Linking iron-sulphur cluster assembly to DNA metabolism

The cytoplasmic protein MMS19 impacts on DNA metabolism by serving as a platform for iron-sulphur cluster transfer to DNA repair and replication proteins.

HFSP Long-Term Fellows Kerstin Gari and Simon Boulton and colleagues
authored on Mon, 02 July 2012

Cells are constantly exposed to factors that damage DNA, either from exogenous sources, such as UV light, or endogenous sources, such as oxidative stress. Moreover, DNA replication itself poses a problem to genome integrity. Not surprisingly, a huge number of proteins work together to faithfully replicate DNA and to detect, signal and repair DNA damage. Failure to do so results in genome instability, one of the hallmarks of cancer.

 

Figure: The cytoplasmic protein MMS19 acts as a platform to facilitate FeS cluster transfer to target FeS proteins involved in DNA metabolism. In this model, the FeS protein DNA polymerase delta (Pol δ) turns from transparent to yellow following FeS cluster transfer mediated by MMS19. After FeS cluster incorporation, Pol δ is fully functional and ready to go to the nucleus.

Over the last years, a surprisingly high number of proteins involved in DNA replication and repair have been identified to bind to an iron-sulphur (FeS) cluster. Maturation of FeS proteins is a multi-step process that takes place in mitochondria and the cytoplasm, but how it is linked to nuclear proteins has remained unclear.

In our study we could establish MMS19 as a key factor that links FeS cluster biogenesis to client FeS proteins.

MMS19 is a highly conserved protein that had previously been shown to impact on DNA repair, transcription, mitotic spindle formation, chromosome segregation, and heterochromatin silencing. However, prior to our study a molecular explanation of how MMS19 could impact on so many different processes had remained elusive.

Using mass spectrometry and co-immunoprecipitation analysis, we could show that MMS19 interacts with multiple FeS proteins including the DNA replication and repair proteins DNA polymerase delta, DNA primase, DNA2, XPD, RTEL1, and FANCJ. We also found that MMS19 forms a complex with proteins of the cytoplasmic FeS assembly machinery, suggesting that MMS19 may physically link FeS cluster biogenesis to client FeS proteins. Consistent with this role, we found that Saccharomyces cerevisiae Mms19 is required for the transfer of FeS clusters to target proteins. Moreover, the stability of multiple FeS proteins involved in either DNA replication or repair was severely affected in the absence of MMS19 in human cells. Consistent with an essential role in DNA repair and replication, MMS19 deficiency in yeast or human cells conferred increased sensitivity to hydroxyurea and an inability to efficiently enter S phase under conditions of limiting nucleotide pools. Moreover, Mms19 knockout mice are embryonically lethal before the implantation stage, suggesting that Mms19 is required for embryonic viability.

Collectively, our data let us to propose that MMS19 functions as part of the cytoplasmic FeS assembly machinery to facilitate FeS cluster transfer to target FeS proteins, many of which play essential roles in DNA replication and repair. Furthermore, our study provides molecular insight to explain the previously reported phenotypes associated with MMS19 deficiency and why defects in mitochondrial FeS cluster biogenesis confer genome instability.

Reference

MMS19 links cytoplasmic iron-sulfur cluster assembly to DNA metabolism. Gari, K., León Ortiz, A.M., Borel, V., Flynn, H., Skehel, J.M. & Boulton, S.J.  Science, 7 June 2012 (10.1126/science.1219664).

Pubmed link