The proteolytic YB-1 fragment interacts with DNA repair machinery and enhances survival during DNA damaging stress.

The Y-box binding protein 1 (YB-1) is a DNA/RNA-binding nucleocytoplasmic shuttling protein whose regulatory effect on many DNA and RNA-dependent events is determined by its localization in the cell. We have shown previously that YB-1 is cleaved by 20S proteasome between E219 and G220, and the truncated N-terminal YB-1 fragment accumulates in the nuclei of cells treated with DNA damaging drugs. We proposed that appearance of truncated YB-1 in the nucleus may predict multiple drug resistance. Here, we compared functional activities of the full-length and truncated YB-1 proteins and showed that the truncated form was more efficient in protecting cells against doxorubicin treatment. Both forms of YB-1 induced changes in expression of various genes without affecting those responsible for drug resistance. Interestingly, although YB-1 cleavage did not significantly affect its DNA binding properties, truncated YB-1 was detected in complexes with Mre11 and Rad50 under genotoxic stress conditions. We conclude that both full-length and truncated YB-1 are capable of protecting cells against DNA damaging agents, and the truncated form may have an additional function in DNA repair.

Proteasome-mediated cleavage of the Y-box-binding protein 1 is linked to DNA-damage stress response.

YB-1 is a DNA/RNA-binding nucleocytoplasmic shuttling protein whose regulatory effect on many DNA- and RNA-dependent events is determined by its localization in the cell. Distribution of YB-1 between the nucleus and the cytoplasm is known to be dependent on nuclear targeting and cytoplasmic retention signals located within the C-terminal portion of YB-1. Here, we report that YB-1 undergoes a specific proteolytic cleavage by the 20S proteasome, which splits off the C-terminal 105-amino-acid-long YB-1 fragment containing a cytoplasmic retention signal. Cleavage of YB-1 by the 20S proteasome in vitro appears to be ubiquitin- and ATP-independent, and is abolished by the association of YB-1 with messenger RNA. We also found that genotoxic stress triggers a proteasome-mediated cleavage of YB-1 in vivo and leads to accumulation of the truncated protein in nuclei of stressed cells. Endoproteolytic activity of the proteasome may therefore play an important role in regulating YB-1 functioning, especially under certain stress conditions.