Investigations on the C1q-calreticulin-phosphatidylserine interactions yield new insights into apoptotic cell recognition

Complement System Part I - Molecular Mechanisms of Activation and Regulation

Complement is a complex innate immune surveillance system, playing a key role in defense against pathogens and in host homeostasis. The complement system is initiated by conformational changes in recognition molecular complexes upon sensing danger signals. The subsequent cascade of enzymatic reactions is tightly regulated to assure that complement is activated only at specific locations requiring defense against pathogens, thus avoiding host tissue damage. Here, we discuss the recent advances describing the molecular and structural basis of activation and regulation of the complement pathways and their implication on physiology and pathology. This article will review the mechanisms of activation of alternative, classical, and lectin pathways, the formation of C3 and C5 convertases, the action of anaphylatoxins, and the membrane-attack-complex. We will also discuss the importance of structure-function relationships using the example of atypical hemolytic uremic syndrome. Lastly, we will discuss the development and benefits of therapies using complement inhibitors.

Complement System Part II: Role in Immunity

The complement system has been considered for a long time as a simple lytic cascade, aimed to kill bacteria infecting the host organism. Nowadays, this vision has changed and it is well accepted that complement is a complex innate immune surveillance system, playing a key role in host homeostasis, inflammation, and in the defense against pathogens. This review discusses recent advances in the understanding of the role of complement in physiology and pathology. It starts with a description of complement contribution to the normal physiology (homeostasis) of a healthy organism, including the silent clearance of apoptotic cells and maintenance of cell survival. In pathology, complement can be a friend or a foe. It acts as a friend in the defense against pathogens, by inducing opsonization and a direct killing by C5b–9 membrane attack complex and by triggering inflammatory responses with the anaphylatoxins C3a and C5a. Opsonization plays also a major role in the mounting of an adaptive immune response, involving antigen presenting cells, T-, and B-lymphocytes. Nevertheless, it can be also an enemy, when pathogens hijack complement regulators to protect themselves from the immune system. Inadequate complement activation becomes a disease cause, as in atypical hemolytic uremic syndrome, C3 glomerulopathies, and systemic lupus erythematosus. Age-related macular degeneration and cancer will be described as examples showing that complement contributes to a large variety of conditions, far exceeding the classical examples of diseases associated with complement deficiencies. Finally, we discuss complement as a therapeutic target.

Agarose gel shift assay reveals that calreticulin favors substrates with a quaternary structure in solution

Here we present an agarose gel shift assay that, in contrast to other electrophoresis approaches, is loaded in the center of the gel. This allows proteins to migrate in either direction according to their isoelectric points. Therefore, the presented assay enables a direct visualization, separation, and prefractionation of protein interactions in solution independent of isoelectric point. We demonstrate that this assay is compatible with immunochemical methods and mass spectrometry. The assay was used to investigate interactions with several potential substrates for calreticulin, a chaperone that is involved in different biological aspects through interaction with other proteins. The current analytical assays used to investigate these interactions are mainly spectroscopic aggregation assays or solid phase assays that do not provide a direct visualization of the stable protein complex but rather provide an indirect measure of interactions. Therefore, no interaction studies between calreticulin and substrates in solution have been investigated previously. The results presented here indicate that calreticulin has a preference for substrates with a quaternary structure and primarily β-sheets in their secondary structure. It is also demonstrated that the agarose gel shift assay is useful in the study of other protein interactions and can be used as an alternative method to native polyacrylamide gel electrophoresis.

The ever-expanding immunomodulatory role of calreticulin in cancer immunity

Calreticulin is a pleiotropic molecule that normally resides in the lumen of the endoplasmic reticulum (ER). Here, it has various functions, ranging from regulation of calcium homeostasis to ensuring proper protein folding. More recently, calreticulin gained special interest for its extracellular functions, where it has direct immunomodulatory activity. In this respect, calreticulin activates dendritic cells and macrophages. In addition, certain anti-cancer therapies induce the translocation of calreticulin from the ER to the cell surface of dying cancer cells, where calreticulin dictates the immunogenicity of these cells. Interestingly, treatment with tumor necrosis factor (TNF)-related apoptosis inducing ligand (TRAIL) also induces membrane calreticulin exposure on cancer cells. As shown here, calreticulin directly interacts with TRAIL and its receptor-signaling complex, as well as with other TNF family members. Of note, TRAIL is a well known immunomodulatory molecule, and is expressed on the surface of natural killer T-cells. Therefore, calreticulin may have an as yet unrecognized wide(r) impact on immunity, with the TNF-ligand family modulating virtually all aspects of the immune response.

Mechanisms of Translocation of ER Chaperones to the Cell Surface and Immunomodulatory Roles in Cancer and Autoimmunity

Endoplasmic reticulum (ER) chaperones (e.g., calreticulin, heat shock proteins, and isomerases) perform a multitude of functions within the ER. However, many of these chaperones can translocate to the cytosol and eventually the surface of cells, particularly during ER stress induced by e.g., drugs, UV irradiation, and microbial stimuli. Once on the cell surface or in the extracellular space, the ER chaperones can take on immunogenic characteristics, as mostly described in the context of cancer, appearing as damage-associated molecular patterns recognized by the immune system. How ER chaperones relocate to the cell surface and interact with other intracellular proteins appears to influence whether a tumor cell is targeted for cell death. The relocation of ER proteins to the cell surface can be exploited to target cancer cells for elimination by immune mechanism. Here we evaluate the evidence for the different mechanisms of ER protein translocation and binding to the cell surface and how ER protein translocation can act as a signal for cancer cells to undergo killing by immunogenic cell death and other cell death pathways. The release of chaperones can also exacerbate underlying autoimmune conditions, such as rheumatoid arthritis and multiple sclerosis, and the immunomodulatory role of extracellular chaperones as potential cancer immunotherapies requires cautious monitoring, particularly in cancer patients with underlying autoimmune disease.

Contribution of Defective PS Recognition and Efferocytosis to Chronic Inflammation and Autoimmunity

The rapid and efficient clearance of apoptotic cells results in the elimination of auto-antigens and provides a strong anti-inflammatory and immunosuppressive signal to prevent autoimmunity. While professional and non-professional phagocytes utilize a wide array of surface receptors to recognize apoptotic cells, the recognition of phosphatidylserine (PS) on apoptotic cells by PS receptors on phagocytes is the emblematic signal for efferocytosis in metazoans. PS-dependent efferocytosis is associated with the production of anti-inflammatory factors such as IL-10 and TGF-β that function, in part, to maintain tolerance to auto-antigens. In contrast, when apoptotic cells fail to be recognized and processed for degradation, auto-antigens persist, such as self-nucleic acids, which can trigger immune activation leading to autoantibody production and autoimmunity. Despite the fact that genetic mouse models clearly demonstrate that loss of PS receptors can lead to age-dependent auto-immune diseases reminiscent of systemic lupus erythematosus (SLE), the link between PS and defective clearance in chronic inflammation and human autoimmunity is not well delineated. In this perspective, we review emerging questions developing in the field that may be of relevance to SLE and human autoimmunity.

In silico and in vitro studies on the protein-protein interactions between Brugia malayi immunomodulatory protein calreticulin and human C1q

Filarial parasites modulate effective immune response of their host by releasing a variety of immunomodulatory molecules, which help in the long persistence of the parasite within the host. The present study was aimed to characterize an immunomodulatory protein of Brugia malayi and its interaction with the host immune component at the structural and functional level. Our findings showed that Brugia malayi Calreticulin (BmCRT) is responsible for the prevention of classical complement pathway activation via its interaction with the first component C1q of the human host. This was confirmed by inhibition of C1q dependent lysis of immunoglobulin-sensitized Red Blood Cells (S-RBCs). This is possibly the first report which predicts CRT-C1q interaction on the structural content of proteins to explain how BmCRT inhibits this pathway. The molecular docking of BmCRT-C1q complex indicated that C1qB chain (IgG/M and CRP binding sites on C1q) played a major role in the interaction with conserved and non-conserved regions of N and P domain of BmCRT. Out of 37 amino acids of BmCRT involved in the interaction, nine amino acids (Pro(126), Glu(132), His(147), Arg(151), His(153), Met(154), Lys(156), Ala(196) and Lys(212)) are absent in human CRT. Both ELISA and in silico analysis showed the significant role of Ca(+2) in BmCRT-HuC1q complex formation and deactivation of C1r2-C1s2. Molecular dynamics studies of BmCRT-HuC1q complex showed a deviation from ∼ 0.4 nm to ∼ 1.0 nm. CD analyses indicated that BmCRT is composed of 49.6% α helix, 9.6% β sheet and 43.6% random coil. These findings provided valuable information on the architecture and chemistry of BmCRT-C1q interaction and supported the hypothesis that BmCRT binds with huC1q at their targets (IgG/M, CRP) binding sites. This interaction enables the parasite to interfere with the initial stage of host complement activation, which might be helpful in parasites establishment. These results might be utilized for help in blocking the C1q/CRT interaction and preventing parasite infection.

Complement, c1q, and c1q-related molecules regulate macrophage polarization

Complement is a critical system of enzymes, regulatory proteins, and receptors that regulates both innate and adaptive immune responses. Natural mutations in complement molecules highlight their requirement in regulation of a variety of human conditions including infectious disease and autoimmunity. As sentinels of the immune system, macrophages are specialized to respond to infectious microbes, as well as normal and altered self, and dictate appropriate immune responses. Complement components such as anaphylatoxins (C3a and C5a) and opsonins [C3b, C1q, mannan binding lectin (MBL)] influence macrophage responses. While anaphylatoxins C3a and C5a trigger inflammasome activation, opsonins such as C1q and related molecules (MBL and adiponectin) downregulate inflammasome activation and inflammation, and upregulate engulfment of apoptotic cells consistent with a pro-resolving or M2 macrophage phenotype. This review summarizes our current understanding of the influence of the complement system on macrophage polarization with an emphasis on C1q and related molecules.

Relative contribution of c1q and apoptotic cell-surface calreticulin to macrophage phagocytosis

C1q has been shown to recognize apoptotic cells, to enhance their uptake and to modulate cytokine release by phagocytes and thus promote immune tolerance. Surface-exposed calreticulin (CRT), known as a C1q receptor, is also considered to be an early eat-me signal that enhances the phagocytosis of apoptotic cells and is capable of eliciting an immunogenic response. However, the molecular mechanisms that trigger these functions are not clear. We hypothesized that CRT and C1q might act together in these processes. We first showed, by means of fluorescence resonance energy transfer (FRET), that CRT interacts with the C1q globular region at the surface of early apoptotic cells. Next, we pointed out that knockdown of CRT on early apoptotic HeLa cells impairs the enhancement effect of C1q on their uptake by THP-1 monocyte-derived macrophages. Furthermore, a deficiency of CRT induces contrasting effects on cytokine release by THP-1 macrophages, increasing interleukin (IL)-6 and monocyte chemotactic protein 1/CCL2 and decreasing IL-8. Remarkably, these effects were greatly reduced when apoptotic cells were opsonized by C1q, which counterbalanced the effect of the CRT deficiency. These results demonstrate that CRT-C1q interaction is involved in the C1q bridging function and they highlight the particular ability of C1q to control the phagocyte inflammatory status, i.e. by integrating the molecular changes that could occur at the surface of dying cells.

The influence of natural lipid asymmetry upon the conformation of a membrane-inserted protein (perfringolysin O)

Eukaryotic membrane proteins generally reside in membrane bilayers that have lipid asymmetry. However, in vitro studies of the impact of lipids upon membrane proteins are generally carried out in model membrane vesicles that lack lipid asymmetry. Our recently developed method to prepare lipid vesicles with asymmetry similar to that in plasma membranes and with controlled amounts of cholesterol was used to investigate the influence of lipid composition and lipid asymmetry upon the conformational behavior of the pore-forming, cholesterol-dependent cytolysin perfringolysin O (PFO). PFO conformational behavior in asymmetric vesicles was found to be distinct both from that in symmetric vesicles with the same lipid composition as the asymmetric vesicles and from that in vesicles containing either only the inner leaflet lipids from the asymmetric vesicles or only the outer leaflet lipids from the asymmetric vesicles. The presence of phosphatidylcholine in the outer leaflet increased the cholesterol concentration required to induce PFO binding, whereas phosphatidylethanolamine and phosphatidylserine in the inner leaflet of asymmetric vesicles stabilized the formation of a novel deeply inserted conformation that does not form pores, even though it contains transmembrane segments. This conformation may represent an important intermediate stage in PFO pore formation. These studies show that lipid asymmetry can strongly influence the behavior of membrane-inserted proteins.

An alternative role of C1q in bacterial infections: facilitating Streptococcus pneumoniae adherence and invasion of host cells

Streptococcus pneumoniae (pneumococcus) is a major human pathogen, which evolved numerous successful strategies to colonize the host. In this study, we report a novel mechanism of pneumococcal-host interaction, whereby pneumococci use a host complement protein C1q, primarily involved in the host-defense mechanism, for colonization and subsequent dissemination. Using cell-culture infection assays and confocal microscopy, we observed that pneumococcal surface-bound C1q significantly enhanced pneumococcal adherence to and invasion of host epithelial and endothelial cells. Flow cytometry demonstrated a direct, Ab-independent binding of purified C1q to various clinical isolates of pneumococci. This interaction was seemingly capsule serotype independent and mediated by the bacterial surface-exposed proteins, as pretreatment of pneumococci with pronase E but not sodium periodate significantly reduced C1q binding. Moreover, similar binding was observed using C1 complex as the source of C1q. Furthermore, our data show that C1q bound to the pneumococcal surface through the globular heads and with the host cell-surface receptor(s)/glycosaminoglycans via its N-terminal collagen-like stalk, as the presence of C1q N-terminal fragment and low m.w. heparin but not the C-terminal globular heads blocked C1q-mediated pneumococcal adherence to host cells. Taken together, we demonstrate for the first time, to our knowledge, a unique function of complement protein C1q, as a molecular bridge between pneumococci and the host, which promotes bacterial cellular adherence and invasion. Nevertheless, in some conditions, this mechanism could be also beneficial for the host as it may result in uptake and clearance of the bacteria.

The scavenger receptor SCARF1 mediates the clearance of apoptotic cells and prevents autoimmunity

Clearance of apoptotic cells is critical for control of tissue homeostasis however the full range of receptor(s) on phagocytes responsible for recognition of apoptotic cells remains to be identified. Here we show that dendritic cells (DCs), macrophages and endothelial cells use scavenger receptor type F family member 1 (SCARF1) to recognize and engulf apoptotic cells via C1q. Loss of SCARF1 impairs uptake of apoptotic cells. Consequently, in SCARF1-deficient mice, dying cells accumulate in tissues leading to a lupus-like disease with the spontaneous generation of autoantibodies to DNA-containing antigens, immune cell activation, dermatitis and nephritis. The discovery of SCARF1 interactions with C1q and apoptotic cells provides insights into molecular mechanisms involved in maintenance of tolerance and prevention of autoimmune disease.

Calreticulin and other components of endoplasmic reticulum stress in rat and human inflammatory demyelination

BACKGROUND

Calreticulin (CRT) is a chaperone protein, which aids correct folding of glycosylated proteins in the endoplasmic reticulum (ER). Under conditions of ER stress, CRT is upregulated and may be displayed on the surface of cells or be secreted. This 'ecto-CRT' may activate the innate immune response or it may aid clearance of apoptotic cells. Our and other studies have demonstrated upregulation of ER stress markers CHOP, BiP, ATF4, XBP1 and phosphorylated e-IF2 alpha (p-eIF2 alpha) in biopsy and post-mortem human multiple sclerosis (MS) samples. We extend this work by analysing changes in expression of CRT, BiP, CHOP, XBP1 and p-eIF2 alpha in a rat model of inflammatory demyelination. Demyelination was induced in the spinal cord by intradermal injection of recombinant mouse MOG mixed with incomplete Freund's adjuvant (IFA) at the base of the tail. Tissue samples were analysed by semi-quantitative scoring of immunohistochemically stained frozen tissue sections. Data generated following sampling of tissue from animals with spinal cord lesions, was compared to that obtained using tissue derived from IFA- or saline-injected controls. CRT present in rat serum and in a cohort of human serum derived from 14 multiple sclerosis patients and 11 healthy controls was measured by ELISA.

RESULTS

Stained tissue scores revealed significantly (p<0.05) increased amounts of CRT, CHOP and p-eIF2 alpha in the lesion, lesion edge and normal-appearing white matter when compared to controls. CHOP and p-eIF2 alpha were also significantly raised in regions of grey matter and the central canal (p<0.05). Immunofluorescent dual-label staining confirmed expression of these markers in astrocytes, microglia or neurons. Dual staining of rat and human spinal cord lesions with Oil Red O and CRT antibody showed co-localisation of CRT with the rim of myelin fragments. ELISA testing of sera from control and EAE rats demonstrated significant down-regulation (p<0.05) of CRT in the serum of EAE animals, compared to saline and IFA controls. This contrasted with significantly increased amounts of CRT detected in the sera of MS patients (p<0.05), compared to controls.

CONCLUSION

This data highlights the potential importance of CRT and other ER stress proteins in inflammatory demyelination.

Human and pneumococcal cell surface glyceraldehyde-3-phosphate dehydrogenase (GAPDH) proteins are both ligands of human C1q protein

C1q, a key component of the classical complement pathway, is a major player in the response to microbial infection and has been shown to detect noxious altered-self substances such as apoptotic cells. In this work, using complementary experimental approaches, we identified the glyceraldehyde-3-phosphate dehydrogenase (GAPDH) as a C1q partner when exposed at the surface of human pathogenic bacteria Streptococcus pneumoniae and human apoptotic cells. The membrane-associated GAPDH on HeLa cells bound the globular regions of C1q as demonstrated by pulldown and cell surface co-localization experiments. Pneumococcal strains deficient in surface-exposed GAPDH harbored a decreased level of C1q recognition when compared with the wild-type strains. Both recombinant human and pneumococcal GAPDHs interacted avidly with C1q as measured by surface plasmon resonance experiments (K(D) = 0.34-2.17 nm). In addition, GAPDH-C1q complexes were observed by transmission electron microscopy after cross-linking. The purified pneumococcal GAPDH protein activated C1 in an in vitro assay unlike the human form. Deposition of C1q, C3b, and C4b from human serum at the surface of pneumococcal cells was dependent on the presence of surface-exposed GAPDH. This ability of C1q to sense both human and bacterial GAPDHs sheds new insights on the role of this important defense collagen molecule in modulating the immune response.

Calreticulin in the immune system: ins and outs

Calreticulin is a calcium-binding chaperone that has several functions in the immune response. In the endoplasmic reticulum (ER), calreticulin facilitates the folding of major histocompatibility complex (MHC) class I molecules and their assembly factor tapasin, thereby influencing antigen presentation to cytotoxic T cells. Although calreticulin is normally ER-resident, it is found at the cell surface of living cancer cells and dying cells. Here, calreticulin promotes cellular phagocytic uptake. In tumor vaccine models, drugs that induce cell surface calreticulin confer enhanced tumor protection in an extracellular calreticulin-dependent manner. Much remains to be understood about the roles of calreticulin in these distinct functions. Further investigations are important towards advancing basic knowledge of glycoprotein-folding pathways, and towards developing new cancer therapeutic strategies.

Proteinase 3, the autoantigen in granulomatosis with polyangiitis, associates with calreticulin on apoptotic neutrophils, impairs macrophage phagocytosis, and promotes inflammation

Proteinase 3 (PR3) is the target of anti-neutrophil cytoplasm Abs in granulomatosis with polyangiitis, a form of systemic vasculitis. Upon neutrophil apoptosis, PR3 is coexternalized with phosphatidylserine and impaired macrophage phagocytosis. Calreticulin (CRT), a protein involved in apoptotic cell recognition, was found to be a new PR3 partner coexpressed with PR3 on the neutrophil plasma membrane during apoptosis, but not after degranulation. The association between PR3 and CRT was demonstrated in neutrophils by confocal microscopy and coimmunoprecipitation. Evidence for a direct interaction between PR3 and the globular domain of CRT, but not with its P domain, was provided by surface plasmon resonance spectroscopy. Phagocytosis of apoptotic neutrophils from healthy donors was decreased after blocking lipoprotein receptor-related protein (LRP), a CRT receptor on macrophages. In contrast, neutrophils from patients with granulomatosis with polyangiitis expressing high membrane PR3 levels showed a lower rate of phagocytosis than those from healthy controls not affected by anti-LRP, suggesting that the LRP-CRT pathway was disturbed by PR3-CRT association. Moreover, phagocytosis of apoptotic PR3-expressing cells potentiated proinflammatory cytokine in vitro by human monocyte-derived macrophages and in vivo by resident murine peritoneal macrophages, and diverted the anti-inflammatory response triggered by the phagocytosis of apoptotic cells after LPS challenge in thioglycolate-elicited murine macrophages. Therefore, membrane PR3 expressed on apoptotic neutrophils might amplify inflammation and promote autoimmunity by affecting the anti-inflammatory "reprogramming" of macrophages.

Eaten alive! Cell death by primary phagocytosis: 'phagoptosis'

Phagoptosis, also called primary phagocytosis, is a recently recognised form of cell death caused by phagocytosis of viable cells, resulting in their destruction. It is provoked by exposure of 'eat-me' signals and/or loss of 'don't-eat-me' signals by viable cells, causing their phagocytosis by phagocytes. Phagoptosis mediates turnover of erythrocytes, neutrophils and other cells, and thus is quantitatively one of the main forms of cell death in the body. It defends against pathogens and regulates inflammation and immunity. However, recent results indicate that inflamed microglia eat viable brain neurons in models of neurodegeneration, and cancer cells can evade phagocytosis by expressing a 'don't-eat-me' signal, suggesting that too much or too little phagoptosis can contribute to pathology. This review provides an overview of the molecular signals that regulate phagoptosis and the physiological and pathological circumstances in which it has been observed.

How apoptotic cells aid in the removal of their own cold dead bodies

Apoptotic cell clearance facilitates the removal of aged, damaged, infected or dangerous cells although minimizing perturbation of surrounding tissues, and is a vital process in the development and homeostasis of multicellular organisms. Importantly, failure to correctly execute programmed cell death and subsequent corpse clearance is broadly associated with chronic inflammatory and/or autoimmune diseases such as systemic lupus erythematosus. Apoptotic cells develop dramatic morphological changes including contraction, membrane blebbing and apoptotic body formation, which were among the first and most readily identifiable features of cellular suicide. However, understanding the purpose of apoptotic cell morphological changes has proven to be elusive, and recent studies have made somewhat surprising, and occasionally opposing, conclusions about the contribution of blebbing to phagocytic clearance and prevention of inflammatory/autoimmune disease. We review the evidence indicating how apoptotic blebs actively promote corpse recognition, uptake, and generation of auto-reactive antibodies.

The human c1q globular domain: structure and recognition of non-immune self ligands

C1q, the ligand-binding unit of the C1 complex of complement, is a pattern recognition molecule with the unique ability to sense an amazing variety of targets, including a number of altered structures from self, such as apoptotic cells. The three-dimensional structure of its C-terminal globular domain, responsible for its recognition function, has been solved by X-ray crystallography, revealing a tightly packed heterotrimeric assembly with marked differences in the surface patterns of the subunits, and yielding insights into its versatile binding properties. In conjunction with other approaches, this same technique has been used recently to decipher the mechanisms that allow this domain to interact with various non-immune self ligands, including molecules known to provide eat-me signals on apoptotic cells, such as phosphatidylserine and DNA. These investigations provide evidence for a common binding area for these ligands located in subunit C of the C1q globular domain, and suggest that ligand recognition through this area down-regulates C1 activation, hence contributing to the control of the inflammatory reaction. The purpose of this article is to give an overview of these advances which represent a first step toward understanding the recognition mechanisms of C1q and their biological implications.

The C1q family of proteins: insights into the emerging non-traditional functions

Research conducted over the past 20 years have helped us unravel not only the hidden structural and functional subtleties of human C1q, but also has catapulted the molecule from a mere recognition unit of the classical pathway to a well-recognized molecular sensor of damage-modified self or non-self antigens. Thus, C1q is involved in a rapidly expanding list of pathological disorders – including autoimmunity, trophoblast migration, preeclampsia, and cancer. The results of two recent reports are provided to underscore the critical role C1q plays in health and disease. First is the observation by Singh et al. (2011) showing that pregnant C1q−/− mice recapitulate the key features of human preeclampsia that correlate with increased fetal death. Treatment of the C1q−/− mice with pravastatin restored trophoblast invasiveness, placental blood flow, and angiogenic balance and, thus, prevented the onset of preeclampsia. Second is the report by Hong et al. (2009) which showed that C1q can induce apoptosis of prostate cancer cells by activating the tumor suppressor molecule WW-domain containing oxydoreductase (WWOX or WOX1) and destabilizing cell adhesion. Downregulation of C1q on the other hand, enhanced prostate hyperplasia and cancer formation due to failure of WOX1 activation. C1q belongs to a family of structurally and functionally related TNF-α-like family of proteins that may have arisen from a common ancestral gene. Therefore C1q not only shares the diverse functions with the tumor necrosis factor family of proteins, but also explains why C1q has retained some of its ancestral “cytokine-like” activities. This review is intended to highlight some of the structural and functional aspects of C1q by underscoring the growing list of its non-traditional functions.

X-ray structure of the human calreticulin globular domain reveals a peptide-binding area and suggests a multi-molecular mechanism

In the endoplasmic reticulum, calreticulin acts as a chaperone and a Ca²⁺-signalling protein. At the cell surface, it mediates numerous important biological effects. The crystal structure of the human calreticulin globular domain was solved at 1.55 Å resolution. Interactions of the flexible N-terminal extension with the edge of the lectin site are consistently observed, revealing a hitherto unidentified peptide-binding site. A calreticulin molecular zipper, observed in all crystal lattices, could further extend this site by creating a binding cavity lined by hydrophobic residues. These data thus provide a first structural insight into the lectin-independent binding properties of calreticulin and suggest new working hypotheses, including that of a multi-molecular mechanism.