N-Terminal Acetylation-Targeted N-End Rule Proteolytic System: The Ac/N-End Rule Pathway

Abstract

Although Nα-terminal acetylation (Nt-acetylation) is a pervasive protein modification in eukaryotes, its general functions in a majority of proteins are poorly understood. In 2010, it was discovered that Nt-acetylation creates a specific protein degradation signal that is targeted by a new class of the N-end rule proteolytic system, called the Ac/N-end rule pathway. Here, we review recent advances in our understanding of the mechanism and biological functions of the Ac/N-end rule pathway, and its crosstalk with the Arg/N-end rule pathway (the classical N-end rule pathway).

Keywords: N-end rule; N-terminal acetylation; degron; proteolysis; ubiquitin.

Fig. 1.

Substrate specificity and subunit compositions of Nt-acetylases. Among 6 Nt-acetylases (NatA-F), NatA, NatB, and NatC mainly Nt-acetylate cellular proteins. NatA consists of a catalytic subunit, Naa10, and an auxiliary subunit, Naa15, and acetylates Ser (S), Ala (A), Cys (C), Gly (G), Thr (T), or Val (V) N-termini of its substrates after Nt-Met removal by methionine aminopeptidases (MetAPs). NatB consists of a catalytic subunit, Naa20, and an auxiliary subunit, Naa25, and Nt-acetylates Met-Asn (MN), Met-Asp (MD), Met-Gln (MQ), or Met-Glu (ME) on cellular proteins. NatC contains a catalytic subunit, Naa30, and auxiliary subunits, Naa35 and Naa38, and Nt-acetylates Met-Ile (MI), Met-Phe (MF), Met-Trp (MW), Met-Leu (ML), or Met-Tyr (MY) on its substrates.

Fig. 2.

Two branches of the N-end rule pathways in eukaryotes. (A) The Arg/N-end rule pathway, which targets unmodified Arg, His, Lys, Leu, Ile, Phe, Trp, Tyr, and Met-Ф (hydrophobic) Nt-residues. Nt-Gln and Asn are destabilizing after Nt-deamidation and subsequent arginylation. Nt-Cys also becomes destabilizing through preliminary oxidation and subsequent Nt-arginylation. (B) The Ac/N-end rule pathway, which targets Nt-acetylated residues of cellular proteins for degradation. Doa10 and Not4 are yeast Ac/N-recognins and Teb4 is a mammalian Ac/N-recognin. In addition to the NatA, NatB, and NatC substrates, other Nt-acetylated proteins are potentially targeted by the Ac/N-end rule pathway for degradation.

Fig. 3.

Functions of the Ac/N-end rule pathway. (A) Control of protein quality and subunit stoichiometry. The Ac/N-end rule pathway targets misfolded proteins or unassembled subunit(s) of complex proteins with Ac/N-degrons. (B) Regulation of blood pressure via G-protein signaling. The Ac/N-end rule pathway degrades Nt-acetylated wild-type MQ-Rgs2. In contrast, the hypertension-related ML-Rgs2 with a Met-Ф degron is targeted by either the Ac/N-end rule pathway or the Arg/N-end rule pathway according to its Nt-acetylation status. Dual targeting of ML-Rgs2 by two branches of the N-end rule pathways dramatically decreases Rgs2 levels, and thereby increases blood pressure via augmented G-protein signaling and subsequent vasoconstriction. (C) Dysregulation of Nt-acetylation most likely affects protein stability, thus causing many cancers. (D) Control of pathogen immunity in plants. Plants contain two SNC1 (suppressors of nod-like protein receptor 1) variants with Met-Met-Asp (MMD) or Met-Asp (MD) N-termini owing to alternative translation. NatA Nt-acetylates MMD-SNC1 for degradation, whereas NatB Nt-acetylates MD-SNC1 for stabilization. Consequently, the steady-state levels of SNC1 are directly involved in plant immunity.