Defective acidification ofendosomes inChinese hamster ovarycell mutants "cross-resistant

ABSTRACT Like manyphysiological ligands, several virusesandtoxins entermammaliancells throughreceptor-mediateden-docytosis. Onceinternalized, the nucleic acids ofseveral virusesandthetoxic subunitofdiphtheriatoxin gainaccess to thecytosolof the host cell through anacidic intracellular compartment. Inthis report,wepresentevidencethatoneclassofmutantsofChinesehamsterovary (CHO)-K1 cells, whichis "cross-resistant" toPseu- domonas exotoxinA, diphtheria toxin, andseveralanimalviruses,hasadefect in acidification ofthe endosome. Cells wereallowedto internalize fluorescein isothiocyanate-conjugated dextran be-fore subcellular fractionation. Fluorescence measurements onsubcellular fractions permitted measurementof the internal pHof the isolated endosomes and lysosomes. Ourresults showthat(i) endosomes andlysosomes fromCHO-KIcells maintainanacidicpH, (ii) acidificationofbothendosomesand lysosomes is mediatedbya Mg2"/ATP-dependent process, (iii) GTPcansatisfytheATPrequirementforacidification of