title: Secretory ribonucleases are internalized by a dynamin-independent endocytic pathway.
abstract: Cytosolic internalization is a requirement for the toxicity of secretory ribonucleases. Here, we investigate the mechanism of internalization of Onconase (ONC), a toxic protein, and ribonuclease A (RNase A), a nontoxic homolog. Microscopy studies indicate that both ribonucleases readily bind to the cell surface and are internalized via acidic vesicles. Blocking dynamin-dependent endocytosis prevents transferrin internalization but does not hinder RNase A internalization. ONC and G88R RNase A, which is a toxic variant, demonstrate enhanced cytotoxicity in the absence of clathrin- and dynamin-mediated endocytosis. The cytosolic entry of ribonucleases does not require an acidic environment or transport to the ER and probably occurs from endosomes. Thus, common proteins - secretory ribonucleases - enter the cytosol by a pathway that is distinct from that of other known toxins.
authors.affiliation: Department of Biochemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.
authors.forename: Marcia C
authors.initials: MC
authors.lastname: Haigis
authors.forename: Ronald T
authors.initials: RT
authors.lastname: Raines
mesh_terms: D012634:Bodily Secretions / Q000502:physiology; D002462:Cell Membrane / Q000378:metabolism*; D003600:Cytosol / Q000378:metabolism; D003603:Cytotoxins / Q000378:metabolism / Q000494:pharmacology; D034281:Dynamins / Q000378:metabolism*; D004705:Endocytosis / Q000502:physiology*; D004721:Endoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism; D011992:Endosomes / Q000187:drug effects / Q000378:metabolism; D005057:Eukaryotic Cells / Q000201:enzymology* / Q000378:metabolism; D006367:HeLa Cells; D006801:Humans; D008954:Models, Biological; D015394:Molecular Structure; D021381:Protein Transport / Q000502:physiology*; D012259:Ribonuclease, Pancreatic / Q000378:metabolism / Q000494:pharmacology; D012260:Ribonucleases / Q000378:metabolism* / Q000494:pharmacology; D022142:Secretory Vesicles / Q000378:metabolism; D014168:Transferrin / Q000378:metabolism
chemical_list: D003603:Cytotoxins; D014168:Transferrin; D012260:Ribonucleases; D012259:Ribonuclease, Pancreatic; D034281:Dynamins; C058703:ranpirnase