Rb pathway and chromatin remodeling genes that antagonize let-60 Ras signaling during C. elegans vulval development

Abstract

The synthetic multivulva (synMuv) class A and class B genes act redundantly to regulate Ras-mediated vulval cell fate specification in the nematode Caenorhabditis elegans. The class B synMuv gene lin-35 encodes a protein similar to the mammalian pRb tumor suppressor protein. The LIN-35 Rb protein is proposed to act with HDA-1, a histone deacetylase homolog with class B synMuv activity, to remodel chromatin and repress transcription of genes that promote vulval development. To further understand how lin-35 Rb and hda-1 regulate vulval cell fate specification, we identified and characterized additional class B synMuv genes. We found that two of these genes, dpl-1 and efl-1, encode homologs of DP and E2F DNA-binding transcription factors, respectively. Loss-of-function mutations in dpl-1 and efl- 1 cause the same synMuv phenotype as do lin-35 Rb loss-of-function mutations, and the DPL-1 and EFL-1 proteins interact with each other and with LIN-35 Rb in vitro. These data suggest that, in the context of vulval development, DPL-1 and EFL-1 recruit LIN-35 Rb, HDA-1 and other synMuv proteins to DNA to repress transcription. We found that the class B synMuv genes lin-52 and lin-54 encode novel, conserved proteins. The cysteine-rich LIN-54 protein is localized to nuclei and interacts with the class B synMuv protein LIN-36 in vitro. Homologs of lin-52 and lin-54 are candidate Rb pathway genes in other organisms. We performed a genetic screen for synMuv mutations and identified seven new synMuv genes.

(cont.) We found that one of these genes, mep-1, encodes a zinc-finger protein. Mutations affecting another gene identified in this screen, trr-1, synergize with either class A or class B mutations, thus defining a new class of synMuv gene. trr-1 encodes a protein similar to the mammalian TRRAP transcriptional adaptor protein. We identified hat-1 and epc-1, which encode homologs of TRRAP-associated histone acetyltransferase and Enhancer of Polycomb-like proteins, respectively, as additional members of this new class of synMuv genes. The synMuv activities of both hat-1 and hda-1 suggest that a combination of histone acetyltransferase and histone deacetylase activities are required to properly specify vulval cell fates.