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Characterization of the Mr Difference between Secreted Murine Fourth Component of Complement and the Major Plasma Form: Evidence for Carboxyl-Terminal Cleavage of the α Chain

David R. Karp, Donald C. Shreffler and John P. Atkinson
Proceedings of the National Academy of Sciences of the United States of America
Vol. 79, No. 21, [Part 1: Biological Sciences] (Nov. 1, 1982), pp. 6666-6670
Stable URL: http://www.jstor.org/stable/13288
Page Count: 5
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Characterization of the Mr Difference between Secreted Murine Fourth Component of Complement and the Major Plasma Form: Evidence for Carboxyl-Terminal Cleavage of the α  Chain
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Abstract

The α -chain of murine fourth component of complement (C4) secreted by cells in vitro and in vivo has a Mr that is larger by ≈ 4,000 than that of the α -chain of the principal form of C4 in plasma. By using in vivo labeling of C4 with [35S]methionine, C4 was shown to be first synthesized with the higher Mr (``secreted'') α -chain, which was then quickly processed (t1/2≈ 1 hr) extracellularly to the mature (``plasma'') C4 possessing the lower Mrα -chain. Both forms of C4 were functional as assayed by the ability of their α -chains to be cleaved by the protease C1̄, to bind methylamine, and to undergo denaturation-dependent autolysis. When secreted C4 and plasma C4 were activated to C4b, the Mr difference of 4,000 was maintained in the α ′-chains. The Mr difference was localized to the carboxyl-terminal autolytic fragment of the α -chain and was unaffected by the removal of carbohydrate. C4 from resident peritoneal macrophage cultures could be converted to the plasma form by incubation with heparin/plasma. This conversion could be blocked by EDTA or 1,10-phenanthroline. These data suggest that an enzyme, presumably a neutral proteinase present in mouse plasma, cleaves the carboxyl terminus of newly synthesized C4 α -chains, thereby creating the major form of C4 in plasma.

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