DREAM Interrupted: Severing MuvB from DREAM’s pocket protein inCaenorhabditis elegansimpairs gene repression but not DREAM chromatin assembly

Abstract

AbstractThe mammalian pocket protein family, which includes the Retinoblastoma protein (pRb) and Rb-like pocket proteins p107 and p130, regulates entry into and exit from the cell cycle by repressing cell cycle gene expression. Although pRb plays a dominant role in mammalian systems, p107 and p130 represent the ancestral pocket proteins. The Rb-like pocket proteins interact with the highly conserved 5-subunit MuvB complex and an E2F-DP transcription factor heterodimer, forming the DREAM (forDp,Rb-like,E2F,andMuvB) complex. DREAM complex formation on chromatin culminates in direct repression of target genes mediated by the MuvB subcomplex. Here, we examined how the Rb-like pocket protein contributes to DREAM formation by disrupting the interaction between the soleCaenorhabditis eleganspocket protein LIN-35 and the MuvB subunit LIN-52 using CRISPR/Cas9 targeted mutagenesis. Disrupting the LIN-35-MuvB association did not affect DREAM chromatin occupancy but did cause a highly penetrant synthetic multivulval (SynMuv) phenotype, indicating that blocking DREAM assembly impairs MuvB function. Some DREAM target genes became derepressed, indicating that for those genes MuvB chromatin binding alone is not sufficient for gene repression and that direct LIN-35-MuvB association potentiates MuvB’s innate repressive activity. In a previous study we showed that in worms lacking LIN-35, E2F-DP and MuvB chromatin occupancy is reduced genome-wide. With LIN-35 present, this study demonstrates that the E2F-DP-LIN-35 interaction promotes E2F-DP’s chromatin localization, which we hypothesize supports MuvB chromatin occupancy indirectly through DNA. Altogether, this study highlights how the pocket protein family may recruit regulatory factors like MuvB to chromatin through E2F-DP to facilitate their transcriptional activity.