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Biophys J, December 2000, p. 2987-3000, Vol. 79, No. 6

Actin Protofilament Orientation in Deformation of the Erythrocyte Membrane Skeleton

Catherine Picart, Paul Dalhaimer, and Dennis E. Discher

Institute for Medicine and Engineering, and School of Engineering and Applied Science, University of Pennsylvania, Philadelphia, Pennsylvania 19104 USA

The red cell's spectrin-actin network is known to sustain local states of shear, dilation, and condensation, and yet the short actin filaments are found to maintain membrane-tangent and near-random azimuthal orientations. When 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 hemispherical cap of the aspirated projection, where the network can be dilated severalfold, filaments have the same membrane-tangent orientation as on a relatively unstrained portion of membrane. Likewise, over the length of the network projection pulled into the micropipette, where the network is strongly sheared in axial extension and circumferential contraction, actin maintains its tangent orientation and is only very weakly aligned with network extension. Similar results are found for the integral membrane protein Band 3. Allowing for thermal fluctuations, we deduce a bound for the effective coupling constant, alpha , between network shear and azimuthal orientation of the protofilament. The finding that alpha  must be about an order of magnitude or more below its tight-coupling value illustrates how nanostructural kinematics can decouple from more macroscopic responses. Monte Carlo simulations of spectrin-actin networks at ~10-nm resolution further support this conclusion and substantiate an image of protofilaments as elements of a high-temperature spin glass.

Biophys J, December 2000, p. 2987-3000, Vol. 79, No. 6
© 2000 by the Biophysical Society   0006-3495/00/12/2987/14  $2.00


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