Binding properties of neuroligin 1 and neurexin 1 beta reveal function as heterophilic cell adhesion molecules

beta-Neurexins and neuroligins are plasma membrane proteins that are displayed on the neuronal cell surface. We have now investigated the interaction of neurexin 1 beta with neuroligin 1 to evaluate their potential to function as heterophilic cell adhesion molecules. Using detergent-solubilized neuroligins and secreted neurexin 1 beta-IgG fusion protein, we observed binding of these proteins to each other only in the presence of Ca2+ and in no other divalent cation tested. Only neurexin 1 beta lacking an insert in splice site 4 bound neuroligins, whereas neurexin 1 beta containing an insert was inactive. Half-maximal binding required 1-3 mu M free Ca2+, which probably acts by binding to neuroligin 1 but not to neurexin 1 beta. To determine if neurexin 1 beta and neuroligin 1 can also interact with each other when present in a native membrane environment on the cell surface, we generated transfected cell lines expressing neuroligin 1 and neurexin 1 beta. Upon mixing different cell populations, we found that cells aggregate only if cells expressing neurexin 1 beta are mixed with cells expressing neuroligin 1. Aggregation was dependent on Ca2+ and was inhibited by the addition of soluble neurexin 1 beta lacking an insert in splice site 4 but not by the addition of neurexin 1 beta containing an insert in splice site 4. We conclude that neurexin 1 beta and neuroligin 1 (and, by extension, other beta-neurexins and neuroligins) function as heterophilic cell adhesion molecules in a Ca2+-dependent reaction that is regulated by alternative splicing of beta-neurexins.