Characterization of the human Suppressor of fused, a negative regulator ofthe zinc-finger transcription factor Gli

Drosophila Suppressor of fused (Su(fu)) encodes a novel 468-amino-acid cyto plasmic protein which, by genetic analysis, functions as a negative regulat or of the Hedgehog segment polarity pathway. Here we describe the primary s tructure, tissue distribution, biochemical and functional analyses of a hum an Su(fu) (hSu(fu)). Two alternatively spliced isoforms of hSu(fu) were ide ntified, predicting proteins of 433 and 484 amino acids, with a calculated molecular mass of 48 and 54 kDa, respectively. The two proteins differ only by the inclusion or exclusion of a 52-amino-acid extension at the carboxy terminus. Both isoforms were expressed in multiple embryonic and adult tiss ues, and exhibited a developmental profile consistent with a role in Hedgeh og signaling. The hSu(fu) contains a high-scoring PEST-domain, and exhibits an overall 37% sequence identity (63% similarity) with the Drosophila prot ein and 97% sequence identity with the mouse Su(fu). The hSu(fu) locus mapp ed to chromosome 10q24-q25, a region which is deleted in glioblastomas, pro state cancer, malignant melanoma and endometrial cancer. HSu(fu) was found to repress activity of the zinc-finger transcription factor Gli, which medi ates Hedgehog signaling in vertebrates, and to physically interact with Gli , Gli2 and Gli3 as well as with Slimb, an F-box containing protein which, i n the fly, suppresses the Hedgehog response, in part by stimulating the deg radation of the fly Gli homologue. Coexpression of Slimb with Su(fu) potent iated the Su(fu)mediated repression of Gli. Taken together, our data provid e biochemical and functional evidence for the hypothesis that Su(fu) is a k ey negative regulator in the vertebrate Hedgehog signaling pathway. The dat a further suggest that Su(fu) can act by binding to Gli and inhibiting Gli- mediated transactivation as well as by serving as an adaptor protein, which links Gli to the Slimb-dependent proteasomal degradation pathway.