Figure 1. NT-3 enhances the binding of PTPσ to cell-surface TrkC. A, Schematic domain structures and protein interactions of TrkC noncatalytic isoform, PTPσ, and NT-3. TrkC LRR and Ig1 are required for PTPσ binding, whereas TrkC Ig2 is responsible for NT-3 binding. SP, Signal peptide; CC, cysteine cluster; TM, transmembrane; NCD, noncatalytic domain; FN, fibronectin type-III; D1 and D2, phosphatase domains. B, Representative images of PTPσ–Fc binding to COS-7 cells expressing HA–TrkC WT, HA–TrkC N366AT369A (NT-3-binding dead mutant), or a negative control HA–CD4, in the absence (left) and presence (right) of 100 ng/ml NT-3. PTPσ–Fc protein at 100 nm was applied. Scale bars, 10 μm. C, Quantification of bound PTPσ–Fc per surface HA expression. NT-3 treatment increased the intensity of PTPσ–Fc bound to HA–TrkC WT but not HA–TrkC N366AT369A. **p < 0.01, ***p < 0.001 in two-way ANOVA with Bonferroni's multiple comparisons, n = 20–40 cells from two independent experiments. D, Quantification of surface HA expression in cells analyzed in C. NS, Not significant. One-way ANOVA with Bonferroni's multiple comparisons.