Contributions of local binding, active transport, and cilium diffusion impediment to OS and synaptic enrichment of PMPs, computed by the DBT model. (A) Schematic of model rod with compartments labeled as in Fig. 2 A. Representative electron micrographs show the geometries of the compartments. Note the many synaptic vesicles filling the spherule (a) and the density and order of OS disc membranes (c). Parameters for each compartment used in all computations are listed: L, length; Ac, area of cross-section; and DGFP, diffusion coefficient of unmodified EGFP. EM images reproduced with permission from a (Schacher et al., 1976), b (Peters et al., 1983), and c (Townes-Anderson et al., 1985). a and b, Scale bars, 1 µm. c, Magnification = 45,000. dia, diameter. (B–E) DBT predictions of EGFP-GRK1ct18 distribution show that active transport and the connecting cilium diffusion impediment, together, are the major contributors to OS enrichment; local OS binding played a minor role. (F) Predictions of OSEI, F(OS)/F(IS) from model traces, and effective DOS, found by fitting model FRAPs as described in Fig 7, plotted versus equivalent binding power. Arrowhead on the DOS line shows that at EBP = 1.5, DOS was approximately twofold lower than that for no binding, as found experimentally for EGFP-GRK1ct18 and EGFP-Far0 versus EGFP (Fig. 7). However, the OSEI was <1. The OSEI line shows that tighter OS binding can lead to fivefold (arrowhead) or better OS enrichment, but at the cost of mobility. BP, binding power. (G) Transport velocity versus predicted OSEI, given OS EBP = 1.5. Arrowhead indicates the velocity that produces the observed approximately fivefold OS enrichment, as found experimentally for EGFP-GRK1ct18. (H–J) Higher synapse affinity resulted in significant enrichment (H) and FRAP recovery t1/2 ∼2 min (I and J), similar to experimental results for Far0 (Fig. 4 G), suggesting the distribution of Far0 within the IS is mediated by equilibrium binding alone.
