Deformation - Enhanced fluctuations in the red cell skeleton with theoretical relations to elasticity, connectivity, and spectrin unfolding

To assess local elasticity in the red cell's spectrin-actin network, nano-p articles were tethered to actin nodes and their constrained thermal motions were tracked. Cells were both immobilized and controllably deformed by asp iration into a micropipette. Since the network is well-appreciated as soft, thermal fluctuations even in an unstressed portion of network were expecte d to be many tens of nanometers based on simple equipartition ideas. Real-t ime particle tracking indeed reveals such root-mean-squared motions for 40- nm fluorescent beads either tethered to actin directly within a cell ghost or connected to actin from outside a cell via glycophorin. Moreover, the el astically constrained displacements are significant on the scale of the net work's internodal distance of similar to 60-80 nm. Surprisingly, along the aspirated projection-where the network is axially extended by as much as tw ofold or more-fluctuations in the axial direction are increased by almost t wofold relative to motions in the unstressed network. The molecular basis f or such strain softening is discussed broadly in terms of force-driven tran sitions. Specific considerations are given to 1) protein dissociations that reduce network connectivity, and 2) unfolding kinetics of a localized few of the red cell's similar to 10(7) spectrin repeats.