CHANGES IN THE SPECTRAL PROPERTIES OF A PLASMA-MEMBRANE LIPID ANALOG DURING THE FIRST SECONDS OF ENDOCYTOSIS IN LIVING CELLS

N-[5-(5,7-dimethyl 1-pentanoyl]-D-erythro-sphingosylphosphorylcholine (C-5-DMB-SM), a fluorescent analog of sphingomyelin, has been used in a study of the formation of very early endosomes in human skin fibrobl asts. This lipid exhibits a shift in its fluorescence emission maximum from green (similar to 515 nm) to red (similar to 620 nm) wavelengths with increasing concentrations in membranes. When cells were incubate d with 5 mu M C-5-DMB-SM at 4 degrees C and washed, only plasma membra ne fluorescence (yellow-green) was observed. When these cells were bri efly (less than or equal to 1 min) warmed to 37 degrees C to allow int ernalization to occur, and then incubated with defatted bovine serum a lbumin (back-exchanged) at 11 degrees C to remove fluorescent lipids f rom the plasma membrane, C-5-DMB-SM was distributed in a punctate patt ern throughout the cytoplasm. Interestingly, within the same cell some endosomes exhibited green fluorescence, whereas others emitted red-or ange fluorescence. Furthermore, the red-orange endosomes were usually seen at the periphery of the cell, while the green endosomes were more uniformly distributed throughout the cytoplasm. This mixed population of endosomes was seen after internalization times as short as 7 s and was also seen over a wide range of C-5-DMB-SM concentrations (1-25 mu M). Control experiments established that the variously colored endoso mes were not induced by changes in pH, membrane potential, vesicle siz e, or temperature. Quantitative fluorescence microscopy demonstrated t hat the apparent concentration of the lipid analog in the red-orange e ndosomes was severalfold higher than its initial concentration at the plasma membrane, suggesting selective internalization (sorting) of the lipid into a subset of early endosomes. Colocalization studies using C-5-DMB-SM and either anti-transferrin receptor antibodies or fluoresc ently labeled low-density lipoprotein further demonstrated that this s ubpopulation of endosomes resulted from receptor-mediated endocytosis. We conclude that the spectral properties of C-5-DMB-SM can be used to distinguish unique populations of early endosomes from one another an d to record dynamic changes in their number and distribution within li ving cells.