The Crystal Structure of the Globular Head of Complement Protein C1q Provides a Basis for Its Versatile Recognition Properties

Abstract

C1q is a versatile recognition protein that binds to an amazing variety of immune and non-immune ligands and triggers activation of the classical pathway of complement. The crystal structure of the C1q globular domain responsible for its recognition properties has now been solved and refined to 1.9 Å of resolution. The structure reveals a compact, almost spherical heterotrimeric assembly held together mainly by non-polar interactions, with a Ca 2؉ ion bound at the top. The heterotrimeric assembly of the C1q globular domain appears to be a key factor of the versatile recognition properties of this protein. Plausible three-dimensional models of the C1q globular domain in complex with two of its physiological ligands, C-reactive protein and IgG, are proposed, highlighting two of the possible recognition modes of C1q. The C1q/human IgG1 model suggests a critical role for the hinge region of IgG and for the relative orientation of its Fab domain in C1q binding.Innate immunity involves a combination of cell-surface receptors and soluble proteins with the ability to recognize microbial pathogens and thereby to generate signals that both orientate subsequent adaptive immune responses and trigger effector mechanisms (1, 2). Most of these molecules are oligomeric and recognize molecular patterns on microorganisms (3). An archetypal molecule of this type is C1q, the recognition subunit of C1, the complex that triggers activation of the classical pathway of complement, a major element of innate immunity. C1q is a 460-kDa protein with the overall shape of a bouquet of flowers, comprising six heterotrimeric collagen-like triple helices that associate in their N-terminal half to form a "stalk," then diverge to form individual "stems", each terminating in a C-terminal heterotrimeric globular domain (4). It is well documented that most of the C1 complex ligands are recognized by these peripheral globular domains, or heads, of C1q, thus triggering activation of C1r and C1s, the proteases associated with C1q (5). It is also established that C1q binds to immune complexes containing IgG or IgM, but not to those having IgA, IgD, or IgE (6). The major C1q binding site on IgG has been mapped to the CH2 domain of the Fc portion of the molecule (7-9). Although C1q shows marked differences in its reactivity toward IgG subclasses, the reason for this selectivity is not known.C1q is traditionally known for its ability to bind antibodies. However, it recognizes an amazing variety of other ligands. These include certain bacteria, viruses, parasites, and mycoplasma (6, 10 -12), underscoring its role as an antibody-independent defense protein. C1q also binds to C-reactive protein (CRP) 1 when complexed with exposed phosphocholine residues on bacteria, providing a further means of host defense (13). C1q is also capable of recognizing aberrant structures from self. Thus, in addition to cellular debris and sub-cellular membranes (14), it is established that C1q binds to, and induces clearance of, apoptotic cells (15), thereby playing a major role ...

Trimeric reassembly of the globular domain of human C1q

“…2a). The native gC1q far-UV CD was typical of a fully β-protein consistent with the X-ray structure of the protein [18,24]. But it had the rare peculiarity of having a positive peak around 235 nm, suggesting the contributions of the side chains of aromatic residues, Trp or Tyr, to the far-UV CD [25].…”

“…To address this question, the gC1q domain generated by collagenase digestion of purified C1q, which is known to remain a heterotrimer after the treatment, was used as a starting material [18]. N-terminal sequence analysis of the purified material indicated that it comprises residues Gly79-Ala223 of the A chain, Gly81-Ala226 of the B chain, and Gly78-Asp217 of the C chain.…”

“…Interestingly, the Trp-scans showed that the maximum emission wavelength of the Trp blueshifted upon acidification (pH 5.0 and pH 1.7), whereas one might have expected a red shift due to exposure of the Trp to the water in the acidified gC1q states. According to the gC1q Xray structure, none of the three Trps are buried in the core of the protein, which might explain the partially-exposed maximum emission wavelength of the native gC1q [18]. Trp 194 and Trp 190 have their benzene ring partially exposed to water whereas that of the Trp-147 is almost fully exposed.…”

“…These residues would have to be accessible within the trimer to be responsible for the loss of the native tertiary structure of the gC1q trimer and they would have to be located also nearby interfaces to be responsible for its dissociation. Using the coordinates from the X-ray structure of the gC1q trimer, we looked for residues or domains within the gC1q molecules that could fulfill those requirements [18]. Because there are several accessible aspartic and glutamic acids within the trimer, it was not possible to isolate particular residues or domain of the protein involved in the loss of the native tertiary structure of the gC1q trimer.…”

The non-inflammatory role of C1q during Her2/neu-driven mammary carcinogenesis

“…6 When C1q binds to IgM or clustered IgG on immunocomplexes, or to surface-bound pentraxins, other complement cascade components are sequentially activated leading to the generation of effector molecules which are able to limit infection by pathogens and play an essential homeostatic role in the clearance of damaged self-antigens, immune complexes, altered self, and apoptotic cells. 7 Besides this role in the activation of the classical complement pathway through the recognition of immune complexes, 8 it is now recognized that C1q can carry out functions that are unrelated to complement activation. Recent studies have demonstrated its involvement in several physiological processes via binding with its specific receptors.…”