The physiological structure of human C-reactive protein and its complex with phosphocholine.

BACKGROUND: Human C-reactive protein (CRP) is the classical acute phase reactant, the circulating concentration of which rises rapidly and extensively in a cytokine-mediated response to tissue injury, infection and inflammation. Serum CRP values are routinely measured, empirically, to detect and monitor many human diseases. However, CRP is likely to have important host defence, scavenging and metabolic functions through its capacity for calcium-dependent binding to exogenous and autologous molecules containing phosphocholine (PC) and then activating the classical complement pathway. CRP may also have pathogenic effects and the recent discovery of a prognostic association between increased CRP production and coronary atherothrombotic events is of particular interest. RESUTLS: The X-ray structures of fully calcified C-reactive protein, in the presence and absence of bound PC, reveal that although the subunit beta-sheet jellyroll fold is very similar to that of the homologous pentameric protein serum amyloid P component, each subunit is tipped towards the fivefold axis. PC is bound in a shallow surface pocket on each subunit, interacting with the two protein-bound calcium ions via the phosphate group and with Glu81 via the choline moiety. There is also an unexpected hydrophobic pocket adjacent to the ligand. CONCLUSIONS: The structure shows how large ligands containing PC may be bound by CRP via a phosphate oxygen that projects away from the surface of the protein. Multipoint attachment of one planar face of the CRP molecule to a PC-bearing surface would leave available, on the opposite exposed face, the recognition sites for C1q, which have been identified by mutagenesis. This would enable CRP to target physiologically and/or pathologically significant complement activation. The hydrophobic pocket adjacent to bound PC invites the design of inhibitors of CRP binding that may have therapeutic relevance to the possible role of CRP in atherothrombotic events.     

Structure and antigenicity of the lipophosphoglycan from Leishmania major amastigotes.

The lipophosphoglycan (LPG) of the intracellular amastigote form of the protozoan parasite Leishmania major is chemically distinct from the LPG on the surface of the extracellular promastigote form. Amastigote LPG is composed of the monosaccharides galactose, glucose, mannose, glucosamine and inositol in the molar ratio 51:30:24:1:1; arabinose is absent. The lipid anchor comprises four alkylglycerols, with alkyl chain lengths 24:0, 22:0, 20:0 and 26:0 in the molar ratio 68:18:8:6. Phosphate is present at 4% w/w of total carbohydrate. HPLC gel permeation reveals LPG to be a polydisperse family of molecules Mr 100-6 kDa. The results from immunological studies with LPG-directed antibodies are consistent with amastigote LPG having the expected tripartite structure of GPI-anchor, a core glycan and the phosphorylated disaccharide repeat backbone. Human sera from L. major patients bound amastigote LPG in enzyme-linked immunosorbent assays.     

Structure of the cell-adhesion fragment of intimin from enteropathogenic Escherichia coli

Enteropathogenic Escherichia coli (EPEC) induce gross cytoskeletal rearrangement within epithelial cells, immediately beneath the attached bacterium. The C-terminal 280 amino acid residues of intimin (Int280; 30.1 kDa), a bacterial cell-adhesion molecule, mediate the intimate bacterial host-cell interaction. Recently, interest in this process has been stimulated by the discovery that the bacterial intimin receptor protein (Tir) is translocated into the host cell membrane, phosphorylated, and after binding intimin triggers the intimate attachment. Using multidimensional nuclear magnetic resonance (NMR) and combining perdeuteration with site-specific protonation of methyl groups, we have determined the global fold of Int280. This represents one of the largest, non-oligomeric protein structures to be determined by NMR that has not been previously resolved by X-ray crystallography. Int280 comprises three domains; two immunoglobulin-like domains and a C-type lectin-like module, which define a new family of bacterial adhesion molecules. These findings also imply that carbohydrate recognition may be important in intimin-mediated cell adhesion.     

Structure of the lipophosphoglycan from Leishmania major

The major cell surface glycoconjugate of the parasitic protozoan Leishmania major is a heterogeneous lipophosphoglycan. It has a tripartite structure, consisting of a phosphoglycan (Mr 5,000-40,000), a variably phosphorylated hexasaccharide glycan core, and a lysoalkylphosphatidylinositol (lysoalkyl-PI) lipid anchor. The structures of the phosphoglycan and the hexasaccharide core were determined by monosaccharide analysis, methylation analysis, fast atom bombardment-mass spectrometry, one- and two-dimensional 500-MHz (correlated spectroscopy (COSY), homonuclear Hartmann-Hahn spectroscopy (HOHAHA] 1H NMR spectroscopy, and exoglycosidase digestions. The phosphoglycan consists of eight types of phosphorylated oligosaccharide repeats which have the general structure, [formula: see text] where R = H, Galp(beta 1-3), Galp(beta 1-3)Galp(beta 1-3), Arap(alpha 1-2)Galp(beta 1-3), Glcp(beta 1-3)Galp(beta 1-3), Galp(beta 1-3)Galp(beta 1-3)Galp(beta 1-3), Arap(alpha 1-2)Galp(beta 1-3)Galp(beta 1-3), or Arap(alpha 1-2)Galp(beta 1-3)Galp(beta 1-3)Galp(beta 1-3)Galp(beta 1-3), and where all the monosaccharides, including arabinose, are in the D-configuration. The average number of repeat units/molecule (n) is 27. Data are presented which suggest that the nonreducing terminus of the phosphoglycan is capped exclusively with the neutral disaccharide Manp(alpha 1-2)Manp alpha 1-. The structure of the glycan core was determined to be, [formula: see text] where approximately 60% of the mannose residues distal to the glucosamine are phosphorylated and where the inositol is part of the lysoalkyl-PI lipid moiety containing predominantly 24:0 and 26:0 alkyl chains. The unusual galactofuranose residue is in the beta-configuration, correcting a previous report where this residue was identified as alpha Galf. Although most of the phosphorylated repeat units are attached to the terminal galactose 6-phosphate of the core to form a linear lipophosphoglycan (LPG) molecule, some of the mannose 6-phosphate residues may also be substituted to form a Y-shaped molecule. The L. major LPG is more complex than the previously characterized LPG from Leishmania donovani, although both LPGs have the same repeating backbone structure and glycolipid anchor. Finally we show that the LPG anchor is structurally related to the major glycolipid species of L. major, indicating that some of these glycolipids may have a function as precursors to LPG.     

Leishmanial protein kinases phosphorylate components of the complement system.

Externally oriented protein kinases are present on the plasma membrane of the human parasite, Leishmania. Since activation of complement plays an important role in the survival of these parasites, we examined the ability of protein kinases from Leishmania major to phosphorylate components of the human complement system. The leishmanial protein kinase-1 (LPK-1) isolated from promastigotes of L. major was able to phosphorylate purified human C3, C5 and C9. Only the alpha-chain of C3 and C5 was phosphorylated. The beta-chain appeared not to be a substrate for this enzyme. C3b which is formed by proteolytic cleavage of C3 was not phosphorylated by LPK-1. Trypsin treatment of phosphorylated C3 (P-C3) resulted in the disappearance of 32P from the alpha-chain. This was correlated with the conversion of the C3 alpha-chain to the alpha'-chain of C3b, and the appearance of a 9 kDa 32P fragment comigrating with the C3a fragment of C3. P-C3 was more resistant to cleavage by trypsin than nonphosphorylated C3. LPK-1 phosphorylated purified C3a and two synthetic peptides, C3a21R and YA-C3a10R, derived from its COOH-terminal end, which contain the C3a binding site to leukocytes and platelets. LPK-1 did not phosphorylate C3a8R. Phosphoamino acid analysis of the synthetic peptides indicated that serine 71 of C3a was phosphorylated by LPK-1. Treatment of C3 with either methylamine or freeze-thaw C3 (H2O) prevented phosphorylation by the LPK-1 suggesting that substrate conformation may be involved in recognition by the leishmanial enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)     

Interplay between site-specific mutations and cyclic nucleotides in modulating DNA recognition by Escherichia coli cyclic AMP receptor protein

Mutagenesis of various amino acids in Escherichia coli cyclic AMP receptor protein (CRP) has been shown to modulate protein compressibility and dynamics [Gekko et al. (2004) Biochemistry 43, 3844-3852]. Cooperativity of cAMP binding to CRP and the apparent DNA binding affinity are perturbed [Lin and Lee (2002) Biochemistry 41, 11857-11867]. The aim of this study is to explore the effects of mutation on the surface chemistry of CRP and to define the consequences of these changes in affecting specific DNA sequence recognition by CRP. Furthermore, the role of the interplay between mutation and specific identity of the bound cyclic nucleotide in this DNA recognition was explored. In the current study, effects of eight site-specific mutations (K52N, D53H, S62F, T127L, G141Q, L148R, H159L, and K52N/H159L) on DNA recognition of four sequences (Class I (site PI of lac), Class II (site PI of gal), and synthetic sequences that are hybrids of Classes I and II sites) modulated by three different cyclic nucleotides (cAMP, cCMP, and cGMP) were investigated. All mutations altered the surface chemistry of CRP as evidenced by the change in elution properties of these proteins from different matrixes. While T127L, S62F, K52N, and H159L exhibited unexpected behavior under combinations of specific experimental conditions, such as the identity of bound cyclic nucleotide and DNA sequence, in general, results showed that the affinities of CRP for DNA were sequence-dependent, increasing in the order of lacgal26 < gal26 < lac26 < gallac26 for all the mutants in the presence of 200 microM cAMP. The apparent association constants significantly increased in the order of no cyclic nucleotide approximately cGMP < cCMP < cAMP for all the examined DNA sequences. Linear correlation between the DeltaG for CRP-DNA complex formation and the cooperativity energy for cAMP binding was observed with gallac26, gal26, and lacgal26; however, the slope of this linear correlation is DNA sequence dependent. Structural information was presented to rationalize the interplay between CRP sequence and cyclic nucleotides in defining the recognition of DNA sequences.     

Phosphorylation of the Synthetic Hexasaccharide Repeating Unit Is Essential for the Induction of Antibodies to Clostridium difficile PSII Cell Wall Polysaccharide

Clostridium difficile is emerging worldwide as a major cause of nosocomial infections. The negatively charged PSII polysaccharide has been found in different strains of C. difficile and, thereby, represents an important target molecule for a possible carbohydrate-based vaccine. In order to identify a synthetic fragment that after conjugation to a protein carrier could be able to induce anti-PSII antibodies, we exploited a combination of chemical synthesis with immunochemistry, confocal immunofluorescence microscopy, and solid state NMR. We demonstrate that the phosphate group is crucial in synthetic glycans to mimic the native PSII polysaccharide; both native PSII and a phosphorylated synthetic hexasaccharide repeating unit conjugated to CRM(197) elicit comparable immunogenic responses in mice. This finding can aid design and selection of carbohydrate antigens to be explored as vaccine candidates.     

Structural analysis of a glycosylphosphatidylinositol glycolipid ofLeishmania donovani

A glycosylphosphatidylinositol (GPI) glycolipid antigen recognized by sera from patients with visceral leishmaniasis was isolated from Leishmania donovani promastigotes. The carbohydrate moiety was cleaved from the lipid part by digestion with specific phosphatidylinositol phospholipase C. After separation, structural analysis was carried out on the phosphorylated inositol oligosaccharide and the alkylacyl glycerol. The following major structures were found: [formula: see text] The presence of the conserved sequence Man alpha 1-2Man alpha 1-6Man alpha 1-4GlcN-PI of glycosyl phosphatidylinositol protein anchors in this antigen may be consistent with a precursor role of Leishmania glycosyl phosphatidylinositol anchored proteins for this glycolipid.     

C-reactive Protein

C-reactive protein (CRP) is a phylogenetically highly conserved plasma protein, with homologs in vertebrates and many invertebrates, that participates in the systemic response to inflammation. Its plasma concentration increases during inflammatory states, a characteristic that has long been employed for clinical purposes. CRP is a pattern recognition molecule, binding to specific molecular configurations that are typically exposed during cell death or found on the surfaces of pathogens. Its rapid increase in synthesis within hours after tissue injury or infection suggests that it contributes to host defense and that it is part of the innate immune response. Recently, an association between minor CRP elevation and future major cardiovascular events has been recognized, leading to the recommendation by the Centers for Disease Control and the American Heart Association that patients at intermediate risk of coronary heart disease might benefit from measurement of CRP. This review will largely focus on our current understanding of the structure of CRP, its ligands, the effector molecules with which it interacts, and its apparent functions.     

Identification of cadherin tyrosine residues that are phosphorylated and mediate Shc association.

Previously, we reported association of the adaptor protein Shc through its SH2 domain with the cytoplasmic domain of the adhesion molecule cadherin (Xu et al. [1997] J. Biol. Chem. 272:13463-13466). This association was dependent on tyrosine phosphorylation of cadherin and could be modulated by extracellular Ca(2+) and epidermal growth factor in intact cells. There are six tyrosine residues in the cytoplasmic domain of cadherin. To define the tyrosine residue(s) that mediate Shc recognition, site-directed mutagenesis was employed to alter Tyr851 and/or Tyr883 in cadherin, which both conform to a predicted Shc SH2 domain recognition sequence. Mutation of either Tyr851 or Tyr883, but mostly the latter, decreased Src phosphorylation of cadherin and the binding of Shc to cadherin, as determined by Sepharose bead binding and gel overlay assays. Of the two tyrosine residues, Tyr883 is the major Src phosphorylation and Shc binding site. However, the double mutant (Tyr851, 883 Phe) exhibited less Shc association than the single Tyr883 Phe mutant, suggesting a role for Tyr851 also. In addition, the binding of Shc to the cadherin cytoplasmic domain was competitively inhibited by tyrosine phosphorylated peptides containing either Tyr851 or Tyr883, but not by the corresponding non-phosphorylated peptides. Mutation of Tyr851 and/or Tyr883 did not alter the capacity of the cytoplasmic domain of cadherin to bind beta-catenin in vitro. However, Shc binding to cadherin did negatively influence beta-catenin binding to the same molecule.     

Determination of a putative phosphate-containing peptide in calreticulin.

Calreticulin is an abundant endo/sarcoplasmic reticulum (ER/SR) protein that may carry out multiple functions inside cells. Except for calreticulin, all of the major ER/SR Ca2+-binding proteins are substrates for protein kinase CK2 in vitro, which led us to hypothesize that native calreticulin might exist in the phosphorylated form. To investigate this possibility, we purified calreticulin from cardiac microsomes and verified its identity by immunoblot analysis and sequencing of tryptic peptides. Purified calreticulin, like cardiac calsequestrin, contained endogenous phosphate as determined by a Malachite green assay for phosphate. Previous analyses of cardiac calsequestrin have localized phosphate to a single tryptic peptide containing serine phosphate on sites phosphorylated by protein kinase CK2. Using a similar procedure, we analyzed calreticulin tryptic peptides with Malachite green, localizing phosphate binding to a single calreticulin peptide 367LKEEEEDKK. As this peptide contains no phosphorylatable residues, our results suggest that calreticulin may tightly bind phosphate or a phosphate-containing molecule at this site.     

C-reactive protein collaborates with plasma lectins to boost immune response against bacteria

Although human C-reactive protein (CRP) becomes upregulated during septicemia, its role remains unclear, since purified CRP showed no binding to many common pathogens. Contrary to previous findings, we show that purified human CRP (hCRP) binds to Salmonella enterica, and that binding is enhanced in the presence of plasma factors. In the horseshoe crab, Carcinoscorpius rotundicauda, CRP is a major hemolymph protein. Incubation of hemolymph with a range of bacteria resulted in CRP binding to all the bacteria tested. Lipopolysaccharide-affinity chromatography of the hemolymph co-purified CRP, galactose-binding protein (GBP) and carcinolectin-5 (CL5). Yeast two-hybrid and pull-down assays suggested that these pattern recognition receptors (PRRs) form pathogen recognition complexes. We show the conservation of PRR crosstalk in humans, whereby hCRP interacts with ficolin (CL5 homologue). This interaction stabilizes CRP binding to bacteria and activates the lectin-mediated complement pathway. We propose that CRP does not act alone but collaborates with other plasma PRRs to form stable pathogen recognition complexes when targeting a wide range of bacteria for destruction.     

Phosphorylation of extracellular carbohydrates by intact cells. Chicken hepatocytes specifically adhere to and phosphorylate immobilized N-acetylglucosamine

Cell-cell adhesion is a multi-step process which may be initiated by binding of cell surface carbohydrates to complementary carbohydrate receptors on apposing cell surfaces. We have modeled such interactions using polyacrylamide gels covalently derivatized with glycosides, to which intact cells specifically adhere; chicken hepatocytes adhere to gels derivatized with N-acetylglucosamine (GlcNAc). Initially adhesion is blocked (or reversed) by soluble GlcNAc, but becomes sugar-resistant rapidly at 37 degrees C, perhaps due to cellular modification of the carbohydrate-derivatized surface (Guarnaccia, S. P., Kuhlenschmidt, M. S., Slife, C. W., and Schnaar, R. L. (1982) J. Biol. Chem. 257, 14293-14299). We report here that, subsequent to recognition and adhesion, intact chicken hepatocytes transfer phosphate covalently to GlcNAc-derivatized gels. Metabolically radiolabeled cells (32Pi) were incubated on polyacrylamide gels derivatized with various aminohexyl glycosides. Noncovalently bound material was then removed from the gels by extensive washing in detergents and salt solutions. Subsequent radiochemical analysis revealed that phosphate was transferred selectively to GlcNAc-derivatized gels (up to 20-fold more than to glucose-, galactose-, or mannose-derivatized gels). Soluble GlcNAc (but not other sugars) or low temperature inhibited phosphate transfer. The phosphorylation was mediated by intact cells; cell lysate was itself incapable of specific phosphate transfer and attenuated specific transfer when added to intact cells. When GlcNAc was immobilized using a cleavable (disulfide-containing) linker arm the transferred phosphate radiolabel could be solubilized by disulfide reduction and recovered for further analysis. The released phosphorylated product migrated as a single low molecular weight species upon gel permeation chromatography, paper electrophoresis, and cellulose thin layer chromatography. Acid hydrolysis of the phosphorylated product generated a compound with the mobility of GlcNAc-6-P in five different separation systems. Treatment with alkaline phosphatase converted the radiolabel to a compound with the properties of inorganic phosphate. These data indicate that; subsequent to carbohydrate recognition and adhesion, intact hepatocytes generate phosphomonoesters of recognized carbohydrates outside of their plasma membranes.     

The mouse macrophage receptor for C3bi (CR3) is a major mechanism in the phagocytosis of Leishmania promastigotes

We examined the role of the macrophage receptor for C3bi, the CR3, in the phagocytosis of Leishmania major promastigotes and report that M1/70, a monoclonal antibody to the CR3, inhibited the binding of leishmania to macrophages both when the assays were performed in the presence of normal serum and in its absence. In serum, leishmania activate complement and fix C3. Fixation and subsequent cleavage to C3bi occurs rapidly, and by as early as 5 min both forms of the molecule can be identified on the parasites' surface. Complement fixation results in an enhanced phagocytosis of leishmania promastigotes by mouse macrophages. In the case of L. major, 63% of this serum-enhanced binding is inhibitable by M1/70. Binding assays were also performed in the absence of serum with the use of thoroughly washed promastigotes. The addition of M1/70 inhibited binding under these conditions by 54%. Two other rat monoclonal antibodies directed against different antigens on the macrophage plasma membrane did not inhibit binding. M1/70 did not inhibit the binding of promastigotes to rat bone marrow cells, nor did it inhibit IgG-SRBC binding to mouse peritoneal macrophages. These data indicate that the inhibition observed in the presence of M1/70 was specific for the CR3 and that the macrophage receptor for C3bi plays a major role in the phagocytosis of Leishmania major promastigotes, even in the absence of serum.     

Determinants that govern the recognition and uptake of Escherichia coli O157 : H7 by Acanthamoeba castellanii

Predation by phagocytic predators is a major source of bacterial mortality. The first steps in protozoan predation are recognition and consumption of their bacterial prey. However, the precise mechanisms governing prey recognition and phagocytosis by protists, and the identities of the molecular and cellular factors involved in these processes are, as yet, ill‐characterized. Here, we show that that the ability of the phagocytic bacterivorous amoebae, Acanthamoeba castellanii, to recognize and internalize Escherichia coli, a bacterial prey, varies with LPS structure and composition. The presence of an O‐antigen carbohydrate is not required for uptake of E. coli by A. castellanii. However, O1‐antigen types, not O157 O‐antigen types, inhibit recognition and uptake of bacteria by amoeba. This finding implies that O‐antigen may function as an antipredator defence molecule. Recognition and uptake of E. coli by A. castellanii is mediated by the interaction of mannose‐binding protein located on amoebae's surface with LPS carbohydrate. Phagocytic mammalian cells also use mannose‐binding lectins to recognize and/or mediate phagocytosis of E. coli. Nonetheless, A. castellanii's mannose binding protein apparently displays no sequence similarity with any known metazoan mannose binding protein. Hence, the similarity in bacterial recognition mechanisms of amoebae and mammalian phagocytes may be a result of convergent evolution.     

Evolution of a carbohydrate binding module into a protein-specific binder

A carbohydrate binding module, CBM4-2, derived from the xylanase (Xyn 10A) of Rhodothermus marinus has been used as a scaffold for molecular diversification. Its binding specificity has been evolved to recognise a quite different target, a human monoclonal IgG4. In order to understand the basis for this drastic change in specificity we have further investigated the target recognition of the IgG4-specific CBMs. Firstly, we defined that the structure target recognised by the selected CBM-variants was the protein and not the carbohydrates attached to the glycoprotein. We also identified key residues involved in the new specificity and/or responsible for the swap in specificity, from xylan to human IgG4. Specific changes present in all these CBMs included mutations not introduced in the design of the library from which the specific clones were selected. Reversion of such mutations led to a complete loss of binding to the target molecule, suggesting that they are critical for the recognition of human IgG4. Together with the mutations introduced at will, they had transformed the CBM scaffold into a protein binder. We have thus shown that the scaffold of CBM4-2 is able to harbour molecular recognition for either carbohydrate or protein structures.     

Purification, composition, and structure of macrophage adhesion molecule

Macrophage adhesion molecule (MAM) is a surface heterodimer consisting of the trypsin- and plasmin-sensitive glycopeptide gp160 (MAM-alpha) and the glycopeptide gp93 (MAM-beta). MAM, which is the guinea pig analogue of Mo1 and Mac-1, was purified from detergent lysates of peritoneal neutrophils by lentil lectin chromatography and M2-antibody chromatography. The pure heterodimer molecule was dissociated by acidic conditions (pH 3.5), and MAM-alpha and MAM-beta were separated by M7-antibody chromatography. MAM-beta is an approximately 640 amino acid residue polypeptide with exceptionally high cysteine content. At 7.2 residues per 100 amino acids, Cys/2 of MAM-beta is more than 3 times the mean for 200 purified proteins. Reactivity with six beta-subunit-specific monoclonal antibodies recognizing at least four epitopes demonstrated that intrapeptide disulfide bonds are required to maintain the structure of MAM-beta. All six antibodies failed to react when MAM-beta was treated with reducing agents. MAM-beta is 18% carbohydrate; the major monosaccharides are mannose, N-acetylglucosamine, galactose, and sialic acid. MAM-beta is estimated to contain five to six N-linked carbohydrate units. MAM-alpha is an approximately 1100-residue polypeptide with lower Cys/2 content (2.0 residues per 100 amino acid residues). MAM-alpha is 21% carbohydrate. The major monosaccharides are mannose, N-acetylglucosamine, galactose, and sialic acid; the mannose content is higher in MAM-alpha than MAM-beta. MAM-alpha is estimated to contain 12 N-linked carbohydrate units.     

Selective ligand recognition by a diversity-generating retroelement variable protein

Diversity-generating retroelements (DGRs) recognize novel ligands through massive protein sequence variation, a property shared uniquely with the adaptive immune response. Little is known about how recognition is achieved by DGR variable proteins. Here, we present the structure of the Bordetella bacteriophage DGR variable protein major tropism determinant (Mtd) bound to the receptor pertactin, revealing remarkable adaptability in the static binding sites of Mtd. Despite large dissimilarities in ligand binding mode, principles underlying selective recognition were strikingly conserved between Mtd and immunoreceptors. Central to this was the differential amplification of binding strengths by avidity (i.e., multivalency), which not only relaxed the demand for optimal complementarity between Mtd and pertactin but also enhanced distinctions among binding events to provide selectivity. A quantitatively similar balance between complementarity and avidity was observed for Bordetella bacteriophage DGR as occurs in the immune system, suggesting that variable repertoires operate under a narrow set of conditions to recognize novel ligands.     

Selective Ligand Recognition by a Diversity-Generating Retroelement Variable Protein

Diversity-generating retroelements (DGRs) recognize novel ligands through massive protein sequence variation, a property shared uniquely with the adaptive immune response. Little is known about how recognition is achieved by DGR variable proteins. Here, we present the structure of the Bordetella bacteriophage DGR variable protein major tropism determinant (Mtd) bound to the receptor pertactin, revealing remarkable adaptability in the static binding sites of Mtd. Despite large dissimilarities in ligand binding mode, principles underlying selective recognition were strikingly conserved between Mtd and immunoreceptors. Central to this was the differential amplification of binding strengths by avidity (i.e., multivalency), which not only relaxed the demand for optimal complementarity between Mtd and pertactin but also enhanced distinctions among binding events to provide selectivity. A quantitatively similar balance between complementarity and avidity was observed for Bordetella bacteriophage DGR as occurs in the immune system, suggesting that variable repertoires operate under a narrow set of conditions to recognize novel ligands.     

Phosphorylation of alpha-tubulin carboxyl-terminal tyrosine prevents its incorporation into microtubules

Insulin receptor kinase phosphorylated tubulin in an insulin-dependent fashion. Two different populations of phosphotubulin were found. In tubulin dimers containing tyrosine at the carboxyl-terminal of their alpha subunit, phosphate was incorporated in that residue, and the phosphorylated protein did not assemble into polymers. In tubulin dimers lacking this tyrosine residue, phosphate was incorporated into different tyrosine residues located in other parts of the molecule, and the phosphoprotein retained its capacity to polymerize.