Cloning and characterization of the complementary DNA for the B chain of normal human serum C1q

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Abstract
Early biosynthetic studies implicated the intestine as one of the major sites of synthesis of C1q in guinea-pigs, humans and pigs (Coltenet al. 1966; Coltenet al. 1968; Dayet al. 1970). Columnar epithelial cells were considered to be the principal cell type involved in the small intestine and this view is supported by the report by Morriset al. (1978) that human fetal intestine columnar epithelial cells can synthesize, and secrete, up to 3700 times more haemolytically active C1 than several other cell types. However, it is well established that a variety of other cell types such as human fibroblasts (Al-Adnani & McGee 1976; Reid & Solomon 1977) human macrophages (Bensaet al. 1983) and mouse or guinea-pig macrophages (see Loos (1983) for a review) synthesize material that is antigentically and functionally similar in some respects to serum C1q. In view of the wide variety of cell types which possess the potential to synthesize C1 functional activity, or Cq immunoreactive material, it was decided to isolate the RNA from both human intestine and human liver for this study. The cell-free translation experiments showed that both the intestine poly A+total RNA and the liver poly A+18S-II RNA fraction produced a main polypeptide chain of 46-48 kDa, and in addition the intestine poly A+total RNA produced two chains of 22—25 kDa. The 46—48 kDa chain is of a similar size to the chains of a C1q-like molecule synthesized and secreted by human fibroblasts (Reid & Solomon 1977; Skoket al. 1981) and could possibly represent a pro-form of the serum C1q chains. The presence of minor chains of 22-25 kDa in the immunoprecipitate from the cell free translation of the intestine poly A+total RNA indicates the possibility that the chains of normal serum C1q (of expected molecular mass 24-28 kDa, Reid 1983) are synthesized with only very short N-terminal extensions but another explanation may be that there has been limited proteolysis of the 46—48 kDa chain during the immunoprecipitation period at 37 °C. Also, the finding that immunoprecipitable C1q-like material was located in the 18S-II fraction of human liver is consistent with the chains of C1q having a larger mRNA than would be expected for polypeptides in the 24—28 kDa range.