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 d isplayed on the neuronal cell surface. We have now investigated the in teraction of neurexin 1 beta with neuroligin 1 to evaluate their poten tial to function as heterophilic cell adhesion molecules. Using deterg ent-solubilized neuroligins and secreted neurexin 1 beta-IgG fusion pr otein, we observed binding of these proteins to each other only in the presence of Ca2+ and in no other divalent cation tested. Only neurexi n 1 beta lacking an insert in splice site 4 bound neuroligins, whereas neurexin 1 beta containing an insert was inactive. Half-maximal bindi ng required 1-3 mu M free Ca2+, which probably acts by binding to neur oligin 1 but not to neurexin 1 beta. To determine if neurexin 1 beta a nd neuroligin 1 can also interact with each other when present in a na tive membrane environment on the cell surface, we generated transfecte d cell lines expressing neuroligin 1 and neurexin 1 beta. Upon mixing different cell populations, we found that cells aggregate only if cell s expressing neurexin 1 beta are mixed with cells expressing neuroligi n 1. Aggregation was dependent on Ca2+ and was inhibited by the additi on of soluble neurexin 1 beta lacking an insert in splice site 4 but n ot by the addition of neurexin 1 beta containing an insert in splice s ite 4. We conclude that neurexin 1 beta and neuroligin 1 (and, by exte nsion, 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.