Limited proteolysis of complement components C2 and factor B. Structural analogy and limited sequence homology

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Abstract
A method was described for simultaneous purification of milligram quantities of [human] complement components C2 and factor B. Both products were homogeneous by the criteria of polyacrylamide-gel electrophoresis and N terminal sequence analysis. C2 was cleaved by serine proteinase C.hivin.1s [activated Cls] at an X-Lys bond to give fragment C2a (approximate MW 74,000) and fragment C2b (approximate MW 34,000). The 2 fragments could be separated by gel filtration without the need for reducing or denaturing agents. Fragment C2b represented the N-terminal end of the molecule. Similar results were seen on cleavage of factor B by factor D in the presence of C3. Again 2 non-covalently linked fragments were formed. The smaller, fragment Ba (approximate MW 36,000), had threonine as the N-terminal residue, as did factor B; the larger, fragment Bb (approximate MW 58,000), had lysine as the N-terminal residue. A similar cleavage pattern was obtained on limited proteolysis of factor B by trypsin, suggesting an Arg-Lys or Lys-Lys bond at the point of cleavage. Although C2 and factor B showed no apparent N-terminal sequence homology, a limited degree of sequence homology was seen around sites of proteolytic cleavage.