DREAM Interrupted: Severing LIN-35-MuvB association in Caenorhabditis elegans impairs DREAM function but not its chromatin localization

Abstract

47The mammalian pocket protein family, which includes the Retinoblastoma protein (pRb) 48 and Rb-like pocket proteins p107 and p130, regulates entry into and exit from the cell 49 cycle by repressing cell cycle gene expression. Although pRb plays a dominant role in 50 mammalian systems, p107 and p130 represent the ancestral pocket proteins. The Rb-51 like pocket proteins interact with the highly conserved 5-subunit MuvB complex and an 52 E2F-DP transcription factor heterodimer, forming the DREAM (for Dp, Rb-like, E2F, and 53 MuvB) complex. DREAM complex formation on chromatin culminates in direct 54 repression of target genes mediated by the MuvB subcomplex. Here, we examined how 55 the Rb-like pocket protein contributes to DREAM formation by disrupting the interaction 56 between the sole Caenorhabditis elegans pocket protein LIN-35 and the MuvB subunit 57 LIN-52 using CRISPR/Cas9 targeted mutagenesis. Disrupting the LIN-35-MuvB 58 association did not affect DREAM chromatin occupancy but did cause a highly 59 penetrant synthetic multivulval (SynMuv) phenotype, indicating that blocking DREAM 60 assembly impairs MuvB function. Some DREAM target genes became derepressed, 61 indicating that for those genes MuvB chromatin binding alone is not sufficient for gene 62 repression and that direct LIN-35-MuvB association potentiates MuvB's innate 63 repressive activity. In a previous study we showed that in worms lacking LIN-35, E2F-64 DP and MuvB chromatin occupancy is reduced genome-wide. With LIN-35 present, this 65 study demonstrates that the E2F-DP-LIN-35 interaction promotes E2F-DP's chromatin 66 localization, which we hypothesize supports MuvB chromatin occupancy indirectly 67 through DNA. Altogether, this study highlights how the pocket protein family may recruit 68 4 regulatory factors like MuvB to chromatin through E2F-DP to facilitate their 69 transcriptional activity.

Auxin-Induced Degradation of Dream Proteins, Lin-9 and Lin-54, in Caenorhabditis Elegans

“…This is likely because C. elegans LIN-52 does not possess the phosphorylation switch of mammalian LIN52 and to date there has not been a BMYB homolog found in C. elegans [6]. In addition, this complex protects developing somatic cells by repressing germline genes and plays a role in cell fate specification through its antagonization of Ras signaling during the animal's vulval development [15,16]. The C. elegans DRM complex presents a unique opportunity to examine how DREAM represses target genes.…”