Fig. 2. Role of dephosphorylation in axis determination.

a, b, d A protein is shown in the elliptical cell firstly at its phosphorylation and detachment site on the membrane and then at the point of its reactivation. The reactivation length gives an average radius (grey circles) how far from the detachment point a protein travels before reactivation. The orange circles around the reactivated protein and the associated arrows sketch some diffusion distance corresponding to a time interval Δt following reactivation, i.e., during this time interval the protein can reattach to the membrane. a If the reactivation length ℓ (radius of grey circle) is small compared with the cell size, the local membrane surface to cytosolic volume ratio strongly affects the position at which detached proteins reattach. Due to the reactivation occurring close to the membrane, within some time interval Δt following reactivation a protein that detaches from a cell pole is more likely to reattach near that same cell pole than a protein detaching from mid-cell is to reattach at mid-cell. Hence, dynamics that are based on membrane–cytosol cycling (such as antagonistic reactions that maintain an interface) are enhanced at the cell poles. b As the reactivation length ℓ approaches the length of the cell, this effect of geometry becomes weaker, and detaching proteins become increasingly unconstrained by the position of detachment (uncaged). c Illustration of the distribution of cytosolic bulk proteins along the long axis. The elliptical cell and the cytosol height is depicted as a function of x, where the x-axis aligns with the long axis (top). The amount of cytosolic bulk proteins for each x varies from the poles to mid-cell as illustrated (bottom). d This effect of cell geometry is completely lost if the reactivation length ℓ exceeds the length of the cell. Hence, detached proteins become uniformly distributed throughout the cell before reactivation occurs. In that case, most will re-encounter the membrane near mid-cell after reactivation, since a delocalised protein will most likely be found in the mid-cell area.