Endosome-lysosome fusion at low temperature

Based on an initial study (Dunn, W. A., Hubbard, A. L., and Aronson, Jr., N. N. (1980) J. Biol. Chem. 255, 5971-5978), low temperature is often used to selectively inhibit fusion between endosomes and lysosomes. Here we have tried to characterize the nature of this inhibition. In addition to endocytic contents markers, we have used a covalent membrane marker to measure the interaction between endosomes and lysosomes over extended periods of time at low temperature. Mouse macrophage cells (P388D1) and human skin fibroblasts were enzymatically labeled with radioactive galactose to provide a covalent marker for plasma-membrane glycoconjugates. Subsequent endocytic membrane traffic for 24 h at 16 degrees C resulted in a significant transfer of membrane marker, as well as of endocytic contents marker, to high density lysosomes, as observed by subcellular fractionation. The kinetics of this transfer have been analyzed for macrophages using the membrane marker, horseradish peroxidase as fluid-phase, and iodinated acetyl low density lipoprotein as receptor-mediated endocytic contents marker. Transfer to lysosomes occurred only about 6 h after application of the respective marker at 16 degrees C. When transfer to lysosomes was initiated by 15 min preincubation at 37 degrees C, subsequent cooling to 16 degrees C did not inhibit ongoing transfer which continued with the same kinetics as when observed after the lag phase. These results show that low temperature delays an unidentified pre-fusion step, but does not inhibit endosome-lysosome fusion as such.