Actin modulates shape and mechanics of tubular membranes

Abstract

The actin cytoskeleton shapes cells and also organizes internal membranous compartments. In particular, it interacts with membranes for intracellular transport of material in mammalian cells, yeast, or plant cells. Tubular membrane intermediates, pulled along microtubule tracks, are formed during this process and destabilize into vesicles. While the role of actin in tubule destabilization through scission is suggested, literature also provides examples of actin-mediated stabilization of membranous structures. To directly address this apparent contradiction, we mimic the geometry of tubular intermediates with preformed membrane tubes. The growth of an actin sleeve at the tube surface is monitored spatiotemporally. Depending on network cohesiveness, actin is able to entirely stabilize or locally maintain membrane tubes under pulling. On a single tube, thicker portions correlate with the presence of actin. These structures relax over several minutes and may provide enough time and curvature geometries for other proteins to act on tube stability.

Polarization modulation with optical lock-in detection reveals universal fluorescence anisotropy of subcellular structures in live cells

“…However, in spine-extending regions, the orientation changed rapidly along the extending direction. Studies showed that the actin cytoskeleton interacted with plasma membrane changes the morphology 54 . OLID-SDOM techniques may even drive forward the studies of synaptic activity regarding to the dynamics of dendritic spine morphology plasticity.…”

Fluctuations of a membrane nanotube covered with an actin sleeve

“…The bottom term πηr bead catches the thermal fluctuations of the isolated bead in the surrounding viscous medium while the upper term expresses the damping of nanotube fluctuations due to the viscosity η I inside the nanotube. Here, an actin sleeve of a few hundred nanometers surrounds the membrane nanotube [22]. We propose that this sleeve affects the membrane nanotube peristaltic modes by increasing the viscosity around the nanotube.…”

“…Under our conditions, tension ranges between 0.2 and 50 × 10 −6 N/m (Fig. 2), while aspirating liposomes in a micropipet gives 10-200 × 10 −6 N/m [14,22]. The detailed effect of membrane tension is assessed in Sec.…”

“…To decorate the membrane with actin, we polymerize a branched actin network at the surface of the membrane nanotube in a two-step procedure [22]. First, we specifically bind an activator of the actin polymerization to the nanotube (see Sec.…”

“…This force depends on the membrane tension σ which ranges in vivo from 5 × 10 −6 N/m for the Golgi membrane to 13 × 10 −6 N/m for the ER membrane [21]. Here, we extrude nanotubes from settled and slightly adherent liposomes using a bead held in an optical trap [22]. We access the nanotube fluctuations through the power spectral density (PSD) of the trapped bead connected to the nanotube.…”