2C), but not subtypes without the splice site 4 insert. The specific interaction with NRXs containing the splice site 4 insert was also observed by the immunoblot analysis (Fig. 2A). In contrast, the extracellular domain of LRRTM2 fused to the Fc fragment (LRRTM2-Fc) bound to HEK293 cells expressing NRX1β(S4−), but not to cells expressing NRX1β(S4+) (Fig. 2D). The extracellular domain of NL1(−) fused to the Fc fragment, i.e., NL1(−) Fc, bound to HEK293 cells expressing NRX1β(S4−) or NRX1β(S4+) (Fig. 2D). The addition of HA-Cbln1, but not HA-CS-Cbln1, significantly inhibited the binding

between NL1(−)-Fc and NRX1β(S4+), whereas it did not affect the binding between NL1(−)-Fc and NRX1β(S4−) (Fig. 2E). Together, these results indicate that, unlike LRRTM2 and NL1(−), hexametric Cbln1 binds to α- and β-isoforms of selleck NRXs in a manner AZD4547 dependent on the splice site 4 insert, which probably determines the interaction

with Cbln1. The binding of NLs and LRRTMs to NRXs has been reported to require extracellular Ca2+ (Ko et al., 2009; Siddiqui et al., 2010), which binds to the interface between these molecules (Koehnke et al., 2008). To examine whether the binding of Cbln1 to NRX(S4+) was also sensitive to extracellular Ca2+, we performed a cell-based binding assay in a medium containing low (56 nm, according to Ca-ethylene glycol tetra-acetic acid calculator) (Schoenmakers et al.,

1992) Ca2+ concentrations. The binding of NL1(−)-Fc to HEK293 cells expressing NRX1β(S4+) under normal Ca2+ concentrations completely disappeared under low Ca2+ concentrations (Fig. 3A and B). Similarly, the binding of LRRTM2-Fc to NRX1β(S4−) was completely inhibited under low Ca2+ concentrations (Fig. 3C and D). In contrast, binding of Cbln1 to NRX1β(S4+) was observed even under low extracellular Ca2+ concentrations (Fig. 3E and F), suggesting that the mode of interaction between NRX1β(S4+) and Cbln1 was distinct from that between NRX1β(S4+) triclocarban and NL1(−). To further confirm the distinct binding mode of Cbln1 to NRX1β(S4+), we mutated Ca2+ binding sites of NRX1β(S4+) (Fabrichny et al., 2007; Reissner et al., 2008). Even under normal extracellular Ca2+ concentrations, NL1(−)-Fc did not bind to HEK293 cells expressing NRX1β(S4+)N238A in which an alanine residue replaced an asparagine residue at position 238 or cells expressing NRX1β(S4+)D137A in which an alanine residue replaced an aspartate residue at position 137 (Fig. 3B and G). In contrast, HA-Cbln1 bound to HEK293 cells expressing NRX1β(S4+)D137A or NRX1β(S4+)N238A in a manner similar to cells expressing wild-type NRX1β(S4+) (Fig. 3F and H). To examine whether Ca2+ concentrations did not affect the direct binding between Cbln1 and NRX1β(S4+), we performed an in vitro binding assay using HA-Cbln1 and NRX1β(S4+)-Fc or CD4-Fc.