Army Medical Research and Material Control (cooperative agreement DAMD17-93-V-3004). REFERENCES 1. that CD81 conformation is usually important for E2 recognition. Regions of E2 Lacidipine involved in the CD81 conversation were analyzed, and our data suggest that the binding site is usually of a conformational nature including aa 480 to 493 and 544 to 551 within the E2 glycoprotein. Finally, we demonstrate that ligation of CD81 by E2661 induced aggregation of lymphoid cells and inhibited B-cell proliferation, demonstrating that E2 conversation with CD81 can modulate cell function. Hepatitis C computer virus (HCV), Lacidipine the major cause of non-A, non-B viral hepatitis, is an enveloped computer virus classified in the family, which also includes the flaviviruses (e.g., tick-borne encephalitis computer virus and dengue computer virus) and pestiviruses (e.g., bovine viral diarrhea computer virus and classical swine fever computer virus) (24). HCV replicates in the liver and induces a chronic contamination, leading to cirrhosis and end-stage liver disease in approximately 20 to 30% of infected individuals. At present no prophylactic steps against HCV contamination are available, and current therapies are unsatisfactory. Early events in cell entry, replication and morphogenesis are not well comprehended (2, 10, 11, 14, 19, 25). One significant impediment to progress in understanding HCV pathogenesis is the absence of suitable small-animal and cell culture models. The structural proteins of HCV Nkx1-2 are believed to comprise the core protein and two Lacidipine predicted envelope glycoproteins (gps), E1 and E2. The majority of E1 and E2 gps expressed in vitro exist as high-molecular-weight disulfide-bridged aggregates (3C5, 7, 23). Hence, in order to study the biological activity of the HCV gps, it is critical to distinguish between gps undergoing productive folding and those following nonproductive pathway(s) resulting in misfolding and aggregation. The E2 gp extends from amino acids (aa) 384 to 746 (position within the polyprotein), and deletions removing the hydrophobic C-terminal region result in secretion of the ectodomain (17, 18, 29, 32). Comparison of E2661 with proteins truncated at position 688, 704, or 715 demonstrates that these deletions result in both reduced secretion and acknowledgement by conformation-dependent monoclonal antibodies (MAbs). Secreted forms of E2661 gp were shown to contain minimal amounts of disulfide-bridged high-molecular-weight aggregates compared to the intracellular form(s) of the antigen (17a, 18). Antigenic characterization of secreted E2661 suggests that it folds in a way comparable to that observed in E1-E2 complexes and therefore Lacidipine makes an ideal soluble mimic of a Lacidipine viral ligand to study cellular receptor interactions. Rosa and colleagues reported that a soluble truncated form of the HCV E2 gp bound to the surface of the T-cell collection Molt-4 and that this conversation could be inhibited both by a MAb specific for the hypervariable region (HVR) and by HCV-infected chimpanzee sera (26). Recently, Pileri and colleagues (22) demonstrated that this cell surface-expressed molecule CD81 could interact with E2, suggesting that it may be the cellular receptor for HCV. CD81 is usually a member of the tetraspanin, or transmembrane 4, family, which traverses the membrane four occasions and has two extracellular (EC) loops of 28 and 80 aa, designated EC1 and EC2, respectively. Engagement of CD81 is usually reported to activate a variety of biologic responses including cell adhesion, morphology, proliferation, activation, and differentiation of T, B, and other cell types (16). In this report, we define a number of potential contact sites between HCV E2 and the EC2 region of CD81, demonstrating that one or more of four amino acids within EC2 of CD81 are critical for this conversation. Various recombinant forms of the CD81 EC2 loop show differences in the ability to bind E2, suggesting that conformation of CD81 is usually important for E2 recognition. Regions of.