Bacterial protein acetylation: the dawning of a new age

Abstract

Protein acetylation has historically been considered a predominantly eukaryotic phenomenon. Recent evidence, however, supports the hypothesis that acetylation broadly impacts bacterial physiology. To explore more rapidly the impact of protein acetylation in bacteria, microbiologists can benefit from the strong foundation established by investigators of protein acetylation in eukaryotes. To help advance this learning process, we will summarize the current understanding of protein acetylation in eukaryotes, discuss the emerging link between acetylation and metabolism and highlight the best-studied examples of protein acetylation in bacteria.

Figure 1. Electron transfer during direct N_ε-acetylation from acetyl-CoA

At neutral pH, the ε-amino group is positively charged. This residue is deprotonated by a base (not shown in figure). (I) The lysine ε-amino group can now act as a nucleophile to attack the electrophilic carbonyl carbon of acetyl-CoA, (II) forming a tetrahedral intermediate. The electron dense oxygen expels the thiolate group (-SCoA). (III) Deprotonation of the amino group (IV) results in an acetylated lysine side chain and CoASH. Figure adapted from (Walsh, 2006).

Figure 2. Relevant central metabolic pathways

Glycolysis (blue) metabolizes glucose to acetyl coenzyme A (acetyl-CoA) in an NAD⁺-dependent manner. GAPDH, glyceraldehyde-3-phosphate dehydrogenase; PDH, pyruvate dehydrogenase. Acetate dissimilation requires the Pta-AckA pathway (red). The enzyme PTA (phosphotransacetylase) converts acetyl-CoA and inorganic phosphate (P_i) into coenzyme A (CoA) and the high-energy pathway intermediate acetyl phosphate (acetyl-P). The enzyme ACKA (acetate kinase) converts acetyl-P and ADP to acetate and ATP. The acetate freely diffuses across the cell envelope into the environment. Acetate assimilation (green) requires the high-affinity enzyme ACS (acetyl-CoA synthetase). In a two-step process that involves an enzyme-bound intermediate (acetyl-AMP), Acs converts acetate, ATP, and CoA into AMP, pyrophosphate (PP_i), and acetyl-CoA. The acetyl-CoA replenishes the NAD⁺-dependent tricarboxylic acid (TCA) cycle (orange). Mammalian ATP-citrate lyase (ACL) uses ATP and CoA to convert the TCA cycle intermediate citrate into oxaloacetate (OAA), ADP, P_i, and acetyl-CoA (purple). Figure adapted from (Wolfe, 2008).

Figure 3. Reversible acetylation of ACS and CheY

The activity of ACS is regulated by reversible acetylation. In a form of feedback inhibition, a GNAT (either Pat or AcuA depending on the organism) uses acetyl-CoA as its acetyl donor to acetylate ACS (^AcACS), which is inactive. When the glycolytic carbon source is depleted, NAD⁺ is recycled (not shown). An NAD⁺-dependent sirtuin (CobB or SrtN, depending on the organism) can now deacetylate ACS, generating the by-product 2'-O-acetyl-ADP-ribose (2'-OAADPr). In B.subtilis, the class III KDAC AcuC also can catalyze deacetylation in an NAD⁺-independent manner (not shown). Deacetylated, and thus reactivated, ACS can now synthesize acetyl-CoA. The activity of CheY is also regulated by reversible acetylation. In this case, acetylation occurs by autoacetylation (auto), ACS, or some unknown acetyltransferase (AT). CobB deacetylates acetyl-CheY (^AcCheY).