Biosynthesis of membrane factor B by mouse peritoneal macrophages

The biosynthesis of many of the complement proteins by cells of the monocyte-macrophage series has been established. Studies on these cells using radiolabelled amino acids demonstrated synthesis of precursor proteins (pro-complement), and the native complement protein similar in size to that found in the plasma. However, synthesis of membrane complement proteins has not been demonstrated, although it has been suggested by previous studies using indirect techniques. In particular, there is evidence for the membrane-associated factor B in human lymphocytes. We report here that resident and thioglycollate-stimulated mouse peritoneal macrophages synthesized, in short-term primary cultures membrane factor B of molecular weight (MW) 95,000 and secreted factor B (MW 90,000) as single chain polypeptides. We also found a large single chain polypeptide with an approximate molecular weight of 195,000, which may be the putative factor B precursor.