A CONSERVED FUNCTIONAL DOMAIN OF DROSOPHILA CORACLE IS REQUIRED FOR LOCALIZATION AT THE SEPTATE JUNCTION AND HAS MEMBRANE-ORGANIZING ACTIVITY

The protein 4.1 superfamily is comprised of a diverse group of cytopla smic proteins, many of which have been shown to associate with the pla sma membrane via binding to specific transmembrane proteins, Coracle, a Drosophila protein 4.1 homologue, is required during embryogenesis a nd is localized to the cytoplasmic face of the septate junction in epi thelial cells, Using in vitro mutagenesis, we demonstrate that the ami no-terminal 383 amino acids of Coracle define a functional domain that is both necessary and sufficient for proper septate junction localiza tion in transgenic embryos, Genetic mutations within this domain disru pt the subcellular localization of Coracle and severely affect its gen etic function, indicating that correct subcellular localization is ess ential for Coracle function. Furthermore, the localization of Coracle and the transmembrane protein Neurexin to the septate junction display an interdependent relationship, suggesting that Coracle and Neurexin interact with one another at the cytoplasmic face of the septate junct ion. Consistent with this notion, immunoprecipitation and in vitro bin ding studies demonstrate that the amino-terminal 383 amino acids of Co racle and cytoplasmic domain of Neurexin interact directly. Together t hese results indicate that Coracle provides essential membrane-organiz ing functions at the septate junction, and that these functions are ca rried out by an amino-terminal domain that is conserved in all protein 4.1 superfamily members.