Efficient Folding of the FcεRI α-chain Membrane-proximal Domain D2 Depends on the Presence of the N-terminal Domain D1

Current Understanding of the Structure and Function of Fungal Immunomodulatory Proteins

Fungal immunomodulatory proteins (FIPs) are a group of proteins found in fungi, which are extensively studied for their immunomodulatory activity. Currently, more than 38 types of FIPs have been described. Based on their conserved structure and protein identity, FIPs can be classified into five subgroups: Fve-type FIPs (Pfam PF09259), Cerato-type FIPs (Pfam PF07249), PCP-like FIPs, TFP-like FIPs, and unclassified FIPs. Among the five subgroups, Fve-type FIPs are the most studied for their hemagglutinating, immunomodulating, and anti-cancer properties. In general, these small proteins consist of 110–125 amino acids, with a molecular weight of ~13 kDa. The other four subgroups are relatively less studied, but also show a noticeable influence on immune cells. In this review, we summarized the protein modifications, 3-dimensional structures and bioactivities of all types of FIPs. Moreover, structure-function relationship of FIPs has been discussed, including relationship between carbohydrate binding module and hemagglutination, correlation of oligomerization and cytokine induction, relevance of glycosylation and lymphocyte activation. This summary and discussion may help gain comprehensive understanding of FIPs' working mechanisms and scope future studies.

Selective targeting of proteins within secretory pathway for endoplasmic reticulum-associated degradation

The endoplasmic reticulum-associated degradation (ERAD) is a cellular quality control mechanism to dispose of misfolded proteins of the secretory pathway via proteasomal degradation. SEL1L is an ER-resident protein that participates in identification of misfolded molecules as ERAD substrates, therefore inducing their ER-to-cytosol retrotranslocation and degradation. We have developed a novel class of fusion proteins, termed degradins, composed of a fragment of SEL1L fused to a target-specific binding moiety located on the luminal side of the ER. The target-binding moiety can be a ligand of the target or derived from specific mAbs. Here, we describe the ability of degradins with two different recognition moieties to promote degradation of a model target. Degradins recognize the target protein within the ER both in secretory and membrane-bound forms, inducing their degradation following retrotranslocation to the cytosol. Thus, degradins represent an effective technique to knock-out proteins within the secretory pathway with high specificity.

IgE-binding properties and selectivity of peptide mimics of the FcvarepsilonRI binding site

FcvarepsilonRIalpha found on the surface of mast cells and basophiles mediates allergic diseases, anaphylaxis and asthma through binding of IgE. Disrupting this interaction with anti-IgE mAbs has proven an efficient approach to control these diseases. The crystallographic structure of the complex formed between the IgE-Fc and FcvarepsilonRIalpha extracellular domain has shown that recognition is mediated by residues in the second Ig-like domain of the receptor (D2) and in the loop connecting the D1 and D2 domains. In an attempt to obtain specific IgE antagonists, we have designed and prepared a polypeptide named IgE-Trap that partially reproduces the IgE receptor-binding sites and binds with micromolar affinity to soluble IgE. The polypeptide contains loops C'-E [residues 129-134] and F-G [residues 151-161] from the D2 domain joined by a linker, and loop B-C [residues 110-113]. Peptide binding to IgE has been assessed by SPR analyses and the data fit with a biphasic model of interaction, in agreement with the two-site mechanism reported for the native receptor. The polypeptide binds to immobilized IgE in a dose-dependent manner with a K(D) estimated to be around 6muM, while it does not recognize IgG nor IgA. Polypeptide sub-domains involved in IgE binding have also been defined, showing that loop C'-E connected to loop B-C, but also the isolated loop B-C alone suffice to bind immunoglobulins E with high selectively though with reduced affinity compared to IgE-Trap. ELISA and cytometric assays on RBL2H3 cells demonstrate that the interacting peptides are able to displace the binding of IgE to receptor, confirming affinity and specificity of these ligands and suggesting a potential application as modulators of disorders associated with inappropriate IgE production.

The superior folding of a RANTES analogue expressed in lactobacilli as compared to mammalian cells reveals a promising system to screen new RANTES mutants

Development of effective topical microbicides for the prevention of HIV-1 sexual transmission represents a primary goal for the control of the AIDS pandemic. The viral coreceptor CCR5, used by the vast majority of primary HIV-1 isolates, is considered a primary target molecule. RANTES and its derivatives are the most suitable protein-based compounds to fight HIV-1 via CCR5 targeting. Yet, receptor activation should be avoided to prevent pro-inflammatory effects and possibly provide anti-inflammatory properties. C1C5 RANTES is a chemokine mutant that exhibits high anti-HIV-1 potency coupled with CCR5 antagonism. However, the need for the formation of an N-terminal intramolecular disulfide bridge between non-natural cysteine residues at positions 1 and 5 represents a challenge for the correct folding of this protein in recombinant expression systems, a crucial step towards its development as a microbicide against HIV-1. We report here a rare case of superior folding in a prokaryote as compared to an eukaryotic expression system. Production of C1C5 RANTES was highly impaired in CHO cells, with a dramatic yield reduction compared to that of wild type RANTES and secretion of the molecule as disulfide-linked dimer. Conversely, a human vaginal isolate of Lactobacillus jensenii engineered to secrete C1C5 RANTES provided efficient delivery of the monomeric protein. This and other reports on successful secretion of complex proteins indicate that lactic acid bacteria are an excellent system for the expression of therapeutic proteins, which can be used as a platform for the engineering of conceptually novel RANTES mutants with potent anti-HIV-1 activity.

Unreliable measurement of basophil maximum leukotriene release with the Bühlmann CAST 2000 enzyme-linked immunosorbent assay kit

The Bühlmann CAST 2000 enzyme-linked immunosorbent assay is a potentially useful assay for measuring sulfidoleukotrienes released in vitro by allergen-challenged basophils. However, we observed that the positive-control reagent yielded positive signals in cell-free systems. These false-positive results depended on using a mouse anti-FcepsilonRI monoclonal antibody and were prevented by degranulation-inducing reagents other than mouse monoclonal antibodies.

Membrane IgE Binds and Activates FcεRI in an Antigen-Independent Manner

Interaction of secretory IgE with FcepsilonRI is the prerequisite for allergen-driven cellular responses, fundamental events in immediate and chronic allergic manifestations. Previous studies reported the binding of soluble FcepsilonRIalpha to membrane IgE exposed on B cells. In this study, the functional interaction between human membrane IgE and human FcepsilonRI is presented. Four different IgE versions were expressed in mouse B cell lines, namely: a truncation at the Cepsilon2-Cepsilon3 junction of membrane IgE isoform long, membrane IgE isoform long (without Igalpha/Igbeta BCR accessory proteins), and both epsilonBCRs (containing membrane IgE isoforms short and long). All membrane IgE versions activated a rat basophilic leukemia cell line transfected with human FcepsilonRI, as detected by measuring the release of both preformed and newly synthesized mediators. The interaction led also to Ca(2+) responses in the basophil cell line, while membrane IgE-FcepsilonRI complexes were detected by immunoprecipitation. FcepsilonRI activation by membrane IgE occurs in an Ag-independent manner. Noteworthily, human peripheral blood basophils and monocytes also were activated upon contact with cells bearing membrane IgE. In humans, the presence of FcepsilonRI in several cellular entities suggests a possible membrane IgE-FcepsilonRI-driven cell-cell dialogue, with likely implications for IgE homeostasis in physiology and pathology.

A 1.7A structure of Fve, a member of the new fungal immunomodulatory protein family

Fve, a major fruiting body protein from Flammulina velutipes, a mushroom possessing immunomodulatory activity, stimulates lymphocyte mitogenesis, suppresses systemic anaphylaxis reactions and edema, enhances transcription of IL-2, IFN-gamma and TNF-alpha, and hemagglutinates red blood cells. It appears to be a lectin with specificity for complex cell-surface carbohydrates. Fve is a non-covalently linked homodimer containing no Cys, His or Met residues. It shares sequence similarity only to the other fungal immunomodulatory proteins (FIPs) LZ-8, Gts, Vvo and Vvl, all of unknown structure. The 1.7A structure of Fve solved by single anomalous diffraction of NaBr-soaked crystals is novel: each monomer consists of an N-terminal alpha-helix followed by a fibronectin III (FNIII) fold. The FNIII fold is the first instance of "pseudo-h-type" topology, a transition between the seven beta-stranded s-type and the eight beta-stranded h-type topologies. The structure suggests that dimerization, critical for the activity of FIPs, occurs by 3-D domain swapping of the N-terminal helices and is stabilized predominantly by hydrophobic interactions. The structure of Fve is the first in this lectin family to be reported, and the first of an FNIII domain-containing protein of fungal origin.

Current progress in the understanding of IgE-FcepsilonRI interaction

The last decade has seen a wealth of studies aimed at the characterization of the binding between IgE and its high-affinity receptor, FcepsilonRI. IgE-FcepsilonRI complex formation is a major molecular event in atopic allergy. IgE-FcepsilonRI binding connects allergen recognition to cellular triggering, ultimately leading to disease manifestations. Consequently, pharmacological intervention at this site is of universal relevance for atopic allergy. Until recent years, the complexity of IgE-FcepsilonRI binding, together with the difficulty in obtaining fully functional recombinant IgE and FcepsilonRI derivatives, often led to confusion and difficulty in data interpretation. Major advances in the understanding of this intricate protein-protein interaction have now been accomplished. Most of the current knowledge on the IgE-FcepsilonRI recognition mode derives from long-lasting efforts in the field of structural biology. Protein engineering, high-throughput screening, immunological and biochemical studies also made relevant contributions in this domain. The data accumulated to date predict that IgE and FcepsilonRI use their modular architecture to approach each other in an asymmetric stepwise manner determining a 1:1 stoichiometry. This recognition appears to be enhanced by conformational changes occurring upon binding, leading to the well-known high-affinity. In conclusion, the vast amount of high-quality data available broadened our knowledge on the IgE-FcepsilonRI system; however, the fine structural details of the recognition process are still largely hypothetical. More studies are necessary to provide the experimental comprehensive picture required to carefully design efficient drugs acting at the IgE-FcepsilonRI interface.