Mutation of BARA/LIN‐9 rescues the CDK4‐null phenotype by releasing the repression on E2F‐regulated genes

Abstract

Genetic studies have previously established that C. elegans LIN-9 functions downstream of cdk-4 and cyd-1, homologues of mammalian CDK4/6 and cyclin D respectively, in a distinct pathway that regulates cell proliferation. In this study, we demonstrate that mammalian BARA/LIN-9 is a nuclear protein that inhibits cell proliferation in a variety of cell lines. More importantly, we determine that BARA/LIN-9 also acts downstream of CDK4/6 and cyclin D in mammalian cells since i) its anti-proliferative effect is partially blocked by co-expression of cyclin D1, and ii) a mutant form that lacks the first 84 amino acids (Δ84) rescues observed phenotypic alterations in fertility and growth in mice null for cdk4. An analysis of cell cycle proteins indicates the Δ84 mutation of BARA/LIN-9 restores the expression of E2F target genes in CDK4-null mouse embryonic fibroblast (MEF) cells. Interestingly, the Δ84 mutation of BARA/LIN-9 had no effect in mice null for cdk2. These data supports a model in which BARA/LIN-9 plays an important role in the repression of transcription of genes required for progression through G1 and entry into S phase downstream of CDK4, but independent of CDK2. Support for this work provided by NIH grant GM54709 (ORC) and DK30667 (RDK).