Characterization of rapid membrane internalization and recycling

Lipids and other membrane constituents recycle between the plasma membrane and intracellular endocytic compartments. In CHO cells, approximately half of the internalized C-6-NBD-SM, a fluorescent lipid analogue widely used as a membrane maker, recycles via the endocytic recycling compartment with a t(1/2) of similar to 12 min (Mayor, S., Presley, J. F,, and Maxfield, F. R, (1993) J, Cell Biol. 121, 1257-1269), Surprisingly, the rest returns to the plasma membrane very quickly. A detailed kinetic study presented in this paper indicates that after a brief internalization pulse, 42-62% of the internalized C-6-NBD-SM returns to the plasma membrane with a t1/2 of 1-2 min. Similar results are obtained using HEp2 and nonpolarized Madin-Darby canine kidney cells. Using FM dyes of different hydrophobicity, we show that rapid recycling involves passage through an endocytic organelle that was subsequently identified as the sorting endosome by co-localization with internalized transferrin and low density lipoprotein, These results imply that the membrane internalization rate is much higher than previously estimated, with a t(1/2) as short as 5-10 min. Rapid internalization and recycling would facilitate processes such as nutrient uptake and cholesterol afflux.