title: The CDM superfamily protein MBC directs myoblast fusion through a mechanism that requires phosphatidylinositol 3,4,5-triphosphate binding but is independent of direct interaction with DCrk.
abstract: Myoblast city (mbc), a member of the CDM superfamily, is essential in the Drosophila melanogaster embryo for fusion of myoblasts into multinucleate fibers. Using germ line clones in which both maternal and zygotic contributions were eliminated and rescue of the zygotic loss-of-function phenotype, we established that mbc is required in the fusion-competent subset of myoblasts. Along with its close orthologs Dock180 and CED-5, MBC has an SH3 domain at its N terminus, conserved internal domains termed DHR1 and DHR2 (or "Docker"), and C-terminal proline-rich domains that associate with the adapter protein DCrk. The importance of these domains has been evaluated by the ability of MBC mutations and deletions to rescue the mbc loss-of-function muscle phenotype. We demonstrate that the SH3 and Docker domains are essential. Moreover, ethyl methanesulfonate-induced mutations that change amino acids within the MBC Docker domain to residues that are conserved in other CDM family members nevertheless eliminate MBC function in the embryo, which suggests that these sites may mediate interactions specific to Drosophila MBC. A functional requirement for the conserved DHR1 domain, which binds to phosphatidylinositol 3,4,5-triphosphate, implicates phosphoinositide signaling in myoblast fusion. Finally, the proline-rich C-terminal sites mediate strong interactions with DCrk, as expected. These sites are not required for MBC to rescue the muscle loss-of-function phenotype, however, which suggests that MBC's role in myoblast fusion can be carried out independently of direct DCrk binding.
authors.affiliation: Stowers Institute for Medical Research, Kansas City, MO 64110, USA.
authors.forename: Lakshmi
authors.initials: L
authors.lastname: Balagopalan
authors.forename: Mei-Hui
authors.initials: MH
authors.lastname: Chen
authors.forename: Erika R
authors.initials: ER
authors.lastname: Geisbrecht
authors.forename: Susan M
authors.initials: SM
authors.lastname: Abmayr
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D002459:Cell Fusion; D002460:Cell Line; D003598:Cytoskeletal Proteins / Q000235:genetics / Q000502:physiology*; D029721:Drosophila Proteins / Q000235:genetics / Q000502:physiology*; D004331:Drosophila melanogaster / Q000166:cytology / Q000196:embryology / Q000201:enzymology; D008969:Molecular Sequence Data; D005810:Multigene Family; D046912:Multiprotein Complexes; D032446:Myoblasts / Q000166:cytology / Q000201:enzymology*; D018129:Phosphatidylinositol Phosphates / Q000378:metabolism*; D011485:Protein Binding; D050737:Proto-Oncogene Proteins c-crk / Q000235:genetics / Q000378:metabolism* / Q000502:physiology; D017386:Sequence Homology, Amino Acid; D015398:Signal Transduction / Q000235:genetics; D018909:src Homology Domains / Q000235:genetics
chemical_list: D003598:Cytoskeletal Proteins; D029721:Drosophila Proteins; D046912:Multiprotein Complexes; D018129:Phosphatidylinositol Phosphates; D050737:Proto-Oncogene Proteins c-crk; C107441:mbc protein, Drosophila; C060974:phosphatidylinositol 3,4,5-triphosphate