Actin protofilament orientation in deformation of the erythrocyte membraneskeleton

The red cell's spectrin-actin network is known to sustain local states of s hear, dilation, and condensation, and yet the short actin filaments are fou nd to maintain membrane-tangent and near-random azimuthal orientations. Whe n calibrated with polarization results for single actin filaments, imaging of micropipette-deformed red cell ghosts has allowed an assessment of actin orientations and possible reorientations in the network. At the hemispheri cal cap of the aspirated projection, where the network can be dilated sever alfold, filaments have the same membrane-tangent orientation as on a relati vely unstrained portion of membrane. Likewise, over the length of the netwo rk projection pulled into the micropipette, where the network is strongly s heared in axial extension and circumferential contraction, actin maintains its tangent orientation and is only very weakly aligned with network extens ion. Similar results are found for the integral membrane protein Band 3. Al lowing for thermal fluctuations, we deduce a bound for the effective coupli ng constant, alpha, between network shear and azimuthal orientation of the protofilament. The finding that alpha must be about an order of magnitude o r more below its tight-coupling value illustrates how nanostructural kinema tics can decouple from more macroscopic responses. Monte Carte simulations of spectrin-actin networks at similar to 10-nm resolution further support t his conclusion and substantiate an image of protofilaments as elements of a high-temperature spin glass.