Deciphering the Molecular Basis of Functional Divergence in AMPylating Enzymes by Molecular Dynamics Simulations and Structure Guided Phylogeny

Structures of a deAMPylation complex rationalise the switch between antagonistic catalytic activities of FICD

The endoplasmic reticulum (ER) Hsp70 chaperone BiP is regulated by AMPylation, a reversible inactivating post-translational modification. Both BiP AMPylation and deAMPylation are catalysed by a single ER-localised enzyme, FICD. Here we present crystallographic and solution structures of a deAMPylation Michaelis complex formed between mammalian AMPylated BiP and FICD. The latter, via its tetratricopeptide repeat domain, binds a surface that is specific to ATP-state Hsp70 chaperones, explaining the exquisite selectivity of FICD for BiP’s ATP-bound conformation both when AMPylating and deAMPylating Thr518. The eukaryotic deAMPylation mechanism thus revealed, rationalises the role of the conserved Fic domain Glu234 as a gatekeeper residue that both inhibits AMPylation and facilitates hydrolytic deAMPylation catalysed by dimeric FICD. These findings point to a monomerisation-induced increase in Glu234 flexibility as the basis of an oligomeric state-dependent switch between FICD’s antagonistic activities, despite a similar mode of engagement of its two substrates — unmodified and AMPylated BiP.

The ER chaperone BiP is regulated by FICD-mediated AMPylation and deAMPylation. Here, the authors characterise the structure of mammalian AMPylated BiP bound to FICD, by X-ray crystallography and neutron scattering, providing insights into the mechanism of BiP AMPylation and deAMPylation.

Evolutionary Diversification of Host-Targeted Bartonella Effectors Proteins Derived from a Conserved FicTA Toxin-Antitoxin Module

Proteins containing a FIC domain catalyze AMPylation and other post-translational modifications (PTMs). In bacteria, they are typically part of FicTA toxin-antitoxin modules that control conserved biochemical processes such as topoisomerase activity, but they have also repeatedly diversified into host-targeted virulence factors. Among these, Bartonella effector proteins (Beps) comprise a particularly diverse ensemble of FIC domains that subvert various host cellular functions. However, no comprehensive comparative analysis has been performed to infer molecular mechanisms underlying the biochemical and functional diversification of FIC domains in the vast Bep family. Here, we used X-ray crystallography, structural modelling, and phylogenetic analyses to unravel the expansion and diversification of Bep repertoires that evolved in parallel in three Bartonella lineages from a single ancestral FicTA toxin-antitoxin module. Our analysis is based on 99 non-redundant Bep sequences and nine crystal structures. Inferred from the conservation of the FIC signature motif that comprises the catalytic histidine and residues involved in substrate binding, about half of them represent AMP transferases. A quarter of Beps show a glutamate in a strategic position in the putative substrate binding pocket that would interfere with triphosphate-nucleotide binding but may allow binding of an AMPylated target for deAMPylation or another substrate to catalyze a distinct PTM. The β-hairpin flap that registers the modifiable target segment to the active site exhibits remarkable structural variability. The corresponding sequences form few well-defined groups that may recognize distinct target proteins. The binding of Beps to promiscuous FicA antitoxins is well conserved, indicating a role of the antitoxin to inhibit enzymatic activity or to serve as a chaperone for the FIC domain before translocation of the Bep into host cells. Taken together, our analysis indicates a remarkable functional plasticity of Beps that is mostly brought about by structural changes in the substrate pocket and the target dock. These findings may guide future structure–function analyses of the highly versatile FIC domains.

An oligomeric state-dependent switch in the ER enzyme FICD regulates AMPylation and deAMPylation of BiP

AMPylation is an inactivating modification that alters the activity of the major endoplasmic reticulum (ER) chaperone BiP to match the burden of unfolded proteins. A single ER-localised Fic protein, FICD (HYPE), catalyses both AMPylation and deAMPylation of BiP. However, the basis for the switch in FICD's activity is unknown. We report on the transition of FICD from a dimeric enzyme, that deAMPylates BiP, to a monomer with potent AMPylation activity. Mutations in the dimer interface, or of residues along an inhibitory pathway linking the dimer interface to the enzyme's active site, favour BiP AMPylation in vitro and in cells. Mechanistically, monomerisation relieves a repressive effect allosterically propagated from the dimer interface to the inhibitory Glu234, thereby permitting AMPylation-competent binding of MgATP. Moreover, a reciprocal signal, propagated from the nucleotide-binding site, provides a mechanism for coupling the oligomeric state and enzymatic activity of FICD to the energy status of the ER.

FIC proteins: from bacteria to humans and back again

During the last decade, FIC proteins have emerged as a large family comprised of a variety of bacterial enzymes and a single member in animals. The air de famille of FIC proteins stems from a domain of conserved structure, which catalyzes the post-translational modification of proteins (PTM) by a phosphate-containing compound. In bacteria, examples of FIC proteins include the toxin component of toxin/antitoxin modules, such as Doc-Phd and VbhT-VbhA, toxins secreted by pathogenic bacteria to divert host cell processes, such as VopS, IbpA and AnkX, and a vast majority of proteins of unknown functions. FIC proteins catalyze primarily the transfer of AMP (AMPylation), but they are not restricted to this PTM and also carry out other modifications, for example by phosphocholine or phosphate. In a recent twist, animal FICD/HYPE was shown to catalyze both AMPylation and de-AMPylation of the endoplasmic reticulum BIP chaperone to regulate the unfolded protein response. FICD shares structural features with some bacterial FIC proteins, raising the possibility that bacteria also encode such dual activities. In this review, we discuss how structural, biochemical and cellular approaches have fertilized each other to understand the mechanism, regulation and function of FIC proteins from bacterial pathogens to humans.

Enzymes Involved in AMPylation and deAMPylation

Posttranslational modifications are covalent changes made to proteins that typically alter the function or location of the protein. AMPylation is an emerging posttranslational modification that involves the addition of adenosine monophosphate (AMP) to a protein. Like other, more well-studied posttranslational modifications, AMPylation is predicted to regulate the activity of the modified target proteins. However, the scope of this modification both in bacteria and in eukaryotes remains to be fully determined. In this review, we provide an up to date overview of the known AMPylating enzymes, the regulation of these enzymes, and the effect of this modification on target proteins.

Biological Diversity and Molecular Plasticity of FIC Domain Proteins

The ubiquitous proteins with FIC (filamentation induced by cyclic AMP) domains use a conserved enzymatic machinery to modulate the activity of various target proteins by posttranslational modification, typically AMPylation. Following intensive study of the general properties of FIC domain catalysis, diverse molecular activities and biological functions of these remarkably versatile proteins are now being revealed. Here, we review the biological diversity of FIC domain proteins and summarize the underlying structure-function relationships. The original and most abundant genuine bacterial FIC domain proteins are toxins that use diverse molecular activities to interfere with bacterial physiology in various, yet ill-defined, biological contexts. Host-targeted virulence factors have evolved repeatedly out of this pool by exaptation of the enzymatic FIC domain machinery for the manipulation of host cell signaling in favor of bacterial pathogens. The single human FIC domain protein HypE (FICD) has a specific function in the regulation of protein stress responses.