The RING finger protein SNURF modulates nuclear trafficking of the androgen receptor

The androgen receptor (AR) is a transcription factor that mediates androgen action. We have used the green fluorescent protein (GFP) technique to inve stigate dynamics of nuclear trafficking of human AR in living cells. In the absence of ligand, the GFP-AR fusion protein is distributed between cytopl asm and nuclei. Androgen exposure leads to a rapid and complete import of G FP-AR to nuclei of CV-1 cells (greater than or equal to 90% nuclear in 30 m inutes), whereas a pure antiandrogen, Casodex, elicits a slower (<40% nucle ar in 30 minutes) and incomplete transfer. Unliganded ARs with mutations in the basic amino acids of the bipartite nuclear localization signal (NLS) w ithin the second zinc finger and the hinge region are predominantly cytopla smic and their androgen-dependent nuclear import is severely compromised (l ess than or equal to 20% nuclear in 30 minutes). Interestingly, substitutio ns of the Leu residues flanking the bipartite NLS lead to inefficient nucle ar transfer in response to androgen (less than or equal to 20% nuclear in 3 0 minutes). The ligand-binding domain of AR, which represses bipartite NLS activity, contains an agonist-specific NLS. The small nuclear RING finger p rotein SNURF, which interacts with AR through a region overlapping with the bipartite NLS, facilitates AR import to nuclei and retards its export on h ormone withdrawal. More AR is associated with the nuclear matrix in the pre sence than absence of coexpressed SNURF, We suggest that the SNURF-mediated tethering of AR in nuclei represents a novel mechanism for activating ster oid receptor functions.