The detection and characterization of a membrane protein with Factor B-like activity on human lymphoid cells.

Factor B-like activity associated with human peripheral blood lymphocytes was first described by Halbwachs & Lachmann (1976), who employed a functional assay. In this present study, clones of B cells from patients with chronic lymphatic leukaemia (CLL) and lymphoblastoid 'Raji' cells were used. The surface of these cells was labelled with 125I by the lactoperoxidase technique and the cell membrane proteins solubilized with the detergent NP40. A single polypeptide chain of molecular weight 103K was precipitated with F(ab')2 anti-serum Factor B, and not with a control antibody. This 103K protein was also found if Raji cells were biosynthetically labelled with [14C]-leucine. When the radiolabelled cells were incubated with cobra venom factor (CVF) and Factor D, two specific polypeptides were precipitated by F(ab')w anti-Factor B. Moreover, the larger fragment forms a complex with CVF, since it could be precipitated by anti-CVF. These events are similar to those involving serum Factor B and CVF.