Tropomyosin and actin: Move!

My penultimate structure of Christmas is actually two molecular partners, which work together to make muscle move.

Most of my Christmas structures have been separable units – some large, some small – that float around in cells or cell membranes. But to move physically, organisms need to have more at their disposal than some things floating in solution. For most life forms, movement is managed by proteins working together. A perfect example of this is the beautiful partnership between actin and tropomyosin.

How it looks

Actin and tropomyosin are both fibre proteins, made of monomers – independent protein units that stick together. Actin is present across all eukaryotes (which have cells with a nucleus), and indeed may have been the linchpin in the evolution of large cells, as compared to the far smaller, actin-free bacterial cells. Its fibres look like double-stranded, twisted ropes of head-to-tail monomers.

The actin strands can be pushed forwards (so long as one is anchored), using energy mainly reserved for building the strand itself. Some cells are able to move about thanks to a web of actin fibres, which polymerize on the forward edge and anchor the back part of the edge to a solid surface somehow. If you’ve seen those beautiful animations of cells moving around a surface, the movement you see is mainly down to this actin polymerization.

Putting some muscle into it

Actin is also used in bigger movements, for example muscles. In muscle, the actin polymerization doesn’t cause the movement, rather another protein, myosin. Actin is laid out in a regular series of contraction units, repeated in order. Myosin hauls itself against the actin and catches it, pulling forwards (consuming energy). It then brings in another myosin, which hauls itself against another actin.

By this ‘strike and pull’ action, each of the tiny contraction units shrink, and the cumulative effect is physical changes in muscle length. This powers every kind of muscle movement, from the beat of a butterfly’s wing beat to each step of Usain Bolt’s 100-metre sprint.

Timing it right

Timing is critical for the myosin-and-actin strike-and-pull cycle to work. To get it right, the actin fibres are wrapped in a third fibre – tropomyosin – which hides the myosin attachment site. The structure here shows tropomyosin wrapped around actin, hiding the myosin sites.

Tropomyosin actually comes in two forms, depending on whether there is calcium around or not.

Without calcium bound: Calcium is normally kept out of cells and without calcium, tropomyosin sits on the actin and prevents myosin getting a hold.

With calcium bound: In muscle cells nerve impulses trigger a temporary release of calcium from special compartments in the cell. Tropomyosin changes shape (due to another protein called troponin), letting the mysosin get hold of the actin and the pulling starts.

Big, bad, fibrous compounds

Such fibrous proteins form really, really large complexes, which are, by their very nature, fibrous! That means they can be badly behaved when scientists try to crystallise them, so many of the structures we have for fibrous proteins are actually of their better-behaved fragments.

It can be very difficult to get a complex to crystallise so that it can hold up to X-rays, so structural biologists are always looking at alternative techniques. One useful structural technique for studying these massive complexes illuminates the subject not with X-rays, but with electrons. In this manner, it allows us to visualise the sample directly.

This technique, Electron Microscopy, is the subject of my ninth and final Structure of Christmas.

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