Study of actin filament ends in the human red cell membrane

Abstract

There is conflicting evidence concerning the state of the actin protofilaments in the membrane cytoskeleton of the human red cell. To resolve this uncertainty, we have analysed their characteristics with respect to nucleation of G-actin polymerization. (1) The effects of cytochalasin E on the rate of elongation of the protofilaments have been measured in a medium containing 0.1 m-sodium chloride and 5 m m-magnesium chloride, using pyrenelabelled G-actin. At an initial monomer concentration far above the critical concentration for the negative (“pointed”) end of F-actin, high concentrations of cytochalasin reduce the elongation rate of free F-actin by about 70%. The residual rate is presumed to correspond to the elongation rate at the negative ends. By contrast, the elongation rate on red cell ghosts or cytoskeletons falls to zero, allowing for the background of self-nucleated polymerization of the G-actin. (2) The critical concentration of the actin in the red cell membrane has been measured after elongation of the filaments by added pyrenyl-G-actin in the same solvent. It was found to be 0.07 μ m, compared with 0.11 μ m under the same conditions for actin alone. This is consistent with prediction for the case of blocked negative ends on the red cell actin. (3) The rate of elongation of actin filaments, free and in the red cell membrane cytoskeleton, has been measured as a function of the concentration of an added actin-capping protein, plasma gelsolin, with a high affinity for the positive ends. The elongation rate falls linearly with increasing gelsolin concentration until it approaches a minimum when the gelsolin has bound to all positive filament ends. The elongation rate at this point corresponds to the activity of the negative ends, and its ratio to the unperturbed polymerization rate (in the absence of capping proteins) is indistinguishable from zero in the case of ghosts, but about 1: 4 in the case of F-actin. When ATP is replaced in the system by ADP, so that the critical concentrations at the two filament ends are equalized, the difference is equally well-marked: for F-actin, the rate at the equivalence point is about 40% of that in the absence of capping protein, whereas for ghosts the nucleated polymerization rate at the equivalence point is again zero, indicating that under these conditions the negative ends contribute little or not at all to the rate of elongation. These results are interpreted most simply in terms of complete or substantial blocking of the negative ends of the protofilaments in the red cell membrane cytoskeleton.