Cell News | Issue 02, 2017 - page 8

Cell News 02/2017
Evgenij Fiskin
Regulation of host-pathogen interactions
by the ubiquitin system
Institute of Biochemistry II, Goethe University School of Medicine,
University Hospital, Frankfurt, Germany
The increased emergence of antibiotics resistance and the dan-
ger it poses to human health prompted the WHO a few months
ago to publish a “Global priority list of antibiotic-resistant
bacteria to guide research, discovery, and development of new
antibiotics” (WHO, 2017). These recent developments clearly
illustrate the over-reliance of current therapies on antibiotics
and call for the establishment of alternative treatment options.
Accumulated evidence indicates that deciphering pathogenic
virulence and host defense mechanisms during bacterial infec-
tions may be the key to future developments of novel antibac-
terial strategies. Being one of the most versatile and prevalent
post-translational modifications in eukaryotic cells, ubiquiti-
nation is also a major regulator of host-pathogen interactions
(Hu and Sun, 2016). As such, ubiquitin (Ub) drives a multitude
of microbicidal programs including for example innate immune
receptor signaling as well as selective autophagy (Gomes and
Dikic, 2014). Given its central role as a regulator of host defense,
ubiquitination is a major target of pathogen-derived virulence
factors. In this work, we performed the first systematic analy-
sis of the cellular ubiquitin landscape upon bacterial infection
to obtain a thorough understanding of molecular mechanisms
of host-pathogen interplay required for the rational design of
novel anti-bacterial strategies.
Ubiquitination in host defense
The ubiquitination reaction requires the sequential action of at
least three enzymes: an Ub-activating enzyme (E1), an Ub-con-
jugating enzyme (E2) and a Ub ligase (E3) (Hershko et al., 2000).
In eukaryotes three distinct classes of Ub ligases mediate the E3
step of ubiquitination: (i) RING-type E3s functioning as scaf-
folds, bringing Ub-charged E2s into close proximity of sub-
strates, thereby facilitating Ub transfer from E2 to a substrate
lysine (ii) HECT-type E3s forming a thioester intermediate with
Ub before transferring it directly to the substrate and (iii) RING-
between-RING (RBR) ligases, which function as RING-HECT hy-
brids (Zheng and Shabek, 2017). Ub itself contains seven lysine
residues (K6, K11, K27, K29, K33, K48, K63) and a methionine
at position 1 (M1), all of which can be used to form Ub chains
in vivo
(Yau and Rape, 2016). Whereas the best studied role of
ubiquitination concerns protein degradation by the proteasome,
which is commonly mediated by K48- or K11-linked Ub chains
and contributes essentially to cellular homeostasis, M1-linked
chains, also designated as linear chains, have recently attracted
a lot of attention due to their critical role in the regulation of
innate and adaptive immune pathways (Rittinger and Ikeda,
2017). Hereby, linear Ub chains are essential to control NF-
signaling downstream of cytokine receptors or upon recognition
of bacteria (Wild et al., 2011). M1-linked Ub chains are formed
by a single dedicated E3 ligase termed the linear Ub chain as-
sembly complex (LUBAC) consisting of heme-oxidized iron-re-
sponsive element binding protein 2 ubiquitin ligase-1 (HOIL-1L),
the catalytic subunit HOIL-1L interacting protein (HOIP/RNF31)
as well as the third subunit SH3 and multiple ankyrin repeat
domains protein (SHANK)-associated RBCK1 homology (RH)-do-
main-interacting protein (SHARPIN) (Kirisako et al., 2006).
Importantly, M1-linked Ub chains were demonstrated to harbor
important anti-bacterial functions as they are formed on the
surface of cytosolic
enterica (Noad et al., 2017; van
Wijk et al., 2012; van Wijk et al., 2017), and are able to recruit
autophagy receptors in order to promote lysosomal clearance of
cytosolic pathogens (Gomes and Dikic, 2014)
Systematic analysis of ubiquitin-driven events upon
Typhimurium infection
Whereas previous efforts focused on the study of a small num-
ber of ubiquitination events triggered by the detection of iso-
lated bacterial products (e.g. lipopolysaccharide or peptidogly-
can), it is now well established that certain host inflammatory
pathways are exclusively stimulated upon live bacterial infection
(Sander et al., 2011; Vance et al., 2009). In similar fashion, the
subversion of cellular cytoskeleton and membrane trafficking
systems upon bacterial invasion were shown to require the ac-
tive and timely secretion of pathogen-derived virulence proteins
(LaRock et al., 2015). In light of these observations, we sought
to provide a global view of ubiquitination during live pathogenic
insult and to uncover novel Ub-involving host-pathogen inter-
actions. In particular, we focused our efforts on the Ub-modified
proteome of cells exposed to the invasive Gram-negative patho-
enterica serovar Typhimurium (S. Typhimurium),
which can infect a broad range of hosts including humans and
elicits severe gastroenteritis representing one of the major
causes of food and waterborne disease. In order to monitor the
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