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Like the stomach, lysosomes soak their contents in an acid bath. Using two new techniques, Steinberg et al. show that lysosomes can hike their acidity because they shed positive ions.
To keep its internal pH between 4 and 5, a lysosome actively pumps in protons. But as more positively charged hydrogen ions enter, continued importation becomes harder. The organelle needs to balance the electrical charge across its membrane. Researchers assumed that lysosomes counteract the increase in positive charge by allowing in negatively charged chloride ions through channels such as the cystic fibrosis transmembrane conductance regulator (CFTR) channel, the protein that's defective in cystic fibrosis.
But Steinberg et al. found that lysosomes could still acidify in mice lacking CFTR or another key chloride transporter, ClC-7. By temporarily punching holes in the cell membrane, the researchers replaced chloride ions in the cytosol with bulky, negatively charged molecules that can't diffuse into the lysosome. Even after this substitution, lysosomes still reduced their pH, indicating that a chloride influx isn't necessary.
Instead, the lysosomes offset the entering protons by losing positively charged sodium and potassium ions. To explore this effect, the researchers temporarily permeabilized the lysosome membrane, allowing them to adjust the organelle's contents. Steinberg et al. replaced the internal potassium with a positively charged molecule that can't pass through the lysosome membrane after it reseals. Organelles altered in this way couldn't increase their acidity.
The results don't mean that chloride has no role, the researchers say. They suggest that lysosomes probably lose some positive ions and gain some chloride, thus avoiding any osmotic swelling or shrinking.