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Colchicine binds tubulin and interferes with microtubule assembly. Effects on intracellular organization and inflammatory-cell activity connect that molecular interaction to its pharmacology. The same broad cellular target also helps explain why exposure matters for toxicity.^[ref]

Electron crystallography determines ordered molecular or atomic structure using electrons interacting with a crystalline specimen. Electron diffraction and suitably resolved electron images provide complementary constraints on a structural model. The periodic order supplies repeated information; interpreting that information still requires a model of scattering and image formation.

An important biological application resolved the alpha/beta tubulin heterodimer. Zinc-induced tubulin sheets provided an ordered specimen for a model reported at $3.7$ angstrom resolution. The resulting structure distinguished the two related protein subunits and their different nucleotide environments.^[ref]

The specimen geometry matters. An ordered sheet is not the same object as an isolated microtubule in a living cell, even when its molecules reveal features relevant to microtubule assembly. The structural experiment answers questions about the molecular arrangement under its preparation conditions; biological interpretation then connects that arrangement to other observations.

Resolution also differs from the size of the object. A sheet may extend over many molecular repeats while the data constrain features on the angstrom scale. Quoting a nominal resolution does not mean every atom is independently located with that exact uncertainty or that every flexible region is equally well resolved.

Electron crystallography belongs to the wider family of electron microscopy and crystallography methods, but should not be used as a synonym for every electron micrograph. A dark spot, an image of a cellular outline and a diffraction-constrained molecular model are different types of evidence.

E. Nogales is the first author of a 1998 study resolving the structure of the alpha–beta tubulin heterodimer, the building block of microtubules. The researchers used electron crystallography of zinc-induced tubulin sheets to obtain a density map at 3.7 angstrom resolution and fit an atomic model to it.^[ref]

The two related protein subunits had similar overall folds but occupied distinct positions in a heterodimer. The model identified a nucleotide-binding region and other structural regions relevant to interactions. It illustrates a practical strategy in structural biology: organize a difficult protein into a repeatable experimental specimen, measure its scattering or imaging signal, and test a molecular model against that evidence. The specimen here was an induced sheet, so interpreting it as the architecture of an intact cellular microtubule requires additional evidence.^[ref]

In protein chemistry, a heterodimer is an assembly of two different protein subunits, conventionally written $AB$. A homodimer has two copies of the same subunit type, $A_2$. Different types can have similar protein folds and nearly equal molecular masses; “different” refers to molecular identity, not simply size or shape. An $A_2{B}_2$ protein complex contains two types but four chains, so it is not itself a heterodimer.

The alpha/beta tubulin heterodimer is the repeating building block of a microtubule. The alpha and beta protein subunits have similar folds but distinct sequences and nucleotide environments. Their structural relationship was resolved by electron crystallography of zinc-induced tubulin sheets: the 1998 model reached $3.7$ angstrom resolution. This is a concrete example of a two-chain protein quaternary structure, rather than the protein tertiary structure of a single chain.^[ref]

Stoichiometry gives a useful upper bound. If an idealized mixture contains $3\mu \mathrm{mol}$ of $A$ and $2\mu \mathrm{mol}$ of $B$, and only $AB$ can assemble, at most $2\mu \mathrm{mol}$ of heterodimer can form; at least $1\mu \mathrm{mol}$ of $A$ remains outside $AB$. The actual amount also depends on binding equilibrium and experimental conditions: chain counting alone does not guarantee complete association.

P. A. Janmey investigated how networks of cytoskeletal proteins bear mechanical loads. A 1991 study compared purified actin, tubulin and vimentin assemblies under deformation. The networks differed in stiffness and resistance to disruption, with vimentin networks especially able to withstand large deformations.^[ref]

The comparison separated the mechanical properties of a filament network from the complexity of an intact cell. A material's response to a small deformation does not completely predict whether it survives a large one. This connects cytoskeleton composition to the distinct questions of stiffness, viscoelasticity and mechanical resilience.

A protein subunit is an individual polypeptide component of a protein complex. The number of protein subunits counts chain copies, whereas the number of subunit types counts distinct kinds of chain. An assembly $A_2{B}_2$ has four protein subunits of two types. A protein domain is a region within a chain and need not be a separate protein subunit; the arrangement of several chains belongs to protein quaternary structure.

The alpha/beta tubulin heterodimer illustrates the distinction: it contains two related but different polypeptides, not two domains cut out of one chain. The 1998 electron-crystallographic model resolved the alpha/beta architecture at $3.7$ angstrom resolution, using zinc-induced tubulin sheets.^[ref]

Subunit counting can also become an optical experiment. In total internal reflection fluorescence microscopy, tagged membrane proteins can be observed as isolated fluorescent spots; successive photobleaching steps reveal individual fluorescent tags disappearing. Subunit counting in membrane-bound proteins by Maximilian H. Ulbrich and Ehud Y. Isacoff used this approach on membrane proteins expressed in frog oocytes. For NR1/NR3B receptor assemblies, tagging one component type mainly gave two steps, while tagging both gave four, supporting a two-plus-two arrangement.^[ref]

A count of visible steps need not equal the true number of protein subunits: some tags fail to fluoresce, and two tags may bleach too close together to resolve. The experiment therefore combines step counts with a model of labeling and detection, rather than interpreting every two-step trace as proof of a two-chain protein complex.^[ref]

Tubulin is the protein building material of microtubules. Alpha- and beta-tubulin form a heterodimer; repeated addition of these units produces polarized protofilaments assembled into a hollow tube. The two ends of a microtubule differ in organization and growth behaviour. GTP binding and hydrolysis by beta-tubulin couple assembly history to stability. ^[ref]

This explains how the same material supports both persistent cellular architecture and rapid rearrangement of a spindle apparatus. Losing a stabilizing end region can trigger rapid shortening even while soluble tubulin remains available. Fluorescence measurements of tubulin amount alone therefore do not determine how much is assembled, the direction of growth or the lifetime of an individual microtubule.