INTRODUCTION
Gram-negative bacterial pathogens such as Shigella flexneri ,Salmonella enterica , Legionella pneumophila , etc., cause a
broad range of communicable human diseases (Jennison & Verma, 2004;
Knodler & Elfenbein, 2019; Gonçalves et al., 2021). Their virulence is
often mediated by dedicated protein secretion systems (i.e., type III
for S. flexneri and S. enterica or type IV for L.
pneumophila ), which can deliver effector proteins directly into host
cells to modulate diverse cellular pathways (Muthuramalingam et al.,
2021; Jennings et al., 2017; Hubber & Roy, 2010; Isberg et al., 2009).
In the last decade, a growing body of studies has shown that many
secreted bacterial effectors harbor enzymatic activities that often lead
to post-translational modifications (PTMs) of their host targets
(Jennings et al., 2017; Mattock & Blocker, 2017; Qiu & Luo, 2017;
Macek et al., 2019; Chambers & Scheck, 2020). From the perspective of
bacterial pathogens, covalently modifying key host proteins represents a
more economic and rapid means (relative to mechanisms altering protein
abundance) to regulate host biological processes including
anti-bacterial defense system, thereby promoting bacterial survival
and/or proliferation (Ashida et al., 2014). Indeed, accumulating
evidence suggests that bacteria-mediated modifications of host targets
are much more prevalent than we initially thought.
Rapid growth of this emerging field (i.e., pathogen-mediated host
modifications) is, at least in part, spurred by technical innovations
that facilitate accurate and precise measurements of these molecular
events such as protein PTMs. Among those tools probing covalent
modifications, mass spectrometry (MS) is arguably the most powerful and
versatile analytical approach thanks to major technological
breakthroughs (e.g., in terms of instrument sensitivity, mass resolution
and throughput) that were witnessed in the last two decades.
Nevertheless, the application of this analytical tool in dissecting
effector-mediated PTMs is, at least in our opinion, still far below
where it is supposed to be. This review will focus on the application of
MS-based approaches in the study of host protein modifications catalyzed
by bacterial effectors. Specifically, we will highlight some recent work
on protein ADP-ribosylation (and related ADP-riboxanation) and classical
phosphorylation as examples to illustrate our current understanding of
this fascinating area in host-pathogen interactions (Figure 1).
Furthermore, we will critically discuss the increasing role and
capability of MS in dissolving some looming challenges for further
advancements in the field.