5. ACP is the donor molecule in chemical acetylation
Many studies on protein acetylation in bacteria have focused on
describing enzymatic acetylation, its implication in cellular
physiology, pathogenesis, in bacterial response to environmental
conditions, etc. However, evidence shows that non-enzymatic acetylation
is also possible (Table 2).
The acetylation by AcP has been mainly described by proteomic studies,
in which a comparative analysis of the acetylome of different strains
allows us to determine if there is any difference in the protein
acetylation levels and infer the acetylation mechanism. This has been
achieved in E. coli, N. gonorrhoeae and Borrelia
burgdorferi (Table 2) (Bontemps-Gallo et al., 2018; Kosono et al.,
2015; Kuhn et al., 2014; Post et al., 2017; Reverdy et al., 2018;
Schilling et al., 2015; Weinert et al., 2013). These studies have
focused on comparing the proteomic data of the acetylated proteins of
the wild-type strain with different isogenic mutant strains. The
acetylome data using Escherichia coli as a model demonstrated
that acetylation depends on acetyl-phosphate (AcP) formation and occurs
at a low level in growth-arrested cells. Mutant cells unable to
synthesize (pta ackA mutant) or metabolize (ackA mutant)
AcP had the opposite behavior, while in the first one, significantly
reduced acetylation levels were observed, and the accumulation of AcP
significantly elevated acetylation levels. Also, the authors
demonstrated that the AcP acetylate lysine residues in vitro at a
concentration comparable to that found in vivo . These data
establish AcP as a critical regulator of acetylation and suggest that
AcP acts nonenzymatically to regulate acetylation levels in response to
glucose (Table 2) (Kuhn et al., 2014; Schilling et al., 2015; Weinert et
al., 2013).
Since AcCoA and AcP are derived from multiple metabolic pathways in
other microorganisms, it is difficult to establish whether both or only
one of these molecules is the acetyl group donor. The bacterium B.
burgdorferi is characterized by producing AcCoA and AcP from a unique
metabolic pathway, the acetate/mevalonate pathway. In this pathway,
acetate is converted to Ac-P by acetate kinase (AckA), which is
metabolized to acetyl-CoA by phosphotransacetylase (Pta) (Bontemps-Gallo
et al., 2018; Richards et al., 2015). The acetylome analysis of
different mutant strains and their respective complements showed that no
acetylation is observed in the strain that does not synthesize either
AcP nor AcCoA (ΔackA mutant). In the ackA complemented strain, an
increase in acetylation was detected, and the Δpta complemented strain
displayed similar levels of acetylation as the wild-type. Remarkably,
hyper-lysine acetylation levels were detected in the Δpta due to the AcP
accumulation. Together these results demonstrated that this molecule is
the primary source of acetylation (Bontemps-Gallo et al., 2018) (Table
2).
To establish the metabolic processes that AcP acetylation regulates, the
modified proteins can be analyzed with different software (PANTHER,
DAVID, ERGO, and KEEG). From the proteomic data of E. coli, the
functional analysis reveals that the elongation factors, most of the
ribosomal subunits and aminoacyl-tRNA ligases, are acetylated in an
AcP-dependent manner (Christensen et al., 2019; Kuhn et al., 2014; Post
et al., 2017). In Borrelia burgdorferi , the acetylated proteins
were involved in genetic information, metabolism and transport, protein
folding and degradation, detoxification, motility, and chemotaxis
(Bontemps-Gallo et al., 2018). Similarly, some metabolic pathways
related to carbohydrate metabolisms (glycolysis/gluconeogenesis, pentose
phosphate pathway, pyruvate metabolism, and the TCA cycle), fatty acid
metabolism, and pantothenate metabolism are sensitive to AcP-dependent
acetylation (Kuhn et al., 2014; Schilling et al., 2015).