We have developed an assay that permits the in vivo monitoring of steroid receptor protein nuclear export. In this assay co-cultured heterologous cells are fused using polyethylene glycol to generate multi-nucleated heterokaryon with shared cytoplasm. Since only one of the two cell types used to generate a heterokaryon will contain receptor protein within its nucleus (i.e. the donor cell), any receptor detected within the heterologous nucleus of the original receptor cell type (i.e. recipient cell) must derive from protein that exported from the donor cell nucleus. This assay will be used to determine whether the glucocorticoid, progesterone, and thyroid hormone receptors possess the capacity to export in isolation when fused to heterologous proteins that do not normally have this capacity. We anticipate that this system of analysis will provide novel insights into the mechanisms that regulate bi-directional movement of proteins across the nuclear envelope.
