The lysis of cells by complement requires only the terminal components C5, C6, C7, C8 and C9 and is initiated by the cleavage of C5 to C5b. Sequential addition of C6, C7, C8, and C9 to C5b leads to the formation of the membrane attack complex (MAC)' which, when inserted into the lipid bilayer, can form transmembrane pores (1-5). It is well known that when complement of one species is activated on homologous erythrocytes, lysis is much less efficient than when it is activated on other species of cell, and even among different heterologous cell species the lytic efficiency may be very different. It has long been known that the basis of this variable lytic efficiency is found, at least in part, at the C8 and/or C9 step (6-9). More recently, specific membrane proteins have been described that appear to protect cells from homologous complement . The first of these to be described was the decayaccelerating factor (DAF), a membrane protein of -70 kD molecular mass (10). This protein interferes with the assembly of the C3 converting enzymes both of the classical and alternative pathway (10, 11) and therefore it has only indirect effects on the cell lytic mechanism. A further membrane protein that does restrict homologous lysis, and that has been described both as the C8-binding protein (C8bp) (7, 12) and as homologous restriction factor (HRF) (13), has also been isolated . It seems likely that both these descriptions apply to a single protein of 65 kD molecular mass. In addition, a 55/65-kD MAC-inhibiting protein (MIP) with the capacity to bind C8 and C9 has been identified both on human erythrocyte membranes and in normal human serum (14). The relationship ofthis to HRF/C8bp is not yet clear. Both DAF and HRF/C8bp are bound on cell membranes by a glycolipid anchor (15, 16) and can be eluted from the cell membrane, at least in part, by phosphatidylinositol-specific phospholipase C. These proteins also have the capacity when they are isolated from

on Sephadex G-75. On Edman degradation S-UIHcarboxymethylcysteine was released at step 9 and yglutamyl4CJ methylamide was released at step 12. We interpret these data to indicate the presence of an internal thiolester bond in native C3. In addition, evidence is presented for an identical reactive site in a2-macroglobulin. The third component of human complement (C3), is composed of two polypeptide chains, a and 3, bridged by one or more interchain disulfide bonds(1). Cleavage of C3 by the classical pathway convertase (C4b2a), characteristic of the first step in C3 activation, results in the formation of two fragments, a vasoactive peptide (C3a) and a macromolecular fragment (C3b) having an a'f3 chain structure (1-3). C3b participates in both the classical and alternative pathways of complement activation as a subcomponent of the respective C5 cleaving enzymes C4b2a3b (4) and C3bnBb (5-7). A bimolecular complex of activated forms of CS and Factor B (C3bBb) further functions in the alternative pathway as a CS convertase (8).Structural analyses of GMa (9) and the polypeptide chains of CS and C(b (10) have indicated that the probable cleavage site is an Arg-Ser bond at positions 77 and 78 in the a chain. Subsequent modulation of C3b biological activities is affected by the concerted actions of 1,H and C3b inactivator (11) and, further, by additional enzymes acting at specific cleavage sites in the a' chain to produce the CSc, CMd, and CMe fragments (12)(13)(14).Interactions of C3 with cell membranes (15) and carbohydrate polymers (16)(17)(18) The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U. S. C. §1734 solely to indicate this fact.

Dysregulation of the alternative complement pathway (AP) predisposes individuals to a number of diseases including paroxysmal nocturnal hemoglobinuria, atypical hemolytic uremic syndrome, and C3 glomerulopathy. Moreover, glomerular Ig deposits can lead to complement-driven nephropathies. Here we describe the discovery of a highly potent, reversible, and selective small-molecule inhibitor of factor B, a serine protease that drives the central amplification loop of the AP. Oral administration of the inhibitor prevents KRN-induced arthritis in mice and is effective upon prophylactic and therapeutic dosing in an experimental model of membranous nephropathy in rats. In addition, inhibition of factor B prevents complement activation in sera from C3 glomerulopathy patients and the hemolysis of human PNH erythrocytes. These data demonstrate the potential therapeutic value of using a factor B inhibitor for systemic treatment of complement-mediated diseases and provide a basis for its clinical development.

The main features of the protein structure are two antiparallel beta-sheets (a central one with three strands and another with two), a short helix that packs against the three-stranded beta-sheet, and a carboxy-terminal region that, although lacking regular secondary structure, is well defined and packs against the three-stranded beta-sheet, on the opposite face to the helix. We have used the structure, in combination with existing biochemical data, to identify residues that may be involved in C8 binding.