NatB modulates Rb mutant cell death and tumor growth by regulating EGFR/MAPK signaling through the N-end rule pathways

Despite the prevalence of N-terminal acetylation (Nt-acetylation), little is known of its biological functions. In this study, we show that NatB regulates Rb mutant cell survival, EGFR/MAPK signaling activity, and EGFR signaling-dependent tumor growth. We identify Grb2/Drk, MAPK, and PP2AC as the key NatB targets of EGFR pathway. Surprisingly, NatB activity increases the levels of positive pathway components Grb2/Drk and MAPK while decreases the levels of negative pathway component PP2AC despite these proteins have the same first two amino acids that are recognized by NatB and N-end rule pathways. Mechanistically, we show that NatB regulates Grb2/Drk protein stability through its N-terminal sequences and that Grb2/Drk and MAPK are selectively degraded by the Arg/N-end rule E3 ubiquitin ligase Ubr4, which targets proteins with free N-terminus. In contrast, PP2AC is selectively degraded by the Ac/N-end rule pathway E3 ubiquitin ligase Cnot4 that targets proteins with acetylated N-terminus. These results reveal a novel mechanism by which NatB-mediated Nt-acetylation and N-end rule pathways modulate EGFR/MAPK signaling by inversely regulating the levels of positive and negative components. Since mutation or overexpression that deregulate the EGFR/Ras signaling pathway are common in human cancers and NatB subunits are significant unfavorable prognostic markers, this study can potentially lead to the development of novel therapeutic approaches.\n\nSignificance StatementNt-acetylation is often regarded as a constitutive, irreversible, and static modification that is not suited to serve regulatory functions. Our observation that Nt-acetylation by NatB coordinately regulate the levels of positive and negative components of the EGFR/MAPK pathway show that Nt-acetylation and N-end rule pathways can play important roles regulating important signaling pathways. As Acetyl-CoA level, which is influenced by cell metabolism, can be rate limiting for Nt-acetylation, our results also suggest a potentially new mechanism by which cellular metabolic status can regulate growth factor signaling.