Structure and function of gC1q-R: a multiligand binding cellular protein

The N-terminal conserved domain of rubella virus capsid interacts with the C-terminal region of cellular p32 and overexpression of p32 enhances the viral infectivity

Cellular 'defense collagens' are produced to launch virus-specific responses to clear the invading viruses. Cellular p32, the C1q binding protein is one such protein. In this report, we identified the interaction of p32 derived from a human lung diploid cell line (WI-38) with rubella virus capsid (RVCP from Therien strain) N-terminal 28-amino acid domain, which is conserved among several RV strains including the vaccine strains. We further identified that the C-terminal 69 aa of the mature p32 is sufficient to interact with the CP. In addition, we observed that in three independent Vero 76-derived cell lines constitutively overexpressing p32, the RV infectivity was enhanced. Our results suggest that RV has evolved a strategy whereby one of its proteins is recruited to interact with, and exploit the cellular defense machinery to its advantage.

The hemopexin-like C-terminal domain of membrane type 1 matrix metalloproteinase regulates proteolysis of a multifunctional protein, gC1qR

Matrix metalloproteinases (MMPs) including membrane type 1 MMP (MT1-MMP) can degrade extracellular matrix and cell surface receptor molecules and have an essential function in malignancy. Recently, we established a functional link between MT1-MMP and the receptor of complement component 1q (gC1qR). The gC1qR is known as a compartment-specific regulator of diverse cellular and viral proteins. Once released by proliferating cells, soluble gC1qR may inhibit complement component 1q hemolytic activity and play important roles in vivo in assisting tumor cells to evade destruction by complement. Here, we report that gC1qR is susceptible to MT1-MMP proteolysis in vitro and in cell cultures. The major MT1-MMP cleavage site (Gly(79) down arrow Gln(80)) is localized within the structurally disordered loop connecting the beta(3) and the beta(4) strands of gC1qR. The recombinant MT1-MMP construct that included the catalytic domain but lacked the hemopexin-like domain lost the proteolytic capacity; however, it retained the ability to bind gC1qR. Inhibition of MT1-MMP activity by a hydroxamate inhibitor converted the protease into a cell surface receptor of gC1qR and promoted co-precipitation MT1-MMP with the soluble gC1qR protein. It is tempting to hypothesize that these novel mechanisms may play important roles in vivo and have to be taken into account in designing hydroxamate-based cancer therapy.

Cooperation of C1q receptors and integrins in C1q-mediated endothelial cell adhesion and spreading

The interaction of C1q with endothelial cells elicits a multiplicity of biologic responses. Although these responses are presumed to be mediated by the interaction of C1q with endothelial cell surface proteins, the identity of the participants is not known. In this study we examined the roles of two C1q binding proteins, cC1q-R/calreticulin and gC1q-R/p33, in C1q-mediated adhesion and spreading of human dermal microvascular endothelial cells (HDMVEC). When HDMVEC were cultured in microtiter plate wells coated with concentrations of C1q ranging from 0 to 50 microg/ml, a specific and dose-dependent adhesion and spreading was observed. The extent of adhesion and spreading was similar to the adhesion seen on collagen-coated wells. Spreading (68 +/- 12%) and to a moderate extent adhesion (47 +/- 9%) were inhibited by anti-gC1q-R mAb 60.11. Similar effects were noted with polyclonal anti-cC1q-R but not with control nonimmune IgG. The two Abs had a slight additive effect (75 +/- 13% inhibition) when mixed together in the proportion of 100 microg/ml anti-gC1q-R and 30 microg/ml anti-cC1q-R. More importantly, a 100% inhibition of spreading, but not adhesion, to C1q-coated wells was observed when HDMVEC were cultured in the presence of 30 microM of the peptide GRRGDSP but not GRRGESP. Furthermore, while anti-beta(1) integrin Ab blocked both adhesion and spreading, anti-alpha(5) integrin blocked only spreading and not adhesion. Ag capture ELISA of endothelial cell membrane proteins using polyclonal anti-gC1q-R showed the presence of not only beta(1) and alpha(5) integrins but also CD44. Taken together these results suggest that endothelial cell adhesion and spreading require the cooperation of both C1qRs and beta(1) integrins and possibly other membrane-spanning molecules.

Cluster analysis of an extensive human breast cancer cell line protein expression map database

In the current study, the protein expression maps (PEMs) of 26 breast cancer cell lines and three cell lines derived from normal breast or benign disease tissue were visualised by high resolution two-dimensional gel electrophoresis. Analysis of this data was performed with ChiClust and ChiMap, two analytical bioinformatics tools that are described here. These tools are designed to facilitate recognition of specific patterns shared by two or more (a series) PEMs. Both tools use PEMs that were matched by an image analysis program and locally written programs to create a match table that is saved in an object relational database. The ChiClust tool uses clustering and subclustering methods to extract statistically significant protein expression patterns from a large series of PEMs. The ChiMap tool calculates a differential value (either as percentage change or a fold change) and represents these graphically. All such differentials or just those identified using ChiClust can be submitted to ChiMap. These methods are not dependent on any particular commercial image analysis program, and the whole software package gives an integrated procedure for the comparison and analysis of a series of PEMs. The ChiClust tool was used here to order the breast cell lines into groups according to biological characteristics including morphology in vitro and tumour forming ability in vivo. ChiMap was then used to highlight eight major protein feature-changes detected between breast cancer cell lines that either do or do not proliferate in nude mice. Mass spectrometry was used to identify the proteins. The possible role of these proteins in cancer is discussed.

Hepatitis C virus core protein inhibits human T lymphocyte responses by a complement-dependent regulatory pathway

Complement proteins are involved in early innate immune responses against pathogens and play a role in clearing circulating viral Ags from the blood of infected hosts. We have previously demonstrated that hepatitis C virus (HCV) core, the first protein to be expressed and circulating in the blood of infected individuals, inhibited human T cell proliferative response through interaction with the complement receptor, globular domain of C1q receptor (gC1qR). To investigate the mechanisms of HCV core/gC1qR-induced inhibition of T cell proliferation, we examined the effect of core protein on the early events in T cell activation. We found that HCV core inhibited phosphorylation of extracellular signal-regulated kinase (ERK) and mitogen-activated ERK kinase (MEK). HCV core-induced impairment of ERK/MEK mitogen-activated protein kinase resulted in the inhibition of IL-2 and IL-2Ralpha gene transcription, which led to the inhibition of IL-2 production and high-affinity IL-2R expression. Importantly, the ability of anti-gC1qR Ab treatment to reverse HCV core-induced inhibition of ERK/MEK phosphorylation reveals that the interaction between HCV core and gC1qR is linked to the interference of ERK/MEK mitogen-activated protein kinase activation. These results imply that HCV core-induced blockage of intracellular events in T cell activation by a complement-dependent regulatory pathway may play a critical role in the establishment of HCV persistence during the acute phase of viral infection.

C1q and mannose binding lectin engagement of cell surface calreticulin and CD91 initiates macropinocytosis and uptake of apoptotic cells

Removal of apoptotic cells is essential for maintenance of tissue homeostasis, organogenesis, remodeling, development, and maintenance of the immune system, protection against neoplasia, and resolution of inflammation. The mechanisms of this removal involve recognition of the apoptotic cell surface and initiation of phagocytic uptake into a variety of cell types. Here we provide evidence that C1q and mannose binding lectin (MBL), a member of the collectin family of proteins, bind to apoptotic cells and stimulate ingestion of these by ligation on the phagocyte surface of the multifunctional protein, calreticulin (also known as the cC1qR), which in turn is bound to the endocytic receptor protein CD91, also known as the alpha-2-macroglobulin receptor. Use of these proteins provides another example of apoptotic cell clearance mediated by pattern recognition molecules of the innate immune system. Ingestion of the apoptotic cells through calreticulin/CD91 stimulation is further shown to involve the process of macropinocytosis, implicated as a primitive and relatively nonselective uptake mechanism for C1q- and MBL-enhanced engulfment of whole, intact apoptotic cells, as well as cell debris and foreign organisms to which these molecules may bind.

C1q and mannose binding lectin engagement of cell surface calreticulin and CD91 initiates macropinocytosis and uptake of apoptotic cells

Removal of apoptotic cells is essential for maintenance of tissue homeostasis, organogenesis, remodeling, development, and maintenance of the immune system, protection against neoplasia, and resolution of inflammation. The mechanisms of this removal involve recognition of the apoptotic cell surface and initiation of phagocytic uptake into a variety of cell types. Here we provide evidence that C1q and mannose binding lectin (MBL), a member of the collectin family of proteins, bind to apoptotic cells and stimulate ingestion of these by ligation on the phagocyte surface of the multifunctional protein, calreticulin (also known as the cC1qR), which in turn is bound to the endocytic receptor protein CD91, also known as the alpha-2-macroglobulin receptor. Use of these proteins provides another example of apoptotic cell clearance mediated by pattern recognition molecules of the innate immune system. Ingestion of the apoptotic cells through calreticulin/CD91 stimulation is further shown to involve the process of macropinocytosis, implicated as a primitive and relatively nonselective uptake mechanism for C1q- and MBL-enhanced engulfment of whole, intact apoptotic cells, as well as cell debris and foreign organisms to which these molecules may bind.

Pathogenic trickery: deception of host cell processes

Microbial pathogens cause a spectrum of diseases in humans. Although the disease mechanisms vary considerably, most pathogens have developed virulence factors that interact with host molecules, often usurping normal cellular processes, including cytoskeletal dynamics and vesicle targeting. These virulence factors often mimic host molecules, and mediate events as diverse as bacterial invasion, antiphagocytosis, and intracellular parastism.

The human gC1qR/p32 gene, C1qBP. Genomic organization and promoter analysis

gC1qR is an ubiquitously expressed cell protein that interacts with the globular heads of C1q (gC1q) and many other ligands. In this study, the 7.8-kilobase pair (kb) human gC1qR/p32 (C1qBP) gene was cloned and found to consist of 6 exons and 5 introns. Analysis of a 1.3-kb DNA fragment at the 5'-flanking region of this gene revealed the presence of multiple TATA, CCAAT, and Sp1 binding sites. Luciferase reporter assays performed in different human cell lines demonstrated that the reporter gene was ubiquitously driven by this 1.3-kb fragment. Subsequent 5' and 3' deletion of this fragment confined promoter elements to within 400 base pairs (bp) upstream of the translational start site. Because the removal of the 8-bp consensus TATATATA at -399 to -406 and CCAAT at -410 to -414 did not significantly affect the transcription efficiency of the promoter, GC-rich sequences between this TATA box and the translation start site may be very important for the promoter activity of the C1qBP gene. One of seven GC-rich sequences in this region binds specifically to PANC-1 nuclear extracts, and the transcription factor Sp1 was shown to bind to this GC-rich sequence by the supershift assay. Primer extension analysis mapped three major transcription start regions. The farthest transcription start site is 49 bp upstream of the ATG translation initiation codon and is in close proximity of the specific SP1 binding site.

The globular heads of C1q specifically recognize surface blebs of apoptotic vascular endothelial cells

Complement protein C1q is required to maintain immune tolerance. The molecular mechanism responsible for this link has not been determined. We have previously demonstrated that C1q binds directly and specifically to surface blebs of apoptotic human keratinocytes, suggesting that it may participate in clearance of self Ags generated during programmed cell death. Here, we demonstrate that C1q also binds directly to apoptotic blebs of vascular endothelial cells and PBMC. These apoptotic cells are recognized by the globular heads of C1q, which bind specifically to the surface blebs, and deposition increases as the blebs mature on the cell surface. These observations suggest that C1q may participate in the clearance of apoptotic cells from the circulation and from the walls of the vascular lumen. The interaction of surface blebs with the globular heads of C1q suggests that surface blebs may be capable of directly activating the classical pathway of complement under certain circumstances, generating C4- and C3-derived ligands for receptors such as CR1, CR2, CR3, and CR4. Appropriate recognition of apoptotic cells by C1q and targeted clearance of the molecular contents of surface blebs to complement receptors may be critical for the maintenance of immune tolerance.

Interaction between complement receptor gC1qR and hepatitis C virus core protein inhibits T-lymphocyte proliferation

Hepatitis C virus (HCV) is an important human pathogen that is remarkably efficient at establishing persistent infection. The HCV core protein is the first protein expressed during the early phase of HCV infection. Our previous work demonstrated that the HCV core protein suppresses host immune responses, including anti-viral cytotoxic T-lymphocyte responses in a murine model. To investigate the mechanism of HCV core-mediated immunosuppression, we searched for host proteins capable of associating with the core protein using a yeast two-hybrid system. Using the core protein as bait, we screened a human T cell-enriched expression library and identified a gene encoding the gC1q receptor (gC1qR). C1q is a ligand of gC1qR and is involved in the early host defense against infection. Like C1q, HCV core can inhibit T-cell proliferative responses in vitro. This core-induced anti-T-cell proliferation is reversed by addition of anti-gC1qR Ab in a T-cell proliferation assay. Furthermore, biochemical analysis of the interaction between core and gC1qR indicates that HCV core binds the region spanning amino acids 188 to 259 of gC1qR, a site distinct from the binding region of C1q. The inhibition of T-cell responsiveness by HCV core may have important implications for HCV persistence in humans.

C1q: structure, function, and receptors

C1q is the first subcomponent of the C1 complex of the classical pathway of complement activation. Several functions have been assigned to C1q, which include antibody-dependent and independent immune functions, and are considered to be mediated by C1q receptors present on the effector cell surface. There remains some uncertainty about the identities of the receptors that mediate C1q functions. Some of the previously described C1q receptor molecules, such as gC1qR and cC1qR, now appear to have less of a role in C1q functions than in functions unrelated to C1q. The problem of identifying receptor proteins with complementary binding sites for C1q has been compounded by the highly charged nature of the different domains in C1q. Although newer candidate receptors like C1qR(p) and CR1 have emerged, full analysis of the C1q-C1q receptor interactions is still at an early stage. In view of the diverse functions that C1q is considered to perform, it has been speculated that several C1q-binding proteins may act in concert, as a C1q receptor complex, to bring about C1q mediated functions. Some major advances have been made in last few years. Experiments with gene targeted homozygous C1q-deficient mice have suggested a role for C1q in modulation of the humoral immune response, and also in protection against development of autoimmunity. The recently described crystal structure of Acrp-30, which is a serum protein secreted from adipocytes, has revealed a new C1q/TNF superfamily of proteins. Although the members of this superfamily may have diverse functions, there may be a common theme in their phylogeny and modular organisation of their distinctive globular domains.

Interactions between Listeria monocytogenes and host mammalian cells

Bacterial pathogens have developed a variety of strategies to induce their own internalization into mammalian cells which are normally nonphagocytic. The Gram-positive bacterium Listeria monocytogenes enters into many cultured cell types using two bacterial surface proteins, InlA (internalin) and InlB. In both cases, entry takes place after engagement of a receptor and induction of a series of signaling events.

gC1q-R/p32, a C1q-binding protein, is a receptor for the InlB invasion protein of Listeria monocytogenes

InlB is a Listeria monocytogenes protein that promotes entry of the bacterium into mammalian cells by stimulating tyrosine phosphorylation of the adaptor proteins Gab1, Cbl and Shc, and activation of phosphatidyl- inositol (PI) 3-kinase. Using affinity chromatography and enzyme-linked immunosorbent assay, we demonstrate a direct interaction between InlB and the mammalian protein gC1q-R, the receptor of the globular part of the complement component C1q. Soluble C1q or anti-gC1q-R antibodies impair InlB-mediated entry. Transient transfection of GPC16 cells, which are non-permissive to InlB-mediated entry, with a plasmid-expressing human gC1q-R promotes entry of InlB-coated beads. Furthermore, several experiments indicate that membrane recruitment and activation of PI 3-kinase involve an InlB-gC1q-R interaction and that gC1q-R associates with Gab1 upon stimulation of Vero cells with InlB. Thus, gC1q-R constitutes a cellular receptor involved in InlB-mediated activation of PI 3-kinase and tyrosine phosphorylation of the adaptor protein Gab1. After E-cadherin, the receptor for internalin, gC1q-R is the second identified mammalian receptor promoting entry of L. monocytogenes into mammalian cells.

Staphylococcus aureus protein A recognizes platelet gC1qR/p33: a novel mechanism for staphylococcal interactions with platelets

The adhesion of Staphylococcus aureus to platelets is a major determinant of virulence in the pathogenesis of endocarditis. Molecular mechanisms mediating S. aureus interactions with platelets, however, are incompletely understood. The present study describes the interaction between S. aureus protein A and gC1qR/p33, a multifunctional, ubiquitously distributed cellular protein, initially described as a binding site for the globular heads of C1q. Suspensions of fixed S. aureus or purified protein A, chemically cross-linked to agarose support beads, were found to capture native gC1qR from whole platelets. Moreover, biotinylated protein A bound specifically to fixed, adherent, human platelets. This interaction was inhibited by unlabeled protein A, soluble recombinant gC1qR (rgC1qR), or anti-gC1qR antibody F(ab')(2) fragments. The interaction between protein A and platelet gC1qR was underscored by studies illustrating preferential recognition of the protein A-bearing S. aureus Cowan I strain by gC1qR compared to recognition of the protein A-deficient Wood 46 strain, as well as inhibition of S. aureus Cowan I strain adhesion to immobilized platelets by soluble protein A. Further characterization of the protein A-gC1qR interaction by solid-phase enzyme-linked immunosorbent assay techniques measuring biotinylated gC1qR binding to immobilized protein A revealed specific binding that was inhibited by soluble protein A with a 50% inhibitory concentration of (3.3 +/- 0.7) x 10(-7) M (mean +/- standard deviation; n = 3). Rabbit immunoglobulin G (IgG) also prevented gC1qR-protein A interactions, and inactivation of protein A tyrosil residues by hyperiodination, previously reported to prevent the binding of IgG Fc, but not Fab, domains to protein A, abrogated gC1qR binding. These results suggest similar protein A structural requirements for gC1qR and IgG Fc binding. Further studies of structure and function using a truncated gC1qR mutant lacking amino acids 74 to 95 demonstrated that the protein A binding domain lies outside of the gC1qR amino-terminal alpha helix, which contains binding sites for the globular heads of C1q. In conclusion, the data implicate the platelet gC1qR as a novel cellular binding site for staphylococcal protein A and suggest an additional mechanism for bacterial cell adhesion to sites of vascular injury and thrombosis.

Structure of the lnlB leucine-rich repeats, a domain that triggers host cell invasion by the bacterial pathogen L. monocytogenes

The L. monocytogenes protein lnlB activates phosphoinositide 3-kinase and induces phagocytosis in several mammalian cell types. The 1.86 A resolution X-ray crystal structure of the leucine-rich repeat domain of lnlB that is both necessary and sufficient to induce phagocytosis is presented here. The structure supports a crucial role for calcium in host cell invasion by L. monocytogenes and supplies a rationale for its function. Calciums are bound to the protein in an unusually exposed manner that suggests that the metals may act as a bridge between lnlB and mammalian cell surface receptors. The structure also identifies surfaces on the curved and elongated molecule that may constitute additional interaction sites in forming a bacterial-mammalian signaling complex.

Interaction of the alpha(1B)-adrenergic receptor with gC1q-R, a multifunctional protein

gC1q-R, a multifunctional protein, was found to bind with the carboxyl-terminal cytoplasmic domain of the alpha(1B)-adrenergic receptor (173 amino acids, amino acids 344-516) in a yeast two-hybrid screen of a cDNA library prepared from the rat liver. In a series of studies with deletion mutants in this region, the ten arginine-rich amino acids (amino acids 369-378) were identified as the site of interaction. The interaction was confirmed by specific co-immunoprecipitation of gC1q-R with full-length alpha(1B)-adrenergic receptors expressed on transfected COS-7 cells, as well as by fluorescence confocal laser scanning microscopy, which showed co-localization of these proteins in intact cells. Interestingly, the alpha(1B)-adrenergic receptors were exclusively localized to the region of the plasma membrane in COS-7 cells that expressed the alpha(1B)-adrenergic receptor alone, whereas gC1q-R was localized in the cytoplasm in COS-7 cells that expressed gC1q-R alone; however, in cells that co-expressed alpha(1B)-adrenergic receptors and gC1q-R, most of the alpha(1B)-adrenergic receptors were co-localized with gC1q-R in the intracellular region, and a remarkable down-regulation of receptor expression was observed. These observations suggest a new role for the previously identified complement regulatory molecule, gC1q-R, in regulating the cellular localization and expression of the alpha(1B)-adrenergic receptors.

Up-regulation of endothelial cell binding proteins/receptors for complement component C1q by inflammatory cytokines

Endothelial cells express a variety of receptor systems involved in humoral defense, including receptors for the collagen-like and globular domains of the complement component C1q, designated cC1qR and gC1qR, respectively. In the present study a microvascular endothelial cell line was used to test the hypothesis that expression of these C1q-binding proteins may be affected by vascular inflammatory reactions. The results demonstrate that the expression of both cC1qR and gC1qR by bone marrow vascular endothelial cells is up-regulated by inflammatory mediators, interferon-gamma, tumor necrosis factor-alpha, and lipopolysaccharide (Escherichia coli, 055:B5) in a dose- and time-dependent manner, as detected by enzyme-linked immunosorbent assay. cC1qR and gC1qR expression increased significantly (P < .05) within 4 to 7 hours and doubled after 22 hours of stimulation. 3H-thymidine incorporation studies and direct cell counts confirmed that increased C1qR expression was not due to increased cell proliferation. Northern blot analysis revealed that the up-regulation of cC1qR and gC1qR protein expression was preceded by increases in corresponding mRNA levels, suggesting increased gene transcription. Indeed C1qR mRNA up-regulation was prevented by actinomycin D, and C1qR protein synthesis was inhibited by cycloheximide. Bone marrow vascular endothelial cell exposure to C1q, however, did not alter cC1qR or gC1qR expression, but up-regulation of the leukocyte adhesion molecule ICAM-1 was noted in the presence of aggregated C1q. The up-regulation of C1qR by inflammatory mediators and the ability of C1q itself to increase ICAM-1 expression suggest a potential role for these binding sites in vascular inflammation and immune injury.

The receptor for the globular "heads" of C1q, gC1q-R, binds to fibrinogen/fibrin and impairs its polymerization

The 33-kDa cellular C1q binding protein, designated gC1q-R was previously shown to bind a number of plasma proteins involved in the coagulation and kinin systems. This study demonstrates the interaction between recombinant gC1q-R and fibrinogen. Using enzyme-linked immunosorbent assays, biotinylated gC1q-R was found to bind to microplate-immobilized fibrinogen in a manner which was specific and inhibited by excess soluble fibrinogen or polyclonal antibodies directed against either gC1q-R or fibrinogen. Moreover, gC1q-R inhibited fibrin polymerization in a dose-dependent manner. Reptilase induced fibrin clot formation was completely inhibited by gC1q-R at a 2:1 molar ratio (gC1q-R:fibrinogen), and repolymerization of thrombin induced fibrin monomers was similarly abrogated. At equivalent molar concentrations, gC1q-R appeared to be a more potent inhibitor of fibrin polymerization than fibrinogen, a well-known inhibitor. Moreover, in the presence of both gC1q-R and soluble fibrinogen, the effect of each inhibitor on fibrin polymerization was additive. When plasmin derived fibrinogen degradation products, including the C-terminal D domain (D-100) or the N-terminal E domain, were immobilized on microtiter plates, gC1q-R bound to fibrinogen fragment D-100, but not to fragment E. Further digestion of fibrinogen fragment D-100 by plasmin to fragment D-60 resulted in loss of gC1q-R binding. Thus, gC1q-R binds to the D domain of fibrinogen/fibrin, and the carboxyterminal segment of at least the fibrinogen/fibrin gamma chain appears important for this interaction. These observations may suggest a potential role for gC1q-R in modulating fibrin formation particularly at local sites of immune injury or inflammation.