Two-dimensional assembly of pentameric rabbit C-reactive proteins on lipid monolayers

Early verification of myocardial ischemia with a novel biomarker of acute tissue damage: C-reactive protein fractional forms

BACKGROUND

We evaluated the utility of an independent biomarker of early ischemic cellular damage-circulating fractional forms of C-reactive protein (fracCRP), to verify the diagnostic relevance of low Troponin I (TnI) values within the context of a workup for Acute Coronary Syndrome (ACS).

METHODS

On a semi-preparative scale, the molecular characteristics of fracCRP were established by electron microscopy and Western Blot, using isolates captured from patient serum on phosphorylcholine beads and purified by size exclusion high-pressure liquid chromatography (SE-HPLC). Captured on an analytical scale, the diagnostic utility of fracCRP was evaluated in first-draw plasma specimens (total CRP not exceeding 6 mg/l) recovered from 300 cardiac emergency patients with final discharge diagnoses of ACS ruled out (N=132) or ruled in (N=168).

RESULTS

At a cutoff value chosen for 97.7% test specificity, the test metric (fracCRP×TnI) identified in the first blood draw 39.9% of all emergency patients ultimately diagnosed with ACS, and 17.9% of ultimately diagnosed patients who arrived with TnI values within the normal reference range (0.01-0.04 ng/ml).

CONCLUSIONS

These findings suggest that the fracCRP test metric could serve as a rule-in test for ACS in a significant proportion of low to moderate risk emergency patients.

The active ring-like structure of SecA revealed by electron crystallography: conformational change upon interaction with SecB

SecA is a multifunctional protein involved in protein translocation in bacteria. The structure of SecA on membrane is dramatically altered compared with that in solution, accompanying with functional changes. We previously reported the formation of a novel ring-like structure of SecA on lipid layers, which may constitute part of the preprotein translocation channel. In the present work, two-dimensional crystallization of Escherichia coli SecA on lipid monolayers was performed to reveal the structural details of SecA on lipid layers and to investigate its function. The 2D crystals composed of ring-like structures were obtained by specific interaction between SecA and negatively charged lipid. The 2D projection map and 3D reconstruction from negative stained 2D crystals exhibited a distinct open channel-like structure of SecA, with an outer diameter of 7 nm and an inner diameter of 2 nm, providing the structural evidence for SecA importance in forming the part of the translocation channel. This pore structure is altered after transferring crystals to the SecB solution, indicating that the lipid-specific SecA structure has the SecB binding activity. The strategy developed here provides a promising technique for studying structure of SecA complex with its ligand on membrane.

Escherichia coli CorA Periplasmic Domain Functions as a Homotetramer to Bind Substrate

CorA is a primary Mg2+ transporter in bacteria, which also mediates influx of Ni2+ and Co2+. Topological studies suggested that it could be divided into a large soluble periplasmic domain (PPD) and three membrane-spanning alpha-helixes. In the present study, glutathione S-transferase (GST) fusion Escherichia coli CorA PPD was purified by GST affinity chromatography, and PPD was obtained by on-column thrombin digestion. Size-exclusion chromatography indicated that purified PPD exists as a homotetramer. Single particle electron microscopy analysis of PPD and two-dimensional crystals of GST-PPD indicated that E. coli CorA PPD is a pyramid-like homotetramer with a central cavity. Comparison of the CD spectra of full-length CorA and PPD also suggested that PPD has similar secondary structure to the full-length CorA. Dissociation constants for CorA and PPD with their substrates, determined by dose-dependent fluorescence quench of ligands, suggested that purified PPD retains its substrate binding ability as native CorA. The CorA PPD structure described here may provide structural information for the E. coli CorA functional oligomeric state.

Two-dimensional crystallization of a small heat shock protein HSP16.3 on lipid layer

As a member of small heat shock proteins, HSP16.3 was identified as the major membrane-bound protein of Mycobacterium tuberculosis during stationary phase. Previous studies revealed that HSP16.3 was in a nonameric form in solution. Here, two-dimensional crystal of HSP16.3 molecules on lipid monolayer was obtained for the first time. The crystal exhibited p422 symmetry with lattice parameters a=b=90A, gamma=90 degrees. The projection map of untilted crystals showed that the basic unit of the crystal was a rod-like structure with two high-density regions. The three-dimensional map at 2.2 nm resolution revealed a rod-like structure with a dimension of 56A x 32A x 25A, similar to the dimeric forms of M. jannaschii HSP16.5 and wheat HSP16.9. Cross-linking experiments confirmed that HSP16.3 nonamers dissociated into dimers upon interaction with the positively charged lipid layer. Surface plasmon resonance measurements revealed that both electrostatic and hydrophobic forces involved in the formation of the 2D crystal on the lipid monolayer. These results provide a basis for further investigation on the unique dimeric structure of HSP16.3 and its functions in vivo.

Ring-like pore structures of SecA: implication for bacterial protein-conducting channels

SecA, an essential component of the general protein secretion pathway of bacteria, is present in Escherichia coli as soluble and membrane-integral forms. Here we show by electron microscopy that SecA assumes two characteristic forms in the presence of phospholipid monolayers: dumbbell-shaped elongated structures and ring-like pore structures. The ring-like pore structures with diameters of 8 nm and holes of 2 nm are found only in the presence of anionic phospholipids. These ring-like pore structures with larger 3- to 6-nm holes (without staining) were also observed by atomic force microscopic examination. They do not form in solution or in the presence of uncharged phosphatidylcholine. These ring-like phospholipid-induced pore-structures may form the core of bacterial protein-conducting channels through bacterial membranes.

Visualization of synaptotagmin I oligomers assembled onto lipid monolayers

Neuronal exocytosis is mediated by Ca(2+)-triggered rearrangements between proteins and lipids that result in the opening and dilation of fusion pores. Synaptotagmin I (syt I) is a Ca(2+)-sensing protein proposed to regulate fusion pore dynamics via Ca(2+)-promoted binding of its cytoplasmic domain (C2A-C2B) to effector molecules, including anionic phospholipids and other copies of syt. Functional studies indicate that Ca(2+)-triggered oligomerization of syt is a critical step in excitation-secretion coupling; however, this activity has recently been called into question. Here, we show that Ca(2+) does not drive the oligomerization of C2A-C2B in solution. However, analysis of Ca(2+).C2A-C2B bound to lipid monolayers, using electron microscopy, revealed the formation of ring-like heptameric oligomers that are approximately 11 nm long and approximately 11 nm in diameter. In some cases, C2A-C2B also assembled into long filaments. Oligomerization, but not membrane binding, was disrupted by neutralization of two lysine residues (K326,327) within the C2B domain of syt. These data indicate that Ca(2+) first drives C2A-C2B.membrane interactions, resulting in conformational changes that trigger a subsequent C2B-mediated oligomerization step. Ca(2+)-mediated rearrangements between syt subunits may regulate the opening or dilation kinetics of fusion pores or may play a role in endocytosis after fusion.

Dissociation and subunit rearrangement of membrane-bound human C-reactive proteins

As one of the most important acute-phase reactants in human serum, C-reactive protein plays its physiological roles mainly on membranes. Here we show that the human C-reactive protein is two-dimensionally crystallized upon specific adsorption on the phosphorylcholine ligand containing membranes by monolayer approach. The 2.0-nm resolution projection structure of the two-dimensional crystals analyzed by electron microscopy and image reconstruction reveals open-ring-like pentamers in the crystals. The electron microscope graphs also show that the dissociated pentamers with open-ring-like structure occur in a closed packing region (not two-dimensionally crystallized). These results indicate a membrane-induced dissociation and rearrangement of hCRP, which may relate to the variety of hCRP's physiological functions.