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

Densest plane group packings of regular polygons

Packings of regular convex polygons (n-gons) that are sufficiently dense have been studied extensively in the context of modeling physical and biological systems as well as discrete and computational geometry. Former results were mainly regarding densest lattice or double-lattice configurations. Here we consider all two-dimensional crystallographic symmetry groups (plane groups) by restricting the configuration space of the general packing problem of congruent copies of a compact subset of the two-dimensional Euclidean space to particular isomorphism classes of the discrete group of isometries. We formulate the plane group packing problem as a nonlinear constrained optimization problem. By means of the Entropic Trust Region Packing Algorithm that approximately solves this problem, we examine some known and unknown densest packings of various n-gons in all 17 plane groups and state conjectures about common symmetries of the densest plane group packings for every n-gon.

Membrane pore formation by pentraxin proteins from Limulus, the American horseshoe crab

The pentraxins are a family of highly conserved plasma proteins of metazoans known to function in immune defence. The canonical members, C-reactive protein and serum amyloid P component, have been identified in arthropods and humans. Mammalian pentraxins are known to bind lipid bilayers, and a pentraxin representative from the American horseshoe crab, Limulus polyphemus, binds and permeabilizes mammalian erythrocytes. Both activities are Ca2+-dependent. Utilizing model liposomes and planar lipid bilayers, in the present study we have investigated the membrane-active properties of the three pentraxin representatives from Limulus and show that all of the Limulus pentraxins permeabilize lipid bilayers. Mechanistically, Limulus C-reactive protein forms transmembrane pores in asymmetric planar lipid bilayers that mimic the outer membrane of Gram-negative bacteria and exhibits a Ca2+-independent form of membrane binding that may be sufficient for pore formation.

Dietary modulation and structure prediction of rat mucosal pentraxin (Mptx) protein and loss of function in humans

Mucosal pentraxin (Mptx), identified in rats, is a short pentraxin of unknown function. Other subfamily members are Serum amyloid P component (SAP), C-reactive protein (CRP) and Jeltraxin. Rat Mptx mRNA is predominantly expressed in colon and in vivo is strongly (30-fold) regulated by dietary heme and calcium, modulators of colon cancer risk. This renders Mptx a potential nutrient sensitive biomarker of gut health. To support a role as biomarker, we examined whether the pentraxin protein structure is conserved, whether Mptx protein is nutrient-sensitively expressed and whether Mptx is expressed in mouse and human. Sequence comparison and 3D modelling showed that rat Mptx is highly homologous to the other pentraxins. The calcium-binding site and subunit interaction sites are highly conserved, while a loop deletion and charged residues contribute to a distinctive "top" face of the pentamer. In accordance with mRNA expression, Mptx protein is strongly down-regulated in rat colon mucosa in response to high dietary heme intake. Mptx mRNA is expressed in rat and mouse colon, but not in human colon. A stop codon at the beginning of human exon two indicates loss of function, which may be related to differences in intestinal cell turnover between man and rodents.

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.

Role of immediate early gene expression in cortical morphogenesis and plasticity

During the development of the central nervous system, there is a fundamental requirement for synaptic activity in transforming immature neuronal connections into organized functional circuits (Katz 1996). The molecular mechanisms underlying activity-dependent adaptive changes in neurons are believed to involve regulated cascades of gene expression. Immediate early genes (IEGs) comprise the initial cascade of gene expression responsible for initiating the process of stimulus-induced adaptive change, and were identified initially as transcription factors that were regulated in brain by excitatory synaptic activity. More recently, a class of neuronal immediate early genes has been identified that encodes growth factors, signaling molecules, extracellular matrix and adhesion proteins, and cytoskeletal proteins that are rapidly and transiently expressed in response to glutamatergic neurotransmission. This review focuses on the neuronal immediate early gene (nIEG) response, in particular, the class of "effector" immediate early gene proteins that may directly modify neuronal and synaptic function.

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.