Coloured scanning electron micrograph (SEM) of cultured osteoclasts (orange).

Living cells can harbour protein aggregates with the properties of soft glasses. Credit: EYE OF SCIENCE/SPL

Biophysics

How bits of ‘glass’ take shape inside a cell

Odd protein droplets that behave like liquids make a smooth shift to having the properties of a solid.

Some living cells contain protein clusters that behave like liquid droplets. But these droplets gradually stiffen with age, and new experiments have revealed that they turn into a curious material: a soft ‘glass’.

Previous research showed that cellular protein droplets can solidify, but how they do so has puzzled researchers. To understand the shift, Anthony Hyman at the Max Planck Institute of Molecular Cell Biology and Genetics in Dresden, Germany, Frank Jülicher at the Max Planck Institute for the Physics of Complex Systems, also in Dresden, and their colleagues prepared droplets of protein isolated from cells and compared newly formed droplets with aged droplets.

The researchers used laser beams to squeeze and stretch the droplets, which became more elastic with age. In separate experiments, droplets became smaller and denser as they grew older, and the movement of tracer beads in the droplets became restricted.

The observations suggest that, over time, the interactions between the proteins making up the droplets grew stronger, restraining the proteins. As the proteins froze into place, the liquid droplets gradually solidified — a process typically seen when glassy materials form.