Tag Archives: axoneme

Is the microtubule alive ??

When does inanimate matter become animate?  How about cilia — they beat and move around.  No one would call  the alpha/beta tubulin dimer from which they are formed alive.  The tubulin proteins contain 450 amino acids or so and form a globule 40 Angstroms (4 nanoMeters) in diameter.  The dimer is then 40 x 80 Angstroms and looks like an oil drum.  Then they form protofilaments stacked end to end — e.g. alpha beta alpha beta.  Then 13 protofilaments then align side by side to form the microtubule (which is 250 Angstroms in diameter, with a central hole about half that size.  Do you think you could design a protein to do this?

Lets make it a bit more complicated, and add another 10 protofilaments forming a second incomplete ring.  This is the microtubule doublet, and each cilium has 9 of them all arranged in a circle.

Hopefully you have access to the 31 October cell where the repeating unit of the microtubule doublet is shown in exquisite detail — https://www.cell.com/action/showPdf?pii=S0092-8674%2819%2931081-5. — Cell 179, 909–922 ’19

The structure is from the primitive eukaryote Chlamydomonas, the structure repeats every 960 Angstroms (e.g every 12 alpha/beta tubulin dimers).  So just for one repeating unit which is just under 1/10 of a micron (10,000 Angstroms) there are (13 + 10) * 12 = 276 dimers.  The cilium is 12 microns long so that’s 12 * 276 * 100 = 298,080 alpha tubulin dimers/microtubule doublet. The cilium has 9 of these + another doublet in the center, so thats 2,980,800 alpha tubulin dimers/cilium.

The cell article is far better than this, because it shows how the motor proteins which climb along the outside of the doublet (such as dynein) attach.The article also describes the molecular ruler (basically a 960 Angstrom coil coil which spans the repeat. They found some 38 different proteins associated with the microtubule repeat.  They repeat as well at 80, 160, 240, 480 and 960 Angstrom periodicity.  The proteins in the hole in the center of the microtubule (e.g. the lumen) are rich in a protein module called the EF hand which binds calcium, and which likely causes movement of the microtubule, at which point the damn thing (whose structure we now know) appears alive.

Because of the attachment of the partial ring (B ring) to the complete ring of protofilaments, each of the 23 protofilaments has a unique position in the doublet, and each of the proteins in the lumen is bound to a specific mitotubule profilament. There are 6 different coiled coil proteins inside the A ring, occupying  specific furrows between the protofilaments.

Staggering complexity built from a simple subunit, but then Monticello is only made of bricks.