A LIM protein, Hic-5, functions as a potential coactivator for Sp1.

Hic-5 is a LIM protein with striking similarity to paxillin, and shuttles between focal adhesions and the nucleus. Our previous study suggested that Hic-5 participates in the transcriptional control of several genes such as the c-fos and p21 genes. In the present study, we examined the function of Hic-5 in the nucleus using the transcriptional promoter region of the p21 gene. When localized to the nucleus, Hic-5 was found to transactivate the p21 promoter through two of five Sp1 sites in the region proximal to the TATA box. The Hic-5 effect was mediated by a transactivation domain of Sp1 and functional interaction with p300 through the LIM4 domain. Hic-5 was also shown to interact functionally and physically with Smad3 through the LIM domains and to potentiate p21 promoter activity together with Smad3 and Sp1. These properties were confirmed in an artificial system using GAL4-fusion protein. Thus, Hic-5 was suggested to have a potential function as a cofactor in the transcriptional complex that contains Sp1, playing a role in gene transcription in the nucleus as well as in integrin signaling at focal adhesion sites.