Monitoring phospholipid dynamics <i>in vivo</i> with a fluorescent dye octadecyl rhodamine B

Phospholipids are major components of biological membranes. They play an essential role in intracellular signaling and organelle dynamics; however, the availability of suitable lipid-specific probes is limited, which has hindered studies on their spatial distribution and functional dynamics in living cells. Previously, we demonstrated that octadecyl rhodamine B chloride (R18) is transported to the endoplasmic reticulum via nonvesicular membrane transport. In this study, we showed that R18 is internalized in a phosphatidylethanolamine (PE)-dependent manner in vivo. The internalization of R18 in Saccharomyces cerevisiae is blocked in PE-deficient mutants, but restored by ethanolamine supplementation, which suggests strict PE dependence. Moreover, R18 delivered to vacuoles through autophagy was not terminally retained, but underwent Pep4- and Atg15-dependent export from the vacuoles. The exported R18 was then redirected to endosomes following prolonged autophagy. These results suggest that R18 may serve as an indicator of PE dynamics and vacuole-endosome lipid transport, which contributes to lipid homeostasis inside vacuoles.Key words: autophagy, in vivo lipid dynamics, octadecyl rhodamine B (R18), phospholipase, phospholipid, vacuole, yeast.