Mouse complement components C3 and C4. Characterization of their messenger RNA and molecular cloning of complementary DNA for C3.

Abstract

Mouse liver mRNA species which direct the cell-free synthesis of pro-C3 and pro44 polypeptides with an apparent molecular weight of 175,000 and 190,000, respectively, were shown to sediment faster than 28 s.By electrophoresis in a denaturing gel the mRNA for C3 was determined to contain approximately 7,500 ribonucleotides.cDNA was synthesized from size-fractionated mouse liver mRNA and cloned in the plasmid pBR 322.Among the cDNA clones recovered three were identified as being complementary to portions of the mRNA for C3.The third and fourth components of the complement system (C3 a n d C4) are serum glycoproteins, which in mammalian organisms participate in defense reactions against infections (1).C 3 is an essential element in both the classical and the alternative pathways of complement utilization.While partial C3 deficiencies in humans have been reported, complete C3 deficiencies are rarely observed, due to fatal childhood infections (2).In contrast, complete homozygous C4 deficiencies without associated serious disease have been detected in humans (3) and guinea pigs (4).Humans have two separate loci for two distinct hemolytically active C4 proteins (5,6).In mice only one gene is known for hemolytically active C4.Mice, however, possess a variant of the C4 protein (Slp or sexlimited protein), which is probably the product of a duplicated gene (7, 8) but which has no C4-hemolytical function (9).Human and mouse C4 genes are located inside the major histocompatibility complex (HLA, H-2), whereas the human C3 gene is not linked to the HLA complex (10) and the mouse C3 gene is located distant from the H-2 complex on the same chromosome ( 11).C3 is synthesized as an intracellular precursor (pro-C3) of an approximate molecular weight of 170,000 which is processed by proteolytic cleavage into two mature chains a, 120,000 and b, 70,000 (12, 13).Similarly, C4 is synthesized as a single chain precursor (pro-C4) with an approximate molecular weight of 200,000.It is cleaved into a disulfide-bonded triple chain with subunits a (98,000), /? (70,000), and y (32,000) with minor species variations (14-16).