Rheology of Membrane-Attached Minimal Actin Cortices

Abstract

The actin cortex is a thin cross-linked network attached to the plasma membrane, which is responsible for the cell's shape during migration, division, and growth. In a reductionist approach, we created a minimal actin cortex (MAC) attached to a lipid membrane to correlate the filamentous actin architecture with its viscoelastic properties. The system is composed of a supported 1-palmitoyl-2-oleoyl- sn-glycero-3-phosphocholine bilayer doped with the receptor lipid phosphatidylinositol(4,5)-bisphosphate (PtdIns(4,5)P) to which a constitutively active mutant of ezrin, which is a direct membrane-cytoskeleton linker, is bound. The formation of the MAC on the supported lipid bilayer is analyzed as a function of increasing PtdIns(4,5)P/ezrin pinning points, revealing an increase in the intersections between actin filaments, that is, the node density of the MAC. Bead tracking microrheology on the membrane-attached actin network provides information about its viscoelastic properties. The results show that ezrin serves as a dynamic cross-linker for the actin cortex attached to the lipid bilayer and that the stiffness of the network is influenced by the pinning point density, relating the plateau storage modulus G to the node density of the MAC.

Ezrin Phosphorylation at T567 Modulates Cell Migration, Mechanical Properties, and Cytoskeletal Organization

“…Through its interaction with transmembrane proteins and the underlying cytoskeleton, it helps provide structural support to strengthen the cell cortex. A plethora of molecular biology studies have focused on characterizing the monomeric or oligomeric configuration of ezrin when bound to the cellular membrane or its binding rates and unbinding forces to the membrane and/or the actin cortex, especially in terms of ezrin's activation state [2][3][4][5][6][7]. Ezrin is also important in the biological regulation of signaling pathways in the vicinity of the cellular membrane, allowing for optimal signaling from membrane-associated receptors, such as hepatocyte and epidermal growth factor receptors, to their respective downstream effector pathways, e.g., Cdc42, MAP [8].…”

Fluctuations of a membrane nanotube covered with an actin sleeve

“…The viscosity of actin networks is highly dependent on the temperature and the actin concentration [35], on the presence of cross linkers and their relative amount [36,37], and on whether the network is in three dimensions [28] or coated on a membrane [38]; therefore, the literature provides values of viscosity that are sparse (η = 0.2-2000 Pa s, [28,[36][37][38][39][40][41]). Comparing our measured value to these references is hard for several reasons.…”

Leaflet-Dependent Distribution of PtdIns[4,5]P2 in Supported Model Membranes

“…With 1% of all phospholipids of the plasma membrane of human erythrocytes, phosphatidylinositol-4,5-bisphosphate (PtdIns­[4,5]­P 2 ) is the most abundant one. , PtdIns­[4,5]­P 2 is known as a source for second messengers and contributes to intracellular signaling processes . It has a pivotal role in eukaryotic cell functions such as cytoskeletal dynamics, endo- and exocytosis, signal transduction and ion channel activation. − Part of these functions is based on the specific interaction of cytosolic proteins with PtdIns­[4,5]­P 2 , − with some of these PtdIns­[4,5]­P 2 -specific interaction partners harboring protein domains of known and well-conserved structures like ENTH-, FERM-, FYVE-, PH-, PX-, and Tubby domains, whereas other binding motifs are rather unstructured. − …”

“…We and others have used this protocol to measure protein binding onto SLBs. Nöding et al produced SLBs with different PtdIns­[4,5]­P 2 concentrations to bind ezrin and F-actin, whereas Harishchandra et al focused on the complex membrane-binding mechanism of the tumor suppressor protein PTEN (phosphatase and tensin homolog). , Drücker et al also used a low pH of 4.6, but increased the salt concentration (150 mM NaCl) to prepare SLBs on SiO 2 to investigate annexin A2 binding …”