The human transcription factor MafF (hMafF) lacks a transactivation domain, it contains a heptad-leucine repeat motif (viz., Leu-zipper) that may mediate protein-protein interactions to regulate transcriptional activities. A protein with a coiled-coil domain encoded by a novel human gene (GenBank Accession No. AF289559) was found to interact with hMafF in vitro and in vivo and is designated as a MafF interacting protein (MIP). Here, we provide evidence that the coiled-coil domain of MIP is essential for binding to the Leu-zipper of hMafF and that the interaction between MIP and hMafF causes the translocalization of MIP from cytoplasm to nucleolus in HELA cells. We used a promoter-reporter system containing six tandem repeats of the US2 element, located in the promoter of the human oxytocin receptor gene and reported to bind specifically to hMafF, to understand effects on transcriptional activation of hMafF, and its interaction with MIP. Expression of hMafF or MIP alone did not alter basal reporter transcription activity, whereas co-expression of hMafF and MIP activated transcription efficiently. Moreover, truncated MIP, still containing the coiled-coil domain, transactivated as well as the full-length MIP did, and highlighting that MIP acts as a co-activator of hMafF.