Abstract
Neurexins mediate protein interactions at the synapse, playing an essential role in synaptic function. Extracellular domains of neurexins, and their fragments, bind a distinct profile of different proteins regulated by alternative splicing and Ca2+. The crystal structure of n1alpha_LNS#2 (the second LNS/LG domain of bovine neurexin 1alpha) reveals large structural differences compared with n1alpha_LNS#6 (or n1beta_LNS), the only other LNS/LG domain for which a structure has been determined. The differences overlap the so-called hyper-variable surface, the putative protein interaction surface that is reshaped as a result of alternative splicing. A Ca2+-binding site is revealed at the center of the hyper-variable surface next to splice insertion sites. Isothermal titration calorimetry indicates that the Ca2+-binding site in n1alpha_LNS#2 has low affinity (Kd approximately 400 microm). Ca2+ binding ceases to be measurable when an 8- or 15-residue splice insert is present at the splice site SS#2 indicating that alternative splicing can affect Ca2+-binding sites of neurexin LNS/LG domains. Our studies initiate a framework for the putative protein interaction sites of neurexin LNS/LG domains. This framework is essential to understand how incorporation of alternative splice inserts expands the information from a limited set of neurexin genes to produce a large array of synaptic adhesion molecules with potentially very different synaptic function.
Highlights
Neurexins are multidomain cell surface proteins found in the brain at the synapse
It is thought that neurexin LNS/LG domains contain special protein interaction surfaces that coincide with the hyper-variable surface that is regulated by alternative splicing [37]
An increasing body of research points toward neurexin LNS/LG domains containing localized regions of variability that coincide with regions of functional significance; in the context of the full-length extracellular domains, these areas will be proffered to the solvent as interaction surfaces
Summary
Neurexins are multidomain cell surface proteins found in the brain at the synapse (recently reviewed in Refs. 1– 4). LNS#6 in neurexin 1␣ (n1␣_LNS#6) and the identical LNS domain in neurexin 1 (n1_LNS) interact with neuroligins, a family of post-synaptic cell surface proteins [9, 14, 17, 24, 25], ␣-latrotoxin, a spider neurotoxin triggering massive neurotransmitter release [21], and ␣-dystroglycan as well [22]. A growing body of work demonstrates, at least in vitro, that alternative splicing of neurexin LNS/LG domains regulates protein-binding specificities. It is thought that neurexin LNS/LG domains contain special protein interaction surfaces that coincide with the hyper-variable surface that is regulated by alternative splicing [37]
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