Subnuclear distribution of SSX regulates its function

Area of Science:

• Molecular Biology
• Cancer Genetics
• Cellular Stress Response

Background:

• The SSX gene family, located on the X chromosome, is recognized as a cancer-testis antigen.
• SSX is a component of the SYT-SSX fusion gene in synovial sarcoma, suggesting a role in tumorigenesis.
• Limited understanding exists regarding the molecular regulation of SSX.

Purpose of the Study:

• To investigate the molecular regulation and cellular localization of SSX.
• To elucidate the interaction between SSX and Bmi1 (a core factor of polycomb repressor complex 1).
• To explore the role of SSX in cellular stress responses.

Main Methods:

• Immunofluorescence microscopy to observe SSX and Bmi1 localization.
• Analysis of SSX domains for nuclear speckle distribution and Bmi1 recruitment.
• Investigation of SSX nucleolar translocation under stress conditions (hydrogen peroxide, heat shock).
• Assessment of Bmi1 activity following SSX translocation.

Main Results:

• SSX and its SYT fusion protein localize to nuclear speckles, co-localizing with Bmi1.
• The C-terminus of SSX is essential for nuclear speckle localization, while the N-terminus mediates Bmi1 recruitment.
• The SSX N-terminus contains a nucleolar localization signal, mediating translocation to the nucleolus under cellular stress.
• Stress-induced SSX translocation is reversible and involves HSP 70 and p14ARF.
• Translocation causes SSX dissociation from Bmi1, leading to down-regulation of Bmi1 activity.

Conclusions:

• SSX interacts with Bmi1 within nuclear speckles, with distinct domains mediating localization and interaction.
• SSX functions as a stress response gene, translocating to the nucleolus under stress.
• This stress-induced translocation modulates Bmi1 activity, offering insights into synovial sarcoma development and SSX's role in cancer.