C1q domain

Abstract: In humoral defence, pathogens are cleared by antibodies acting as adaptor molecules: they bind to antigen and trigger clearance mechanisms such as phagocytosis, antibody-dependent cell-mediated cytotoxicity and complement lysis. The first step in the complement cascade is the binding of C1q to the antibody. There are six heads on C1q, connected by collagen-like stems to a central stalk, and the isolated heads bind to the Fc portion of antibody rather weakly, with an affinity of 100 microM (ref. 3). Binding of antibody to multiple epitopes on an antigenic surface, aggregates the antibody and this facilitates the binding of several C1q heads, leading to an enhanced affinity of about 10 nM (ref. 1). Within the Fc portion of the antibody, C1q binds to the CH2 domain. The interaction is sensitive to ionic strength, and appears to be highly conserved throughout evolution as C1q reacts with IgG from different species (for example see ref. 8). By systematically altering surface residues in the mouse IgG2b isotype, we have localized the binding site for C1q to three side chains, Glu 318, Lys 320 and Lys 322. These residues are relatively conserved in other antibody isotypes, and a peptide mimic of this sequence is able to inhibit complement lysis. We propose that this sequence motif forms a common core in the interactions of IgG and C1q.

Abstract: Adiponectin is a major insulin-sensitizing, multimeric hormone derived from adipose tissue that acts on muscle and liver to regulate whole-body glucose and lipid metabolism. Here, we describe a novel and highly conserved paralog of adiponectin designated as C1q/TNF-related protein (CTRP) 9. Of all the CTRP paralogs, CTRP9 shows the highest degree of amino acid identity to adiponectin in its globular C1q domain. CTRP9 is expressed predominantly in adipose tissue and females expresses higher levels of the transcript than males. Moreover, its expression levels in ob/ob mice changed in an age-dependent manner, with significant up-regulation in younger mice. CTRP9 is a secreted glycoprotein with multiple post-translational modifications in its collagen domain that include hydroxylated prolines and hydroxylated and glycosylated lysines. It is secreted as multimers (predominantly trimers) from transfected cells and circulates in the mouse serum with levels varying according to sex and metabolic state of mice. Furthermore, CTRP9 and adiponectin can be secreted as heterooligomers when cotransfected into mammalian cells, and in vivo, adiponectin/CTRP9 complexes can be reciprocally coimmunoprecipitated from the serum of adiponectin and CTRP9 transgenic mice. Biochemical analysis demonstrates that adiponectin and CTRP9 associate via their globular C1q domain, and this interaction does not require their conserved N-terminal cysteines or their collagen domains. Furthermore, we show that adiponectin and CTRP9 form heterotrimers. In cultured myotubes, CTRP9 specifically activates AMPK, Akt, and p44/42 MAPK signaling pathways. Adenovirus-mediated overexpression of CTRP9 in obese (ob/ob) mice significantly lowered serum glucose levels. Collectively, these results suggest that CTRP9 is a novel adipokine, and further study of CTRP9 will yield novel mechanistic insights into its physiological and metabolic function.

Abstract: A partial cDNA clone for the A-chain of human complement subcomponent C1q was isolated from a monocyte library. Use of the A-chain cDNA clone, and a previously characterized B-chain cDNA clone [Reid (1985) Biochem. J. 231, 729-735] allowed the isolation of overlapping cosmid clones that were shown to contain the genes encoding the A-, B- and C-chains of human C1q. The three genes were found to be aligned, 5'----3', in the same orientation, in the order A-C-B on a 24 kb stretch of DNA on chromosome 1p. The A-, B- and C-chain genes are approx. 2.5, 2.6 and 3.2 kb long respectively, and each contains one intron, located within a codon for a glycine residue found half-way along the collagen-like region present in each chain. These glycine residues are located just before the point where the triple-helical portions of the C1q molecule appear to bend when viewed in the electron microscope. Southern-blot analyses indicated that there is only one gene per chain, and preliminary examination of genomic DNA from several C1q-deficient patients showed no evidence for major deletions or insertions within the A-, B- or C-chain genes. The DNA sequence of the coding region of the C-chain gene allows the completion of the entire derived amino acid sequence for the human C1q molecule. The globular, C-terminal, regions of the chains of C1q show a strong similarity in amino acid sequence to the non-collagen-like, C-terminal, regions of the type VIII and type X collagens, indicating structural and evolutionary relationships between these three molecules.