A Long Lost Key Opens an Ancient Lock: Drosophila Myb Causes a Synthetic Multivulval Phenotype in Nematodes

Abstract

The five-protein MuvB core complex (LIN9/Mip130, LIN37/Mip40, LIN52, LIN54/Mip120, and LIN53/p55CAF1/RBBP4) has been highly conserved during the evolution of animals. This nuclear complex interacts with proteins encoded by the RB tumor suppressor gene family and its associated E2F-DP transcription factors to form DREAM complexes that repress the expression of genes that regulate cell cycle progression and cell fate. The MuvB core complex also interacts with proteins encoded by the Myb oncogene family to form the Myb-MuvB complexes that activate many of the same target genes. We show that animal-type Myb genes and proteins are present in Bilateria, Cnidaria, and Placozoa, the latter including some of the simplest known animal species. However, bilaterian nematode worms appear to have lost their animal-type Myb genes hundreds of millions of years ago. Nevertheless, the amino acids in the LIN9 and LIN52 proteins that directly interact with the MuvB-binding domains of human B-Myb and Drosophila Myb are conserved in C. elegans. Here we show that, despite greater than 500 million years since their last common ancestor, the Drosophila melanogaster Myb protein can bind to the nematode LIN9 and LIN52 family proteins in vitro and can cause a synthetic multivulval (synMuv) phenotype in vivo. This phenotype is similar to that caused by loss-of-function mutations in C. elegans synMuvB class genes including those that encode homologs of the MuvB core, RB, E2F, and DP. Furthermore, amino acid substitutions in the MuvB-binding domain of Drosophila Myb that disrupt its functions in vitro and in vivo also disrupt its activity in C. elegans. We speculate that nematodes and other animals may contain another protein that can bind to LIN9 and LIN52 in order to activate transcription of genes repressed by DREAM complexes.