C-reactive protein is a critical checkpoint that limits destructive activation of complement in acetaminopheninduced liver injury, and could be exploited as a promising therapeutic approach to treat hepatotoxicity caused by drug overdose.BACKGROUND AND AIMS: C-reactive protein (CRP) is a hepatocyte-produced marker of inflammation yet with undefined function in liver injury. We aimed to examine the role of CRP in acetaminophen-induced liver injury (AILI). METHODS:The effects of CRP in AILI were investigated using CRP knockout mice and rats combined with human CRP rescue. The mechanisms of CRP action were investigated in vitro and in mice with Fcg receptor 2B knockout, C3 knockout, or hepatic expression of CRP mutants defective in complement interaction. The therapeutic potential of CRP was investigated by intraperitoneal administration at 2 or 6 hours post-AILI induction in wild-type mice.RESULTS: CRP knockout exacerbated AILI in mice and rats, which could be rescued by genetic knock-in, adeno-associated virus-mediated hepatic expression or direct administration of human CRP. Mechanistically, CRP does not act via its cellular receptor Fcg receptor 2B to inhibit the early phase injury to hepatocytes induced by acetaminophen; instead, CRP acts via factor H to inhibit complement overactivation on injured hepatocytes, thereby suppressing the late phase amplification of inflammation likely mediated by C3a-dependent actions of neutrophils. Importantly, CRP treatment effectively alleviated AILI with a significantly extended therapeutic time window than that of N-acetyl cysteine.CONCLUSION: Our results thus identify CRP as a crucial checkpoint that limits destructive activation of complement in acute liver injury, and we argue that long-term suppression of CRP expression or function might increase the susceptibility to AILI.

C-reactive protein (CRP) is an acute phase reactant secreted by hepatocytes as a pentamer. The structure formation of pentameric CRP has been demonstrated to proceed in a stepwise manner in live cells. Here, we further dissect the sequence determinants that underlie the key steps in cellular folding and assembly of CRP. The initial folding of CRP subunits depends on a leading sequence with a conserved dipeptide that licenses the formation of the hydrophobic core. This drives the bonding of the intra-subunit disulfide requiring a favorable niche largely conferred by a single residue within the C-terminal helix. A conserved salt bridge then mediates the assembly of folded subunits into pentamer. The pentameric assembly harbors a pronounced plasticity in inter-subunit interactions, which may form the basis for a reversible activation of CRP in inflammation. These results provide insights into how sequence constraints are evolved to dictate structure and function of CRP.

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Biophysical models suggest a dominant role of structural over functional constraints in shaping protein evolution. Selection on structural constraints is linked closely to expression levels of proteins, which together with structure-associated activities determine in vivo functions of proteins. Here we show that despite the up to two orders of magnitude differences in levels of C-reactive protein (CRP) in distinct species, the in vivo functions of CRP are paradoxically conserved. Such a pronounced level-function mismatch cannot be explained by activities associated with the conserved native structure, but is coupled to hidden activities associated with the unfolded, activated conformation. This is not the result of selection on structural constraints like foldability and stability, but is achieved by folding determinants-mediated functional selection that keeps a confined carrier structure to pass the stringent eukaryotic quality control on secretion. Further analysis suggests a folding threshold model which may partly explain the mismatch between the vast sequence space and the limited structure space of proteins.

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Urea amidolyase (UA), a bifunctional enzyme that is widely distributed in bacteria, fungi, algae, and plants, plays a pivotal role in the recycling of nitrogen in the biosphere. Its substrate urea is ultimately converted to ammonium, via successive catalysis at the C-terminal urea carboxylase (UC) domain and followed by the N-terminal allophanate hydrolyse (AH) domain. Although our previous studies have shown that Kluyveromyces lactis UA (KlUA) functions efficiently as a homodimer, the architecture of the full-length enzyme remains unresolved. Thus how the biotin carboxyl carrier protein (BCCP) domain is transferred within the UC domain remains unclear. Here we report the structures of full-length KlUA in its homodimer form in three different functional states by negatively-stained singleparticle electron microscopy. We report here that the ADP-bound structure with or without urea shows two possible locations of BCCP with preferred asymmetry, and that when BCCP is attached to the carboxyl transferase domain of one monomer, it is attached to the biotin carboxylase domain in the second domain. Based on this observation, we propose a BCCP-swinging model for biotin-dependent carboxylation mechanism of this enzyme. K E Y W O R D Selectron microscopy, negative staining, single-particle analysis, structural biology

C-reactive protein (CRP) is a circulating marker of inflammation yet with ill-defined biological functions. This is partly due to the uncharacterized activities of endogenous CRP in mice, the major animal model used to define protein function. The hurdles for purification and characterization of mouse CRP are its low circulating levels and the lack of specific antibodies. To clear these hurdles, here we developed an efficient expression system by constructing recombinant Pichia pastoris cells for secretion of native conformation mouse CRP. The recombinant expression of mouse CRP in Escherichia coli failed to yield sufficient amount of native protein, reflecting the importance of post-translational modification of glycosylation in aiding proper folding. By contrast, sufficient amount of native mouse CRP was successfully purified from P. pastoris. Preliminary purification was performed by Nickel Chelating Sepharose Fast-Flow affinity chromatography with 6 × His tags attached to the protein. Subsequently, p-Aminophenyl Phosphoryl Choline Agarose resin affinity chromatography was used for tandem purification. The purified mouse CRP showed native pentamer and capabilities of PC binding. Moreover, the 6 × His tag provides a convenient tool for detecting the interactions of mouse CRP with ligands.

Solid gastric emptying rates (GER) were determined with scintigraphic techniques in 20 patients with non-ulcer dyspepsia (NUD) and 9 healthy volunteers. GER were significantly decreased in NUD patients compared with controls, especially 45 min (P < 0.05), 60 and 90 min (P < 0.01) and 120 min (P < 0.05) after ingestion. In 13 out of 20 NUD patients who demonstrated lower GER, only 4 cases gave a lower GER at all stages throughout the determination, the other 9 showed their abnormal GER only after 60 min. In 3 cases who received repeated GER studies after cisapride therapy, 2 patients showed symptomatic relief accompanied by GER improvement. It is concluded that gastric emptying delay may be present with a high percentage in patients with non-ulcer dyspepsia. Scintigraphic gastric emptying test is a safe and reliable technique with good reproducibility. It may be helpful in quantitative study about gastric motion disorders.