Actb knockout in primary mouse embryonic fibroblasts LC-MS/MS

The cytoplasmic actin proteins, beta- and gamma-actin, are 99% identical but perform non-redundant functions. Genes encoding the cytoplasmic actins, Actb and Actg1, respectively, are less similar but still share 89% of their nucleotide sequences. Knockout (KO) of Actb by deletion of first coding exons 2 and 3 in mice is embryonic lethal while KO embryonic fibroblasts (MEFs) fail to proliferate. In contrast, KO of Actg1 is viable but mice lacking Actg1 present with increased perinatal lethality and Actg1 KO MEFs present with a much milder defect in cell proliferation. Recent studies have identified important protein-independent functions for both Actb and Actg1 and demonstrate that deletions within the Actb nucleotide sequence, and not loss of the beta-actin protein, cause the most severe phenotypes in KO mice and cells. Here, we use a multi-omics approach to better understand what drives the phenotypes of Actb KO MEFs. RNA-sequencing and mass spectrometry of Actb KO MEFs reveal largescale changes to the transcriptome, proteome, and phosphoproteome. Pathway analysis of genes and proteins differentially expressed upon Actb KO shows widespread dysregulation of genes involved in the cell cycle. Together, these data suggest novel, protein-independent roles for Actb in regulating gene expression associated with control of cell proliferation.