Mutational analysis of the neurexin/neuroligin complex reveals essential and regulatory components

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Abstract
Neurexins are cell-surface molecules that bind neuroligins to form a heterophilic, Ca2+-dependent complex at central synapses. This transsynaptic complex is required for efficient neurotransmission and is involved in the formation of synaptic contacts. In addition, both molecules have been identified as candidate genes for autism. Here we performed mutagenesis experiments to probe for essential components of the neurexin/neuroligin binding interface at the single-amino acid level. We found that in neurexins the contact area is sharply delineated and consists of hydrophobic residues of the LNS domain that surround a Ca2+ binding pocket. Point mutations that changed electrostatic and shape properties leave Ca2+ coordination intact but completely inhibit neuroligin binding, whereas alternative splicing in α- and β-neurexins and in neuroligins has a weaker effect on complex formation. In neuroligins, the contact area appears less distinct because exchange of a more distant aspartate completely abolished binding to neurexin but many mutations of predicted interface residues had no strong effect on binding. Together with calculations of energy terms for presumed interface hot spots that complement and extend our mutagenesis and recent crystal structure data, this study presents a comprehensive structural basis for the complex formation of neurexins and neuroligins and their transsynaptic signaling between neurons.