LRRTM2 binding to neurexin-1β(−S4) is calcium dependent and competitive with neuroligin-1 binding. A, Recombinant LRRTM2-AP bound to COS7 cells expressing Nrx1β(−S4)-CFP in normal buffer but not in calcium-free buffer containing 10 mm EGTA. Also see Figure 6 for lack of LRRTM2-AP binding to Nrx1β(−S4) with mutations at known calcium coordination residues, D137 and N208. B, Recombinant NLG1-AP containing only an HA tag for detection bound to Nrx1β(−S4)-CFP expressed in COS7 cells. Preincubation of Nrx1β(−S4)-expressing COS7 cells with LRRTM2-AP reduced subsequent binding of NLG1-AP. C, Recombinant LRRTM2-AP containing only an Myc tag for detection bound to Nrx1β(−S4)-CFP expressed in COS7 cells. Preincubation of Nrx1β(−S4)-expressing COS7 cells with NLG1-AP reduced subsequent binding of LRRTM2-AP. D, Quantitation of the experiments in B and C (t test, *p < 0.0001; n = 20). Results are expressed as mean ± SEM. Scale bars: 10 μm.