Fig. 5

SPR analysis of SALM5 interacting with PTPδ splice code variants. a–d are SPR sensorgrams of immobilized human SALM5 LRR-Ig binding to human PTPδ Ig1–3 WT (a) as well as vairants MeA⁻ (b), MeB⁻ (c) and MeA⁻/MeB⁻ (d). For clarity, human PTPδ Ig1–3 is labeled as “PTPδ.” For each analyte, five concentrations were used as indicated. The WT human PTPδ Ig1–3 and its variants MeA⁻, MeB⁻, and MeA⁻/MeB⁻ bind to SALM5 with affinities of 66.0, 99.9, 1780.0, and 1866.0 nM, respectively. The binding affinity for SALM5 and PTPδ Ig1–3 (MeA⁻) determined here (with SALM5 immobilized) is slightly higher than the reversed interaction (with PTPδ immobilized) shown in Fig. 4b (99.9 vs 281.8 nM), likely due to avidity effect of the SALM5 dimer immobilized on the chip. Different from the previous L-cell aggregation assay-based report¹⁵ that MeB in PTPδ additively suppressed PTPδ binding to SALM5, our SPR experiments revealed that the addition of MeB to PTPδ significantly enhanced its binding to SALM5. This difference might derive from the different methods employed to assess SALM5/PTPδ interaction