Crystal Structures of β-Neurexin 1 and β-Neurexin 2 Ectodomains and Dynamics of Splice Insertion Sequence 4

Presynaptic neurexins (NRXs) bind to postsynaptic neuroligins (NLs) to form Ca2+-dependent complexes that bridge neural synapses. β-NRXs bind NLs through their LNS domains, which contain a single site of alternative splicing (splice site 4) giving rise to two isoforms: +4 and Δ. We present crystal structures of the Δ isoforms of the LNS domains from β-NRX1 and β-NRX2, crystallized in the presence of Ca2+ ions. The Ca2+-binding site is disordered in the β-NRX2 structure, but the 1.7 Å β-NRX1 structure reveals a single Ca2+ ion, ∼12 Å from the splice insertion site, with one coordinating ligand donated by a glutamic acid from an adjacent β-NRX1 molecule. NMR studies of β-NRX1+4 show that the insertion sequence is unstructured, and remains at least partially disordered in complex with NL. These results raise the possibility that β-NRX insertion sequence 4 may function in roles independent of neuroligin binding.