C-Terminal Properties of Teneurin, a Bacterial Toxin Homologue at the Synapse

The brain processes information by relaying signals between neurons at the synapse, creating a diverse network of neuronal circuitry. Protein interactions between receptors, ligands, and cell adhesion molecules in the synaptic cleft enable the development of this highly intricate network. A family of highly conserved cell-adhesion proteins called teneurins (TENs) play a role in axon guidance, target selection, and synapse formation via transsynaptic interactions. TENs are comprised of a transmembrane region (TM) and an extracellular region that forms a barrel-like structure followed by a toxin-like domain at the C-terminus. This fold is not found in other eukaryotic proteins but instead bears resemblance to the large barrel structure of bacterial Tc toxins, a set of triple-protein complexes with a C-terminal cytotoxic region that plays an important role in Tc toxin insecticidal function. Furthermore, sequence alignment reveals that a catalytic site in Tc toxins involved in autoproteolysis of the C-terminus is mostly conserved in TENs. Due to this structural and biochemical homology, we investigated the importance of the C-terminus of TENs by targeting the toxin-like domain of the C. elegans orthologue, ten-1. We observed endogenous expression of Ten-1 at the nerve ring, ventral nerve cord, and tail, and we found that truncation of the toxin-like domain intensified expression. Moreover, defects in M4, NSM, and HSN neurons were observed in both truncation and null mutants. We also observed that protein purification of human TEN2 yielded a C-terminal product when the EGF- and Ig-like domains were deleted, which was abolished when the conserved catalytic residues were mutated. Together, we characterized C. elegans ten-1 expression and significance of its C-terminus in vivo and the autoproteolytic potential of human TEN2 in vitro, providing insight into the importance of the toxin-like domain, which may be worthwhile for studying TENs in human disease.