Two-dimensional crystallization of rabbit C-reactive protein on lipid monolayers

The Multiple Faces of C-Reactive Protein-Physiological and Pathophysiological Implications in Cardiovascular Disease

C-reactive protein (CRP) is an intriguing protein which plays a variety of roles in either physiological or pathophysiological states. For years it has been regarded merely as a useful biomarker of infection, tissue injury and inflammation, and it was only in the early 80s that the modified isoforms (mCRP) of native CRP (nCRP) appeared. It soon became clear that the roles of native CRP should be clearly discriminated from those of the modified form and so the impacts of both isoforms were divided to a certain degree between physiological and pathophysiological states. For decades, CRP has been regarded only as a hallmark of inflammation; however, it has since been recognised as a significant predictor of future episodes of cardiovascular disease, independent of other risk factors. The existence of modified CRP isoforms and their possible relevance to various pathophysiological conditions, suggested over thirty years ago, has prompted the search for structural and functional dissimilarities between the pentameric nCRP and monomeric mCRP isoforms. New attempts to identify the possible relevance between the diversity of structures and their opposing functions have initiated a new era of research on C-reactive protein. This review discusses the biochemical aspects of CRP physiology, emphasizing the supposed relevance between the structural biology of CRP isoforms and their differentiated physiological and pathophysiological roles.

Biological cryo-electron microscopy in China

Cryo‐electron microscopy (cryo‐EM) plays an increasingly more important role in structural biology. With the construction of an arm of the Chinese National Protein Science Facility at Tsinghua University, biological cryo‐EM has entered a phase of rapid development in China. This article briefly reviews the history of biological cryo‐EM in China, describes its current status, comments on its impact on the various biological research fields, and presents future outlook.

The Functional Role of C-Reactive Protein in Aortic Wall Calcification

As an ongoing effort to elucidate the mechanisms involved in bioprosthetic heart valve (BHV) calcification, the role of C-reactive protein (CRP) in the tissue calcification process was investigated. The profile of calcium-associated proteins (CAP) on glutaraldehyde-preserved (0.6%) porcine aortic wall, which were subcutaneously implanted in rats for up to 8 weeks, showed a temporal appearance pattern. The total extracted amount of proteins from the control tissues implanted for 8 weeks was significantly greater than that from ethanol-treated tissues (1.78+/-0.2 vs. 1.27+/-0.18 microg/mg), indicating that the binding affinity of CAP for BHV pretreated with an anticalcification agent was significantly decreased (p<0.05). The dye Stains-All method showed that the dark-blue colored bands, representing high calcium binding and phosphorylated proteins, were stained from the extract of the control BHV at the molecular weight varying from 4 to 250 kDa, but rarely seen in the extract of BHV pretreated with ethanol. One of those proteins was exclusively immunoreactive with CRP antibody, while there was no immunoreaction in less calcified tissues. When aortic wall was exposed to an excess amount of CRP in an in vitro simulating model, the calcification rate of aortic wall increased as the concentration of CRP increased. The results of this work clearly revealed that CRP has indirect vascular effects, leading to an increased rate of aortic wall calcification.

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.

Structure analysis of soluble proteins using electron crystallography

Electron crystallography as a structural determination technique has grown dramatically in use over recent years. Improvements in microscopes, equipment, practical techniques, computation facilities and image processing methods are reflected in the increasing number of near-atomic resolution structures that have been published. In this review we shall summarize the techniques involved in structure determination of soluble proteins using electron crystallography. Many soluble protein structures have been investigated in this manner over the past two decades. Here we present several examples where a variety of approaches have been used to gradually increase the information obtained.

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

The problem of pentamer packing on a two-dimensional plane is of concern not only in physics and mathematics but also in biology. The packing styles of pentamers may either be related to or reflect the physiological or biochemical properties of biological macromolecules. C-reactive protein (CRP), one of the classical members of the petraxin family, was recently two-dimensionally (2D) crystallized by us on lipid monolayers by specific adsorption (Wang, H. W., and Sui, S. F., 1999, J. Struct. Biol. 127, 283-286). Another type of the protein's 2D crystal under the same conditions was obtained in the present work. The new 2D crystal was studied using electron microscopy of negatively stained specimens followed by image processing. A projection map at 2.2-nm resolution was obtained. The previous 2D crystal (PI) and the current 2D crystal (PII) show different pentamer-packing styles. Both of them are closely related to the fivefold symmetry of the molecule itself. The coexistence and the spatial contiguity of the two types of pentamer assembly were observed in a visual field. The fivefold symmetrical macromolecule can form a pentiling pattern on a two-dimensional plane, which has never been reported in biological system before. The possible mechanism of the two-dimensional assembly of pentameric CRP on lipid monolayers is discussed.

Pentameric two-dimensional crystallization of rabbit C-reactive protein on lipid monolayers

As a member of the pentraxin family, C-reactive protein plays various roles in the nonspecific immunity of animals. Though soluble, C-reactive protein always functions on membranes. In order to study the structure of the membrane-bound protein and the reaction between protein and membranes, two-dimensional (2D) crystallization of rabbit C-reactive protein on lipid monolayers was performed. The 2D crystals composed of pentameric proteins were obtained on lipid monolayers by specific adsorption for the first time. The projection map at 26-A resolution is presented, which exhibits P2 symmetry with lattice parameters a = 158(+/-3) A, b = 92(+/-1) A, and gamma = 107(+/-1) degrees. The current work may give a basis for the further study on the structure of complexes made up of C-reactive protein with its functional binding molecules on membranes.

Calcium-dependent binding of rabbit C-reactive protein to supported lipid monolayers containing exposed phosphorylcholine group

The interaction of rabbit C-reactive protein (rCRP) with a supported monolayer containing a phosphorylcholine moiety was studied. Three types of phospholipids were synthesized, each containing a insertion spacer of eight, six, or three atoms between the phosphorylcholine group and hydrophobic tail. By varying the length of the insertion spacer, we can vary the extension of the phosphorylcholine group from the membrane surface. By varying the monolayer composition, we can control the lateral distance between the exposed phosphorylcholine groups. Using the surface plasmon resonance technique (SPR), we demonstrated that the calcium-dependent binding of rCRP to the model membrane is governed not only by the ability of the ligand to access the binding pocket fully (spacer length), but also by lateral hindrance within the two-dimensional plane of the membrane. The value of the apparent binding constant was estimated by theoretical analysis, which is obviously dependent on the composition of the lipid mixture, and a maximum of (9.9 +/- 1.5) x 10(6) M-1 was obtained.

Interaction of rabbit C-reactive protein with phospholipid monolayers studied by microfluorescence film balance with an externally applied electric field

C-reactive protein (CRP) is one of the most characteristic acute-phase proteins in humans and many other animals. It binds to phosphorylcholine in a calcium-dependent manner. In addition, CRP activates the complement systems via the classical pathway. The interaction between rabbit CRP (rCRP) and model biological membrane is studied using dimyristoylphosphatidylethanolamine and dipalmitoylphosphatidylcholine monolayers. Observations with fluorescence microscopy indicate that rCRP is more likely to be incorporated in the liquid phase of monolayers. Such incorporation does not depend on the presence of calcium and is not inhibited by phosphocholine. The area occupied by the protein when incorporated into the monolayer was estimated. The dipole moment density of the protein crossing the air/water interface was measured by applying an external electric field. Our results indicate that calcium binding leads to a conformational change in CPR, which might modify the orientation of CRP in the monolayer. In addition, a negative charge or negative difference in dipole moment density facilitates the incorporation of CPR into the monolayer.