Posterior enrichment of PIE-1 is driven by binary counterdiffusion. (a) We propose that free PIE-1 (P) undergoes a heterogeneous surface reaction in the posterior of the zygote that causes a reduction in its effective diffusion coefficient. This resultant “slow” species of PIE-1 (P*) homogenously reverts back to its initial form in the cytoplasm. (b) Analytical solutions of the concentration profiles of both species of PIE-1, x = 0 to x = L, reveal an overall concentration gradient along the A/P axis. The experimental concentration profiles were taken from the central 30 µm of the A/P axis (where the shape of the gradient is most pronounced) to avoid intensity spikes from P granules in the posterior and large plateaus in intensity in the anterior, which interfere with data fitting. (c) The half-times of recovery of GFP::PIE-1 intensity in 4.5-µm-diameter circular photobleached region experiments from the anterior and posterior cytoplasm were 0.7 ± 0.3 (n = 38) and 4.5 ± 0.3 s (n = 7), respectively. (d) Normalized autocorrelation curves of FCS fluctuation data showed differences between anterior and posterior diffusion of GFP::PIE-1. Theoretical diffusion curves suggest the presence of two species of PIE-1 in the anterior cytoplasm, with diffusion coefficients of 8.7 ± 0.6 and 0.6 ± 0.2 µm²/s, respectively. The posterior cytoplasm is well described by a three-component model with diffusion coefficients of 14.2 ± 7.8, 0.6 ± 0.3, and 0.006 ± 0.002 µm²/s. FRAP and FCS analysis is summarized in panel e and deviations from fit are provided in Fig. S3 (available). Error bars represent SEM.