Neurexins were identified in mammals as a highly polymorphic famliy of presynaptic transmembrane proteins with high affinity for alpha Latrotoxin. A C.elegans neurexin,
nrx-1, shows similarities to all three rat neurexins at the level of the primary structure, genomic organisation, alternative splicing, and neuronal localization. The number of protein isoforms created by this single neurexin gene is much smaller than in mammals; secreted or !-neurexin isoforms have not been found so far. We have identified deletion mutants of
nrx-1 in a frozen library of TMP-mutagenized worms and are currently isolating these mutants. Recently a novel neurexin, Nrx IV, was identified in D.melanogaster (Baumgartner et al. (1996). Cell 87, 1059). In contrast to other neurexins this gene is not expressed in neurons and its transcript is not alternatively spliced. Nrx IV appears to play a role in the formation of septate junctions in epithelia and glia. We have identified a homologue of D.melanogaster Nrx IV in C.elegans, called
nlp-1 (neurexin-like protein). Comparison of all known neurexins suggests that two classes of neurexins exist. Neurexins of class A (mammalian neurexins I-III and C.elegans
nrx-1) contain 6 Laminin G domains and 3 EGF domains, are alternatively spliced, and are expressed in neurons, whereas neurexins of class B (D.melanogaster Nrx IV and C.elegans
nlp-1) contain only 5 laminin G domains and 2 EGF domains, are not alternatively spliced, and are expressed in a variety of cells except neurons.