The androgen receptor (AR) acts as an androgen-dependent transcription factor controlling the development of prostate tissue. Upon binding to androgen, AR undergoes a dynamic Structural change leading to interaction between the NH2- and COOH-terminal regions of AR (N-C interaction). ARA24/Ran, which is a small GTPase, functions as an AR coactivator. Here, We report that ARA24/Ran enhances the N-C interaction of AR. The constitutively GTP- or GDP-bound form of ARA24/Ran repressed the AR N-C interaction. ARA24/Ran did not enhance the transcriptional activities of AR mutants that disrupt the N-C interaction. ARA24/Ran formed an endogenous protein complex with nuclear AR, but not cytoplasmic AR. Unlike SRC-1 with the positive activity for AR N-C interaction, ARA24/Ran did not enhance the transcriptional activity of the COOH-terminal domain-deleted AR mutant that is constitutively localized in the nucleus. These data demonstrate that ARA24/Ran increases AR transactivation by enhancing the AR N-C interaction in the nucleus. (c) 2008 Elsevier Inc. All rights reserved.