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

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Abstract
Human .alpha.2-macroglobulin (.alpha.2M)was adsorbed to colloidal Au and used as a new tool in the study of receptor-mediated endocytosis. .alpha.2M-Au is easy to prepare and is clearly visualized at the EM level. When cells [normal rat kidney NRK-2T cell and Swiss mouse fibroblast 3T3 cell] were incubated with .alpha.2M-Au at 0.degree. C, Au was visualized diffusely over the cell surface and concentrated in coated pits. After cells to which .alpha.2M-Au had been bound at 0.degree. C were warmed, the Au was rapidly internalized into uncoated vesicles, previously termed receptosomes. After 30 min of incubation or longer, Au 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.2M antibodies. By incubating cells with both .alpha.2M-Au and vesicular stomatitis virus (VSV), the internalization of these 2 markers were studied simultaneously. VSV and .alpha.2M-Au rapidly clustered in the same coated pits and were internalized in the same receptosomes. Proteins and hormones adsorbed to Au may be useful in the study of receptor-mediated endocytosis.