We have identified a Drosophila member of the deleted in colorectal cancer (DCC) gene family. The frazzled gene encodes transmembrane proteins that contain four immunoglobulin C2 type domains, six fibronectin type III repeats, and a cytoplasmic domain of 278 amino acids. Like vertebrate members of the DCC family, Frazzled is expressed on axons in the embryonic central nervous system and on motor axons in the periphery. Frazzled is also expressed on epidermis and gut epithelium. Null mutants in frazzled are defective in axon guidance in the central nervous system and in motor axon guidance and targeting in the periphery. The phenotypes strongly resemble those of a deletion of the two Drosophila Netrin genes. We have rescued the frazzled CNS and motor axon defects by expressing Frazzled specifically in neurons; expression in target tissues does not rescue the phenotype. These data, together with vertebrate studies showing binding of DCC to netrin, suggest that Frazzled may function in vivo as a receptor or component of a receptor mediating Netrin-dependent axon guidance.

No abstract

As organisms have evolved in size and complexity, tubular systems have developed to enable the efficient transport of substances into and out of tissues. These tubular systems are generated using strategies that are based on common elements of cell behaviour, including cell polarization, tube migration to target sites, cell-fate diversification and localization of specialized cells to different regions of the tube system. Using examples from both invertebrate and vertebrate systems, this review highlights progress in understanding these basic principles and briefly discusses the possible evolution of strategies to regulate the morphogenesis of tubular systems.

EB1 is a member of a conserved protein family that localizes to growing microtubule plus ends. EB1 proteins also recruit cell polarity and signaling molecules to microtubule tips. However, the mechanism by which EB1 recognizes cargo is unknown. Here, we have defined a repeat sequence in adenomatous polyposis coli (APC) that binds to EB1's COOH-terminal domain and identified a similar sequence in members of the microtubule actin cross-linking factor (MACF) family of spectraplakins. We show that MACFs directly bind EB1 and exhibit EB1-dependent plus end tracking in vivo. To understand how EB1 recognizes APC and MACFs, we solved the crystal structure of the EB1 COOH-terminal domain. The structure reveals a novel homodimeric fold comprised of a coiled coil and four-helix bundle motif. Mutational analysis reveals that the cargo binding site for MACFs maps to a cluster of conserved residues at the junction between the coiled coil and four-helix bundle. These results provide a structural understanding of how EB1 binds two regulators of microtubule-based cell polarity.