alpha 2-macroglobulin adsorbed to colloidal gold: a new probe in the study of receptor-mediated endocytosis.

alpha 2-Macroglobulin (alpha 2 M) was adsorbed to colloidal gold and used as a new tool in the study of receptor-mediated endocytosis. alpha 2 M-gold is easy to prepare and is clearly visualized at the electron microscope level. When cells were incubated with alpha 2 M-gold at 0 degrees C, gold was visualized both diffusely over the cell surface and concentrated in coated pits. After cells to which alpha 2 M-gold had been bound at 0 degrees C were warmed, the gold was rapidly internalized into uncoated vesicles, previously termed receptosomes. After 30 min of incubation or longer, gold was found in small lysosomes and, later, in large lysosomes and very small vesicles in the region of the Golgi complex. This pattern of localization is similar to that previously described, using peroxidase-labeled anti-alpha 2 M antibodies. By incubating cells with both alpha 2 M-gold and vesicular stomatitis virus (VSV), we studied the internalization of these two markers simultaneously. VSV and alpha 2 M-gold rapidly clustered in the same coated pits and were internalized in the same receptosomes. Proteins and hormones adsorbed to gold may be useful in the study of receptor-mediated endocytosis.