Direct measurement of the area expansion and shear moduli of the human redblood cell membrane skeleton

The area expansion and the shear moduli of the free spectrin skeleton, fres hly extracted from the membrane of a human red blood cell (RBC), are measur ed by using optical tweezers micromanipulation. An RBC is trapped by three silica beads bound to its membrane. After extraction, the skeleton is defor med by applying calibrated forces to the beads. The area expansion modulus K, and shear modulus CL, of the two-dimensional spectrin network are inferr ed from the deformations measured as functions of the applied stress. In lo w hypotonic buffer (25 mOsm/kg), one finds K-C = 4.8 +/- 2.7 muN/m, mu (C) = 2.4 +/- 0.7 muN/m, and K-C/mu (C) = 1.9 +/- 1.0. In isotonic buffer, one measures higher values for K-C, mu (C), and K-C/mu (C), partly because the skeleton collapses in a high-ionic-strength environment. Some data concerni ng the time evolution of the mechanical properties of the skeleton after ex traction and the influence of ATP are also reported. In the Discussion, it is shown that the measured values are consistent with estimates deduced fro m experiments carried out on the intact membrane and agree with theoretical and numerical predictions concerning two-dimensional networks of entropic springs.