The coiled-coil coactivator (CoCoA) is a transcriptional coactivator for nuclear receptors and enhances nuclear receptor function by the interaction with the bHLH-PAS domain (AD3) of p160 coactivators. The C-terminal activation domain (AD) of CoCoA possesses strong transactivation activity and is required for the coactivator function of CoCoA with nuclear receptors. To understand how CoCoA AD transmits its activating signal to the transcription machinery, we defined specific subregions, amino acid motifs and protein binding partners involved in the function of CoCoA AD. The minimal transcriptional AD was mapped to approximately 91 C-terminal amino acids and consists of acidic, serine/proline-rich and phenylalanine-rich subdomains. Transcriptional activation by the CoCoA AD was p300-dependent, and p300 interacted physically and functionally with CoCoA AD and was recruited to a promoter by the interaction with CoCoA AD. The FYDVASAF motif in the CoCoA AD was critical for the transcriptional activity of CoCoA AD, the interaction of CoCoA with p300, the coactivator function of CoCoA for estrogen receptor alpha and GRIP1 and the transcriptional synergy among coactivators GRIP1, CARM1, p300 and CoCoA. Taken together these data extend our understanding of the mechanism of downstream signaling by the essential C-terminal AD of the nuclear receptor coactivator CoCoA; they indicate that p300 is a functionally important interaction partner of CoCoA AD and that their interaction potentiates transcriptional activation by the p160 coactivator complex.