Huge thanks to all people involved, @smoa-cbm.bsky.social and flow cytometry facilities from @cniostopcancer.bsky.social and @cbm-csic-uam.bsky.social.

So, to sum up: VCP/p97 ➡️ POLA/PRIM on chromatin ➡️ priming ➡️ basal RSR activation! This work was led by @emlecona.bsky.social from @cbm-csic-uam.bsky.social in collaboration with Juan Mendez' group from @cniostopcancer.bsky.social. Want to dive a little bit more in our work? Click here! rdcu.be/eu1yo

And the activation of the RSR induced by VCPi is relevant, as the phosphorylation of CHK1 leads to an arrest in G2/M. More importantly, CHK1 activation is lost when we block the priming action of POLA/PRIM.

What are the consequences of more POLA/PRIM on chromatin? We observed that the inhibition of VCP/p97 also increased priming by POLA/PRIM, as observed in the EdU comet assay.

In fact, we showed that VCP/p97 and POLA/PRIM do interact. Interestingly, VCP/p97 inhibition could not induce the accumulation of POLA/PRIM on chromatin if origin firing is inhibited with CDC7i.

It struck our attention that the increased activation of the RSR induced when VCP/p97 is inhibited is mirrored by the accumulation of POLA/PRIM on chromatin. Could our two guests be working together?

First, we found that VCP/p97 is necessary for DNA replication and its inhibition leads to defects in fork progression and origin firing that result in the activation of the RSR.

Our second guest, VCP/p97, is an AAA ATPase that extracts proteins from chromatin when they are no longer needed to allow their degradation or recycling. What part do they play in the ATR story?

In the replication of the lagging strand, multiple priming events by POLA/PRIM are required to generate short pieces of replicated DNA or Okazaki fragments.

When an origin of replication is fired, the CMG helicase unwinds the parental DNA duplex, allowing access to the replication machinery. There, the DNA polymerase alpha/primase (POLA/PRIM) complex synthesizes the primers to be extended by DNA polymerase delta and epsilon.

The question is, how is the basal activation of ATR during an unperturbed S phase? We found two unexpected players in this process. Let us introduce to you VCP/p97 and POLA/PRIM, our guests in today’s show.

The basal activation of ATR and CHK1 kinases keeps the activation of replication origins in check to prevent the exhaustion of the replication factors and the onset of genomic instability.

While copying the DNA, replication forks face obstacles that stall their progression. To handle this, cells rely on the replication stress response (RSR) pathway but… the RSR is essential even during normal DNA replication!