Measurement and Origin of a Force that Pushes and Pulls the Plasma Membrane

Abstract

We report the measurement of a dynamic force that pulls and pushes the plasma membrane in a living cell. We form long F-actin filled membrane tubes from a cell and monitor the axial membrane force by use of laser tweezers where the optically trapped bead is attached to the end of the tube. We observe a sawtooth force (up to 75 pN in magnitude) that rides on top of the stationary force (∼10-15 pN). The sawtooth force increases slowly (tens of seconds) and decays rapidly back (tens of ms) to the stationary value. This sawtooth force was not detected after addition of drugs that bind to the barbed end of actin filaments, but it was insensitive to drugs that affect microtubules and myosin II. We determine the force per working object is 3 to 5 pN, with 15 to 30 working objects in the tube, from the magnitude and direction of the discrete force. We calculate a length of ∼ 3 to 6 nm from the measured force per working object and the time course of the force; this is equivalent to G-actin subunit half- and full-lengths. We propose the detected objects are working-actin filaments and the rise and decay phases of the sawtooth force are due to depolymerization and polymerization of actin. Our data supports a type of end-tracking motor model that operates in both the forward (polymerization) and reverse (depolymerization) directions; linker proteins attach the actin filaments to the membrane and transduce the chemical energy of polymerization and depolymerization to the mechanical motion of the membrane. Theory predicts that the actin motor should work in both directions. Our data provides experimental evidence that depolymerization of actin pulls the plasma membrane.