Structural Analysis of the Synaptic Protein Neuroligin and Its β-Neurexin Complex: Determinants for Folding and Cell Adhesion

Abstract

The neuroligins are postsynaptic cell adhesion proteins whose associations with presynaptic neurexins participate in synaptogenesis. Mutations in the neuroligin and neurexin genes appear to be associated with autism and mental retardation. The crystal structure of a neuroligin reveals features not found in its catalytically active relatives, such as the fully hydrophobic interface forming the functional neuroligin dimer; the conformations of surface loops surrounding the vestigial active center; the location of determinants that are critical for folding and processing; and the absence of a macromolecular dipole and presence of an electronegative, hydrophilic surface for neurexin binding. The structure of a beta-neurexin-neuroligin complex reveals the precise orientation of the bound neurexin and, despite a limited resolution, provides substantial information on the Ca2+-dependent interactions network involved in trans-synaptic neurexin-neuroligin association. These structures exemplify how an alpha/beta-hydrolase fold varies in surface topography to confer adhesion properties and provide templates for analyzing abnormal processing or recognition events associated with autism.

Characterization of the neuroligin gene family expression and evolution in zebrafish

“…They bind to presynaptic alpha and beta forms of Neurexins (Ichtchenko et al, 1995(Ichtchenko et al, , 1996Missler et al, 1998;Boucard et al, 2005) and the alternative splicing of both protein families controls their binding affinities (Boucard et al, 2005;Chih et al, 2006;Comoletti et al, 2006;Fabrichny et al, 2007;Koehnke et al, 2008a,b;Shen et al, 2008). The current data suggest that Neurexin-Neuroligin binding is governed by a complex code that is based on the type of isoforms and splice variants involved, calcium binding, and glycosylation (Sudhof, 2008).…”

“…The two helices involved in dimerization are highlighted in yellow, the purple box indicates the main region of interaction with beta-Neurexins, and the pair of residues involved in Ca 2þ binding is boxed in red. Modified from Koehnke et al (2008) and Fabrichny et al (2007). Ac, A. carolinensis; Dr, D. rerio (zebrafish); Ga, G. aculeatus; Mm, M. musculus; Tn, T. nigroviridis; Tr, T. rubripes.…”

“…As shown in Figure 4, the region between a11 and b11 (residues QGEFLN, highlighted with a violet box in Fig. 4) represents the main binding site for beta-Neurexin 1 (Fabrichny et al, 2007). Notably, the specific intron loss event in T. nigroviridis produced the insertion in nlgn2b CDS of a specific sequence absent in all the other Neuroligins (see Fig.…”

“…3B) and we compared their different exonic structures to the secondary structure elements of crystallized Neuroligins (Fabrichny et al, 2007;Koehnke et al, 2008a). In Neuroligins, most of the intron gain/loss events are localized outside or at the borders of secondary structures and only few events are found inside alphahelices.…”

“…4); this nucleotide sequence encodes 27 amino acids that break the main binding region (Q-GEFLN, where dash indicates the insertion). Nevertheless, surprisingly, the amino acid insertion terminates with a glutamine (Q), thus reforming the correct pair of residues (QG) involved in Ca 2þ coordination at the interface between Neuroligin and beta-Neurexin 1 (Fabrichny et al, 2007). The effect of this sequence insertion on the Neuroligin structure and, therefore, on the Neurexin-Neuroligin binding requires further analyses.…”