Fibronectin immobilized to plastic tubes binds soluble Clq with a Kd of 82 ± 2.6 nM. The binding of fibronectin to Clq is relatively insensitive to pH but is sensitive to ionic conditions. Clq covalently bound to Sepharose selectively binds cellular fibronectin produced by a hamster fibroblast cell line. The globular head regions ofClq have no effect on the binding of Clq to fibronectin but the collagenous tails of Clq interfere competitively with a K; of 59 nM. We conclude that fibronectin binds Clq via its collagen-like tail region and thus the process resembles the binding offibronectin to gelatin. This is further emphasized by our observation that gelatin binds to fibronectin immobilized on plastic tubes with a Kd of 131 nM. Because fibronectin stimulates endocytosis in several systems and promotes the clearance of particulate material from the circulation, these results suggest the possibility that fibronectin could function in the clearance of Clqcoated material such as immune complexes or cellular debris.Fibronectins are large glycoproteins that are found at cell surfaces, in extracellular matrices, and in blood plasma (reviewed in refs.
BerneThis work was supported by grants from the "Schweizeriscber Nationalfonds zur Forderung wissenschaftlicher Forschung", the "Kommission zur Forderung der EiweiBforschung an der Universitat Bern", and the "Sandoz-Stiftung zur Forderung der medizinisch-biologischen Forschung".
Five differently isolated and purified human C1q preparations were examined by electron microscopy and analyzed by polyacrylamide gel electrophoresis in 0.1% sodium dodecyl sulfate and 0.5 M urea. The amino acid and carbohydrate composition of C1q purified by the DNA method are reported and compared with results obtained on C1q isolated by other procedures. Electron microscopy showed that all C1q preparations had six peripheral subunits connected by fibrillar strands to a central subunit. The presence of small amounts of dimers was also observed. The physico-chemical properties of the molecule are independent of the purification method used. The five C1q preparations labeled with 125I in presence of lactoperoxidase formed two types of noncovalently linked subunits. In each case the smaller (central) subunit contained over thirty times as much radioactivity as the larger (peripheral) subunit supposed to interact with immune complexes. Reduction and alkylation confirmed for each preparation the presence of three polypeptide chains, the smaller of which contained essentially all radioactivity.
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