actin filament untwirled

The molecules involved in muscle movement have been studied in great detail, zooming in from the microscopic detail of the sliding filament model to single molecule studies and crystal structures in the 1990s. One crucial piece was still missing, though – the structure of the actin filament which serves as the rail on which the myosin engine travels.

Even though Ken Holmes’s group reported the crystal structure of the soluble actin molecule (G actin) in 1990, it took nearly two decades to arrive at a high resolution model of the fibrillar form (F actin). Toshiro Oda and coworkers used a frightening array of expensive modern tools to get there, beginning by making the fibres in a very strong magnetic field created by a superconducting magnet, then firing extremely powerful synchrotron radiation at it, using the Spring-8 synchrotron run by RIKEN in Japan, and finally applying clever ways of molecular modelling to make the structure fit the data.

The resulting structure isn’t all that surprising, as it vaguely resembles previous, cruder models, and also the bacterial actin homologue, MreB. The key feature is that the two domains of the protein, which have a propeller-like twist in the globular version, are untwisted and more neatly stacked in the fibrillar protein. And then there is a single loop that is arranged in a different way, and that’s all the changes that are to be observed.

Still, it’s a valuable piece of information, as it was one of the last missing links in a molecular scale description of muscle movement.

T. Oda et al., Nature 2009, 457, 441.

Protein Data Bank entry
3D view of the structure