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 α- and γ-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 α-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 α- and γ-chains. N-terminal sequences for this fragment and C4α are identical, as are those for the 137,000-dalton slp fragment and Slp α. Although the α-γ fragment, like C4α, undergoes denaturation-dependent autolytic cleavage, unlike C4α, it cannot be cleaved by C1̄. This indicates 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 α-γ fragments represent an intermediate step in the processing of the C4 and Slp precursor polypeptides.