We investigated the protein composition of J774-E clone macrophage phagosomes isolated at different stages of phagolysosome biogenesis. Phagosomes formed by internalizing antibody-coated Staphylococcus aureus for 3 min followed by chase for 0, 4, 9, or 15 min were isolated by density gradient centrifugation. Enrichment and purity of the phagosome preparations were quantitated by radiolabeled ligand recovery, enzyme markers, and electron microscopy. One-dimensional SDS-PAGE analyses of the isolated phagosomes revealed virtually identical protein compositions. However, Western blot analyses with antibodies directed against selected proteins of known itineraries along the endocytic pathway demonstrated distinct differences in phagosome protein compositions. Accumulating within the maturing phagosome were the 31-kD subunit of the vacuolar proton pump, cathepsin D, jl-glucuronidase, the cation dependent mannose 6-phosphate receptor, and LAMP-1. Decreasing within the maturing phagosome were the FcII receptor, the mannose receptor, and alpha-adaptin. These results indicate that although the macrophage phagosome's total protein composition changes little during phagolysosome formation, the maturing phagosome both receives and eliminates, possibly by protein recycling, specific membrane and sequestered proteins. (J. Clin. Invest. 1992. 90:1978

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We have isolated phage clones from Drosophila melanogaster genomic and cDNA libraries containing a sequence homologous to the murine Int-i protooncogene. The Drosophila gene is represented by a single locus at position 28A1-2 on chromosome 2. The gene is expressed as a 2.9-kilobase-long polyadenylylated mRNA in embryo, larval, and pupal stages. It is hardly detectable in adult flies. The longest open reading frame of the cDNA clone corresponds to a protein 469 amino acids long. Alignment of the predicted amino acid sequences shows that the Drosophila protein is 86 amino acids longer than its murine counterpart. In spite of the difference in length, the two proteins are highly conserved with an overall sequence homology of 54%. Both Drosophila and murine Int-i proteins begin with a hydrophobic leader sequence and contain cysteine residues and sites for glycosylation (four in the murine protein and one in the Drosophila protein) in conserved positions, suggesting that they play important functional roles. Cellular protooncogenes have been highly conserved by evolution, suggesting that they may serve important household functions (1). Although their precise function is not known or is only incompletely known, it is already clear that they fall into several groups, such as growth factor or growth factor receptor genes, signal transducers, and DNA-binding nuclear proteins (1). They are believed to participate in the regulation of cell division, although at different levels. Some oncogenes have been shown to hybridize with highly homologous sequences in such genetically well-known organisms as yeast or Drosophila. This is of great interest because the knowledge and the possibility of genetic manipulations provide unparalleled possibilities to obtain important functional clues. The pioneering studies of Shilo have identified 4 oncogene homologs in Drosophila DNA (2). The total number has now increased to 11 and includes three major families, the tyrosine kinases, the ras-group, and the nuclear oncogenes (3). In a search for previously unidentified oncogene homologs, we have screened Drosophila genomic libraries with cDNA probes of mammalian c-myc, N-myc, c-fos and c-int-i. We report the isolation and preliminary characterization of a Drosophila int-i homolog. § The cellular homolog of the viral v-int-i has been discovered and implicated in mouse mammary tumorigenesis on the basis of its frequent insertional activation in murine mammary tumor virus (MMTV)-associated tumors (4); the murine homolog is called lnt-i. It is expressed in the fetal central nervous system (5, 6) and during the late stages of spermatogenesis (6) but not in most normal adult tissues. The murine Int-1 protein contains 370 amino acids and is rich in cysteine residues (7). The active product of the gene and its biochemical features in normal and malignant cells are not known. MATERIALS AND METHODSMolecular Cloning. A recombinant Drosophila genomic library cloned into Charon 4A (8) was screened with a 32P-labeled murine Int-1 cDNA probe (9) b...