Frames from Video 1 showing cortical MKLP1 distributions caused by stable and dynamic MTs. The 80-μm spherical cell has red centrosomes. MT segment color encodes attached Clip170 concentration so recently polymerized segments are green and 25-s-old ones are orange (stable MTs become entirely orange). White dots show 33,000 MKLP1s; each moves at 0.2 μm/s along, has a binding half-life of 20 s to MTs, and stalls at MT tips. Soluble MKLP1s have a molecular diffusivity of 0.5 μm2/s. The main text explains how the gray, white, and red radial density plots surrounding the spherical cell (and their straightened out versions in F) show concentrations of MKLP1s adjacent to the cortex (gray and white) and in soluble form throughout the cell (red). (A–E) Selected time points (15, 115, 340, 390, and 480 simulated seconds from the start of anaphase) used in all figures. Simulations start at t = 0, with no MTs and all MKLP1s in soluble form distributed among randomly located, thus possibly overlapping, cytoplasmic domains. In A, at 15 s, cytoplasmic domains have moved so none intrudes into others; diffusion has equilibrated the concentration of soluble MKLP1 to spatial uniformity at the highest concentration ever reached (because there are as yet few MTs to bind to). By 115 s in B, enough MTs have formed to bind ∼50% of the MKLP1s, and the first stable MTs have reached the cortex; slight elevations of the white radial density plot at the equator occur where they touch the cortex. By 340 s in C, the asters and the cortical MKLP1 pattern have reached a dynamic equilibrium, with most MKLP1s bound to MTs (the red curve is lowest). The fall of the red profile, in A to B to C as the amount of polymerized tubulin increases, shows the effectiveness of MTs at sponging up most MKLP1s. The pronounced MKLP1 accumulation near the cortex at the equatorial spindle midplane at 340 s arises because only the stable MTs (aimed at the equator) remain in place long enough for MKLP1s to reach the cortex along them. We model nocodazole treatment by reducing the MT polymerization on-rate by a factor of 10; this commences at 345 s, and, in D at 390 s, most of the dynamically unstable MTs have depolymerized, leaving only stable MTs. The red radial density plot rises sharply as all MKLP1s previously bound to just-depolymerized MTs are cast adrift into soluble form. The amplitude of the equatorial accumulation of MKLP1s begins to rise because each stable MT now binds (and delivers to the cortex) more MKLP1s than formerly, when it had to compete with the 10-fold more numerous dynamically unstable MTs. At 480 s in E, just as simulated nocodazole washout allows MTs to start regrowing, more previously diffusing MKLP1s have bound to the remaining stable MTs and have motored to the equator, further boosting the equatorial accumulation of MKLP1s. Foe and von Dassow (2008) saw the same gradual increase in the activated myosin signal at the equator after nocodazole treatment. Painting a swarm of MKLP1s stalled at the tip of an MT in the single pixel the tip occupies would not show how many are in the swarm, so we took artistic license to draw all the MKLP1s stalled at the tip of an MT as if spread out along the terminal μm of the MT. The tiny cell above the 10 μm scale bar is the cell from the Fig. 3 simulation, drawn at the same magnification as the large cells here.
