Growing an actin gel on spherical surfaces

Inspired by the motility of the bacteria Listeria monocytogenes, we have ex perimentally studied the growth of an actin gel: around spherical beads gra fted with ActA, a protein known to be the promoter of bacteria movement. On ActA-grafted beads F-actin is formed in a spherical manner, whereas on the bacteria a "comet-like" tail of F-actin is produced. We show experimentall y that the stationary thickness of the gel depends On the radius of the bea ds. Moreover, the actin gel is not formed if the ActA surface density is to o low. To interpret our results, we propose a theoretical model to explain how the mechanical stress (due to spherical geometry) limits the growth of the actin gel. Our model also takes into account treadmilling of actin, We deduce from our work that the force exerted by the actin gel on the bacteri a is of the order of 10 pN. Finally, we estimate from our theoretical model possible conditions for developing actin comet tails.