A KRAB-RELATED DOMAIN AND A NOVEL TRANSCRIPTION REPRESSION DOMAIN IN PROTEINS ENCODED BY SSX GENES THAT ARE DISRUPTED IN HUMAN SARCOMAS

SSX genes show extensive nucleotide sequence conservation but little i s known of their function. Disruption of SSX1 or SSX2, by chromosome t ranslocation and 'in-frame' fusion to SYT, is a consistent feature of synovial sarcomas, The resulting SYT-SSX1/SSX2 proteins are activators of transcription; transactivation function is located in SYT, Unrearr anged SSX1 can repress transcription, and this has been attributed to a putative Kruppel associated box (KRAB) repression domain at the N-te rminus. Here we isolated SSX-KRAB domains to specifically measure repr ession activity, using a previously characterized KOX1-KRAB domain as a control. In our repressor assay SSX1- and SSX2-KRAB domains down-mod ulated the transactivation of reporter gene by threefold, compared wit h 83-fold repression achieved by KOX1-KRAB in the assay. Yeast two-hyb rid analysis showed that SSX1-KRAB, unlike KOX1-KRAB, fails to interac t with the KRAB co-repressor TIF1 beta. These results raise questions about the evolutionary and functional relationship of SSX-KRAB and typ ical KRAB domains of Kruppel zinc finger genes. We found that full-len gth SSX1 showed potent (74-fold) repression in our repressor assay, in dicating the existence of a repression domain distinct from SSX-KRAB. By assaying deletion constructs of SSX1 we localized repression activi ty to 33 amino acids at the C-terminus. This novel domain is conserved between SSX family members, and, unlike the KRAB-related domain, is r etained on fusion with SYT, This has important implications in underst anding the mechanism by which the SYT-SSX fusion protein could contrib ute to neoplasia.