MOLECULAR CHARACTERIZATION AND DEVELOPMENTAL EXPRESSION OF A RETINOID-BINDING AND FATTY-ACID-BINDING GLYCOPROTEIN FROM DROSOPHILA - A PUTATIVE LIPOPHORIN

A detailed understanding of the mechanism of Lipid transport in insect s has been hampered by the inability to identify the proapolipophorin gene that encodes apolipophorins I and II, the principal protein compo nents of lipophorin, the lipid transport vehicle, Here we provide the first molecular description of the Drosophila gene encoding a retinoid - and fatty acid-binding glycoprotein (RFABG) and present evidence tha t it is a member of the proapolipophorin gene family, The gene, locali zed to the chromosome 4 (102 F region), encodes a 3351-amino acid prot ein that could serve as the precursor for the similar to 70-kDa and > 200-kDa polypeptides associated with RFABG, The N-terminal sequence of the similar to 70-kDa polypeptide and that predicted for the > 200-kD a polypeptide showed high sequence similarity to blowfly apolipophorin II and apolipophorin I, respectively. The RFABG precursor contains a signal peptide and exhibits a multidomain mosaic protein structure, wh ich is typical of extracellular proteins. It has structural domains si milar to lipid-binding proteins, namely vitellogenins and apolipoprote in B. The protein also contains a domain similar to the D domain of vo n Willebrand factor and mucin, The gene is expressed in the Drosophila embryo during development in cells that make up the amnioserosa and f at bodies. Immunolocalizations using specific antibodies against RFABG reveal that the protein is initially dispersed through the embryonic amnioserosa sac and latter concentrated at skeletal muscle-epidermis a podemeal contact junctions during larval development, This novel gene may play an important role in the transport of lipids, including retin oids and fatty acids, in insects.