Abstract

Cell Functions .................................................................................................. 135 7.3.2 Actin Waves are Comparable to Trigger Waves in Bistable Systems .... 137 7.4 Results ............................................................................................................. 137 7.4.1 Structure and Dynamics of Self-Organizing Actin Waves .................... 137 7.4.2 Actin Waves are Confined by Different States of Actin Organizationand by Membrane Areas of Different Phosphoinositide Composition ............ 139 7.4.3 Protein and PIP3 Patterns in Phagocytic Cups Correspond to thePatterns in Wave-Forming Cells ...................................................................... 142 Keywords ................................................................................................................. 143 Acknowledgments .................................................................................................... 143 References ................................................................................................................ 1437.1 INTRODUCTIONThis chapter deals with actin waves that are spontaneously generated on the planar, substrate-attached surface of Dictyostelium cells. These waves have the following characteristics. (1) They are circular structures of varying shape, capable of changing the direction of propagation. (2) The waves propagate by treadmilling with a recovery of actin incorporation after photobleaching of less than 10 s. (3) The waves are associated with actin-binding proteins in an ordered 3-dimensional organization: with myosin-IB at the front and close to the membrane, the Arp2/3 complex throughout the wave, and coronin at the cytoplasmic face and back of the wave. Coronin is a marker of disassembling actin structures. (4) The waves separate two areas of the cell cortex that differ in actin structure and phosphoinositide composition of the mem-brane. The waves arise at the border of membrane areas rich in phosphatidylinositol (3,4,5) trisphosphate (PIP3). The inhibition of PIP3 synthesis reversibly inhibits wave formation. (5) The actin wave and PIP3 patterns resemble 2-dimensional projections of phagocytic cups, suggesting that they are involved in the scanning of surfaces for particles to be taken up.

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