Figure 5.

Illustrative summary of theory on spindle-cell size scaling. (A) Recapitulation of major rate-limiting processes related to the spatiotemporal regulation of SAC proteins: 1) diffusion into spindle and 2) poleward convection inside the spindle. The two processes scale with spindle size with opposite trends. (B) Illustration of how the rate-limiting step affects the spindle pole signal. The rate-limiting step changes from poleward convection to diffusion into spindle as the cell size increases. Hence, in small cells, a large spindle slows down convection and reduces spindle pole signal. In large cells, however, a large spindle facilitates diffusion into spindle and enhances spindle pole signal. (C) Event horizon for spindle-cytoplasm exchange. In large cells where diffusion into spindle is rate-limiting, the diffusion process imposes a gradient with a characteristic length scale. The length scale corresponds to the critical cell size. Above the critical cell size, extra cytoplasm outside the event horizon (dashed outline of the orange area) cannot exchange proteins effectively with the spindle, and is thus effectually unseen by the spindle.