What a Glowing Protein Made Visible in Dividing Cells

A Jellyfish, a Protein, a Trail of Light

Before anything can be described, it has to be measurable. For an ion that sits at nanomolar levels in a resting cell and climbs only for seconds, that was long not the case. The way out came from a sea creature.

A Substance That Answers Calcium With Light

In the nineteen-sixties a protein was won from the jellyfish Aequorea victoria that gives off blue light as soon as calcium binds to the places on the molecule set aside for it. It needs neither a lamp nor any prompting from outside; the energy sits in the molecule itself and comes free on binding. Bring it into a cell and a change of concentration turns into something a sensitive detector can count.

Why the First Recordings Were Made on Egg Cells

The method called for the protein to be brought into the cell with a fine needle. At first only unusually large cells came into question for that: egg cells of fish and sea urchins, easy to hit under the microscope and dividing at short intervals. On recordings from the nineteen-seventies and eighties the brightness travels across the cell as a narrow band, shortly before it pinches in.

Later work on cultured mammalian cells managed without a needle. Dyes were at hand for it that enter the cell of their own accord and change their color or their brightness when calcium binds to them. The trail of light from the egg cells nevertheless stayed the picture along which the process is most easily set out.

The Gradient Every One of These Recordings Rests On

A trail of light presupposes that it was dark beforehand. Carried over to the chemistry that reads: the resting value in the cell fluid lies so low that a short climb is noticeable at all. That low resting value is the real piece of work.

Reckoning in Nanomoles

The orders of magnitude lie far apart, and the units change along the way. The overview below puts them next to one another so the distance becomes visible.

Rounded textbook figures from cell physiology. They shift with cell type and method of measurement, and not one of these numbers turns up in the sentence this Excursus arrives at.

Pumps That Work Against the Gradient

Two sorts of pump hold the low value in the cell fluid. The one sits in the outer cell envelope and moves the ion outside; the other sits in the wall of the inner store and fills that one back up. Both use energy, and both keep running even when nothing in the cell seems to be happening.

From that follows a reversal of the usual picture: the expensive part is not the signal, the expensive part is the quiet before it. The signal itself consists in a gate opening for a short while, with the gradient taking over part of the work that had been put in beforehand.

The Membrane Network Arranges Itself Anew Before a Division

The inner store is no container at a fixed spot but a widely branched net of membranes running through the whole cell. Ahead of a division this net does not stay where it was.

Where the Store Moves To

The cell-biology literature describes sections of this net shifting about before and during the division and collecting close to the spindle poles — that is, where the apparatus stands that draws the two sets of chromosomes apart. In many cell types the envelope around the nucleus belongs to the same membrane system and is taken apart at the start of the division; its components pass over into this net.

Channels That Listen for a Second Molecule

The store does not empty of its own accord. Channels sit in its wall that open only once a small messenger molecule binds to them. That molecule arises when a particular fatty building block of the outer membrane is split; in the specialist literature it goes by the abbreviation IP3, and the matching channel by IP3 receptor. Two steps are needed, then, before the concentration climbs, and both are limited in time.

The Receiver and What Becomes of the Pattern

A climb on its own brings nothing about. It has to be read, and there are proteins for that which change their shape as soon as the ion binds to them.

Four Pockets, Two Shapes

The best known of these proteins is called calmodulin and consists of around one hundred and fifty building blocks. It carries four pockets, each of which holds one calcium ion, spread over the two ends of an elongated molecule. Once the pockets are occupied, the molecule folds together and lays itself around a section of the next protein. A change of concentration thereby becomes a change of shape, and the change of shape a chain of further steps.

When It Is Not the Height That Counts but the Sequence

Alongside division, the authorized sentence names a second process: specialisation. What is meant by it is that cells settle on a particular task. For this field the specialist literature describes, among other things, climbs that differ in height, in length and in spacing, and downstream proteins that answer different patterns differently. The sentence itself does not go into any of that; it names the process and stops there.

The Sentence in Which None of This Stands

Before a health-related wording may be used across the Union, a scientific review at Union level goes through the work on exactly one nutrient and exactly one subject area. The wording settled that way then moves onto a list. From that moment the wording is fixed, and whoever uses it uses it character for character like this:

The verb is what stands out. A large share of the entries on this list is phrased with “contributes to”; here it reads “has a role in” instead. Both turns of phrase were settled that way, and neither may step into the place of the other. Anyone advertising calcium also needs a product that counts as a source of calcium under European law in the first place; where that line runs is laid down in a separate EU labelling rule.

The left column repeats what occurs in the wording. The right one collects what is now and then read into it and does not stand there.