In conclusion, we reveal a structural dependency within the SAGA CORE that regulates KAT2A protein stability, and define a degradation pathway that eliminates orphan KAT2A. These results provide insight into why CORE components might be vulnerabilities in cancer, such as AML and neuroblastoma. 9/n

Finally, we asked whether loss of TADA1 would lead to collateral loss of other SAGA components, including its co-translational assembly partner TAF12. Indeed, using TMT proteomics we detected progressive depletion of several TADA1 neighbours in the CORE and HAT modules, including TAF12. 8/n

We identified UBR5, an E3 ligase involved in orphan protein degradation, and OTUD5 as hits whose loss resulted in stabilisation of KAT2A when the SAGA CORE is disrupted, a result which we confirmed by immunofluorescence of endogenous KAT2A in both TAF5L KO and TADA1 KO cells. 7/n

We next wanted to know what mechanism is driving KAT2A protein loss. Since proteasome inhibition rescues KAT2A back to wild-type levels, we teamed up with Caroline Schaetz and @georgwinter.bsky.social to look for factors responsible for the degradation of non-complexed KAT2A. 6/n

To test this hypothesis, we turned to sucrose gradient fractionation. In TAF5L and TADA1 KO cells, KAT2A is lost accumulates in low molecular weight fractions, indicating that the HAT module dissociates from SAGA. Thanks to @supertifurgalab.bsky.social and Gabriel Onea for their help with this. 5/n

Intriguingly, in TAF5L KO cells residual KAT2A was not identified around its normal binding sites at promoters on chromatin, suggesting that the HAT module might have dissociated from SAGA. 4/n

We focused our validation on three CORE components, TAF5L, TAF6L, and TADA1. In KO clones we confirmed a ~50% reduction in KAT2A signal, which could be rescued by cDNA overexpression. Interestingly, this was sufficient to phenocopy KAT2A KO at the level of H3K9ac. 3/n

Using a fluorescent KAT2A stability reporter to monitor KAT2A abundancy upon KO of other members of SAGA, we found that loss of HAT and of specific CORE components results in a reduction in KAT2A protein abundance. 2/n