Actin and PIP 3 waves in giant cells reveal the inherent length 1 scale of an excited state

21 The membrane and actin cortex of a motile cell can autonomously differentiate into two states, 22 one typical of the front, the other of the tail. On the substrate-attached surface of 23 Dictyostelium cells, dynamic patterns of front-like and tail-like states are generated that are 24 best suited to monitor transitions between these states. To image large-scale pattern dynamics 25 independent of boundary effects, we produced giant cells by electric-pulse induced cell fusion. 26 In these cells actin waves are coupled to the front and back of PIP3-rich bands that have a 27 finite width. These composite waves propagate across the plasma membrane of the giant cells 28 with undiminished velocity. After any disturbance, the bands of PIP3 return to their intrinsic 29 width. Upon collision, the waves locally annihilate each other and change direction; at the cell 30 border they are either extinguished or reflected. Accordingly, expanding areas of progressing 31 PIP3 synthesis become unstable beyond a critical radius, their center switching from a front32 like to a tail-like state. Our data suggest that PIP3 patterns in normal-sized cells are segments 33 of the self-organizing patterns that evolve in giant cells. 34