Quantitative analysis of mouse interferon-beta receptor-mediated endocytosis and nuclear entry

The dynamics of internalization and transport of murine interferon-beta (MuIFN-beta) to the cell nucleus was studied in cultured L929 mouse fibroblasts employing postembedding, ultrastructural quantitative immunocytochemistry. Cells preincubated with MuIFN-beta at 4 degrees C were transferred to 37 degrees C for various times, and then fixed and embedded in Lowicryl K4M. Internalized native IFN molecules were identified and quantified on the surface of ultrathin sections with monoclonal (MAb) or polyclonal (PAb) anti-IFN antibodies and a colloidal gold-protein A (CGPA) conjugate. Immunocytochemically recognizable MuIFN-beta molecules entered the cytoplasm by receptor-mediated endocytosis and were translocated via transport vesicles to the nuclear envelope. IFN molecules entered the nucleus through nuclear pores and accumulated maximally in the dense chromatin within 3 min of incubation of the cells at 37 degrees C. The data presented constitute the first direct observation of the dynamics of the extremely rapid transfer of ligand to its intracellular target, in this case, the cell nucleus.