Figure 6.

The binding face on neurexin-1 β (−S4) for LRRTM2 strongly overlaps that for neuroligin-1. Twenty-one point mutants of Nrx1β(−S4)-CFP altered at one to five residues were expressed in COS7 cells, and binding of recombinant LRRTM2-AP or NLG1-AP was measured. A, Representative examples of Nrx1β(−S4) mutants showing binding to recombinant LRRTM2-AP or NLG1-AP. Most mutants affected binding of LRRTM2 and NLG1 to a similar extent, but S209R differentially reduced binding to NLG1-AP, and β4β5 differentially reduced binding to LRRTM2-AP. Scale bar, 10 μm. B, Quantitation of the binding data (ANOVA, p < 0.0001; n = 15). Results are expressed as mean ± SEM. Mutations in Nrx1β(−S4) reduced binding of LRRTM2 and NLG1 to a similar extent for S107A, R109A, D137A, N153A, D158A, N184A, R202A, Q203A, L204A, T205A, I206A, N208A, β2β3 loop, β6β7 loop, and β10β11 loop. Binding was unaffected for both LRRTM2 and NLG1 for T156A, Y198A, and β3β4 loop. Mutation of Nrx1β(−S4) β4β5 loop did not affect binding of NLG1-AP but reduced binding of LRRTM2-AP, whereas mutation S209R did not affect binding of LRRTM2-AP but reduced binding of NLG1-AP. C, Diagram of crystal structure of Nrx1β(−S4) (Protein Data Bank ID#3BIW) showing the common binding face for neuroligin-1 and LRRTM2. The left structure shows point mutations at specific residues (green), including those involved in coordinating calcium (blue) that reduced binding of both LRRTM2 and NLG1 to similar levels. Mutation S209R (red), however, reduced binding of NLG 1 but not of LRRTM2 to Nrx1β(−S4). The right structure shows mutations in loops (green) that reduced the binding of LRRTM2 and NLG1 to similar levels. Mutations in the β4β5 loop (red) reduced binding of LRRTM2 but not of NLG1 to Nrx1β(−S4).