Structural and functional characterization of an incompletely processed form of murine C4 and Slp.
The properties of a 128,000-dalton polypeptide, corresponding to the uncleaved alpha- and gamma- chains of the fourth component of murine complement (C4), were studied. A fragment of similar size (137,000 daltons) from the structurally related sex-limited protein (Slp) was also found. These polypeptides have methylamine and iodoacetamide binding sites. suggesting that they, like the alpha-chains of C4 and Slp, possess an internal thiolester. In tryptic peptide map analysis, extensive homology is seen between the 128,000-dalton fragment and native C4 alpha- and gamma-chains. N-terminal sequences for this fragment and C4 alpha are identical, as are those for the 137,000-dalton Slp fragment and Slp alpha. Although the alpha-gamma fragment, like C4 alpha, undergoes denaturation-dependent autolytic cleavage, unlike C4 alpha, it cannot be cleaved by C1  This indicate that some of the properties of C4 do not require the native three-chain structure, whereas others do require it. These findings suggest that the alpha-gamma fragments represent an intermediate step in the processing of the C4 and Slp precursor polypeptides.
D000818:Animals; D000941:Antigens; D003167:Complement Activation; D003172:Complement C1 / Q000378:metabolism; D003181:Complement C4 / Q000276:immunology; D008744:Methylamines / Q000494:pharmacology; D051379:Mice; D008809:Mice, Inbred C3H; D008970:Molecular Weight; D010455:Peptides / Q000276:immunology; D011817:Rabbits; D014357:Trypsin / Q000494:pharmacology
D000941:Antigens; D003172:Complement C1; D003181:Complement C4; D008744:Methylamines; D010455:Peptides; D014357:Trypsin