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. 2008 Feb 29;94(12):5009–5017. doi: 10.1529/biophysj.107.118554

FIGURE 2.

FIGURE 2

Sketch of the vicinity of the tip of a growing membrane tube (top) and its coded representation in the simulations (bottom). The boxes along the microtubule (dark green) represent binding sites for motors. The membrane tube (yellow) is also discretized, using the same one-dimensional lattice as for the microtubule. The sites are labeled from 0 to N, with 0 and N corresponding respectively to the left boundary and to the position of the leading motor, which defines the position of the tip. In the coded representation, the number at each site corresponds to the number of motors occupying that site. Along the microtubule, the sites may be either empty or occupied by one bound motor (light blue and red) at most, which corresponds, respectively, to occupation numbers 0 and 1 in the code. Along the membrane tube, each site may be occupied by several unbound motors (light green), corresponding to an integer occupation number in the code. (A and B) Examples of the tube retraction dynamics (A.1 and A.2) and examples of the growth dynamics for each organization scheme (B.1–B.3). A portion of the tube in the vicinity of the tip is shown in the coded representation used in the simulations. For each case, the configurations before (top) and after (bottom) a particular motor transition (indicated by an orange arrow) are shown. The bound motors along the membrane tube do not change their positions upon tube motion (light green dashed box). Unbound motors are dragged by the membrane tube and change their positions accordingly upon tube motion (light blue dashed box). (A.1 and A.2) Tube retraction caused by the detachment of the leading motor. Two different motor configurations are shown. The membrane tube retracts up to the position of the closest bound motor. (B) Dynamics of tube growth for the three different organizations of motors pulling the membrane tube at the tip. Forward stepping of the leading motor for the three coordination schemes discussed in the main text: (B.1) cluster-tip, (B.2) variable-tip, and (B.3) fixed-tip.