw1561