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The important role of furin in the proteolytic activation of many pathogenic molecules has made this endoprotease a target for the development of potent and selective antiproteolytic agents. Here, we demonstrate the utility of the protein-based inhibitor ␣ 1 -antitrypsin Portland (␣ 1 -PDX) as an antipathogenic agent that can be used prophylactically to block furin-dependent cell killing by Pseudomonas exotoxin A. Biochemical analysis of the specificity of a bacterially expressed Hisand FLAG-tagged ␣ 1 -PDX (␣ 1 -PDX͞hf) revealed the selectivity of the ␣ 1 -PDX͞hf reactive site loop for furin (K i , 600 pM) but not for other proprotein convertase family members or other unrelated endoproteases. Kinetic studies show that ␣ 1 -PDX͞hf inhibits furin by a slow tight-binding mechanism characteristic of serpin molecules and functions as a suicide substrate inhibitor. Once bound to furin's active site, ␣ 1 -PDX͞hf partitions with equal probability to undergo proteolysis by furin at the Cterminal side of the reactive center -Arg 355 -Ile-Pro-Arg 358 -2 or to form a kinetically trapped SDS-stable complex with the enzyme. This partitioning between the complex-forming and proteolytic pathways contributes to the ability of ␣ 1 -PDX͞hf to differentially inhibit members of the proprotein convertase family. Finally, we propose a structural model of the ␣ 1 -PDX-reactive site loop that explains the high degree of enzyme selectivity of this serpin and which can be used to generate small molecule furin inhibitors.

Activation of furin requires autoproteolytic cleavage of its 83‐amino acid propeptide at the consensus furin site, Arg‐Thr‐Lys‐Arg107↓. This RER‐localized cleavage is necessary, but not sufficient, for enzyme activation. Rather, full activation of furin requires exposure to, and correct routing within, the TGN/endosomal system. Here, we identify the steps in addition to the initial propeptide cleavage necessary for activation of furin. Exposure of membrane preparations containing an inactive RER‐localized soluble furin construct to either: (i) an acidic and calcium‐containing environment characteristic of the TGN; or (ii) mild trypsinization at neutral pH, resulted in the activation of the endoprotease. Taken together, these results suggest that the pH drop facilitates the removal of a furin inhibitor. Consistent with these findings, following cleavage in the RER, the furin propeptide remains associated with the enzyme and functions as a potent inhibitor of the endoprotease. Co‐immunoprecipitation studies coupled with analysis by mass spectrometry show that release of the propeptide at acidic pH, and hence activation of furin, requires a second cleavage within the autoinhibitory domain at a site containing a P6 arginine (‐Arg70‐Gly‐Val‐Thr‐Lys‐Arg75↓‐). The significance of this cleavage in regulating the compartment‐specific activation of furin, and the relationship of the furin activation pathway to those of other serine endoproteases are discussed.

Following correct folding/assembly, many eukaryotic proteins undergo single or multiple endoproteolytic cleavages during transport through the secretory pathway, resulting in the release of smaller, bioactive products. The proprotein convertases (PCs) 1 are a family of calcium-dependent serine endoproteases that catalyze these cleavages at sites containing doublets or clusters of basic amino acids. The PC family is evolutionarily related to bacterial subtilisin and includes seven members expressed in secretory compartments of mammalian cells (see Refs. 1-4 for reviews). Furin, the most intensively studied member of the PC family, is a type I membrane protein localized primarily to the TGN (5-8). Furin is not statically retained in this compartment, but rather it traffics between two local cycling loops, one at the TGN and the other at the cell surface (9, 10). The dynamic trafficking of furin enables it to cleave and activate numerous cellular and pathogen proproteins in both the biosynthetic and endocytic pathways (reviewed in Refs. 1 and 4). Endoproteolysis of these substrates occurs primarily at the consensus furin cleavage site, -Arg-X-Lys/Arg-Arg2-, containing a P1 and P4 Arg. However, in some cases at acidic pH, furin cleaves substrates at the motif -Arg-X-X-X-Lys/Arg-Arg2-, in which a P6 Arg is present in place of the P4 Arg (11, 12).Before it can act on proprotein substrates, furin itself must go through a complex process of activation. Furin is translated as an inactive zymogen with an 83-amino acid N-terminal propeptide. Attempts to eliminate or substitute the native furin proregion produced inactive enzyme, suggesting that the furin propeptide may play a critical role in folding and activation (13)(14)(15). This is consistent with research that shows that the folding of many evolutionarily unrelated classes of protease (e.g. serine-, aspartyl-, cysteinyl-and metalloproteases) is mediated by (typically N-terminal) propeptides that act as "intramolecular chaperones" (IMCs). IMC-mediated folding has been most thoroughly investigated in the secreted bacterial serine endoproteases ␣-lytic protease and subtilisin. These IMCs apparently increase the folding rate of their cognate protease domains by lowering a specific kinetic barrier very late in the folding pathway (reviewed in . IMC propeptides are autoproteolytically excised when their cognate enzymes have only partially folded. In subtilisin, propeptide excision initiates significant conformational changes that result in the loss of hydrophobic surface exposure to solvent (21-23). Although no longer covalently attached, these postexcision conformational changes are mediated by the IMC (22). In the absence of the IMC propeptide, the enzyme folds into an inactive and kinetically stable "molten globule"-like intermediate (16,25). The addition of the propeptide in trans causes rapid conversion of this folding intermediate to the native state (16,25,27,28).Following excision, IMCs remain noncovalently bound to

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Pro bone morphogenetic protein-4 (BMP-4) is initially cleaved at a consensus furin motif adjacent to the mature ligand domain (the S1 site), and this allows for subsequent cleavage at an upstream motif (the S2 site). Previous studies have shown that S2 cleavage regulates the activity and signaling range of mature BMP-4, but the mechanism by which this occurs is unknown. Here, we show that the pro-and mature domains of BMP-4 remain noncovalently associated after S1 cleavage, generating a complex that is targeted for rapid degradation. Degradation requires lysosomal and proteosomal function and is enhanced by interaction with heparin sulfate proteoglycans. Subsequent cleavage at the S2 site liberates mature BMP-4 from the prodomain, thereby stabilizing the protein. We also show that cleavage at the S2, but not the S1 site, is enhanced at reduced pH, consistent with the possibility that the two cleavages occur in distinct subcellular compartments. Based on these results, we propose a model for how cleavage at the upstream site regulates the activity and signaling range of mature BMP-4 after it has been released from the prodomain. INTRODUCTIONBone morphogenetic protein-4 (BMP-4) is a signaling molecule that acts as a morphogen to influence cell fate in a concentration-dependent manner. BMP-4 was originally identified as a protein that is capable of inducing ectopic bone formation, but more recent studies have shown that it plays many different roles during embryonic development and in adults (Hogan, 1996).BMP-4 function is essential for normal embryogenesis as illustrated by the fact that mice homozygous for a null allele of BMP-4 form little or no mesoderm and die near the time of gastrulation (Winnier et al., 1995). BMP-4 heterozygous mutant mice are viable but display a variety of birth defects, including reduced numbers of primordial germ cells, polydactyly, and kidney, eye, and craniofacial abnormalities (Dunn et al., 1997;Lawson et al., 1999;Miyazaki et al., 2000;Chang et al., 2001). These data indicate that control of BMP-4 gene dosage is essential for normal embryonic patterning.Excess BMP-4 activity also leads to birth defects. Mice mutant for the BMP antagonists gremlin, noggin, and/or chordin show early lethality and/or defects in the spinal cord, forebrain, somites, skeleton, and kidney (Brunet et al., 1998;McMahon et al., 1998;Gong et al., 1999;Khokha et al., 2003). In humans, mutations in the noggin gene are responsible for multiple synostoses syndrome, a genetic disease characterized by fusion of the joints (Gong et al., 1999), and abnormally high levels of BMP-4 protein are a key feature of fibrodysplasia ossificans progressiva, a crippling hereditary disorder in which ectopic bone forms throughout the body (Kaplan and Shore, 1998).The requirement for strict regulation of BMP-4 dosage is met by controlling BMP-4 activity at multiple levels. At the extracellular level, BMP-4 is regulated by binding proteins, such as chordin and noggin, that block activation of cell surface receptors, and by the prote...

Aerolysin is secreted as an inactive dimeric precursor by the bacterium Aeromonas hydrophila. Proteolytic cleavage within a mobile loop near the C terminus of the protoxin is required for oligomerization and channel formation. This loop contains the sequence KVRRAR 432 , which should be recognized by mammalian proprotein convertases such as furin, PACE4, and PC5/6A. Here we show that these three proteases cleave proaerolysin after Arg-432 in vitro, yielding active toxin. We also investigated the potential role of these enzymes in the in vivo activation of the protoxin. We found that Chinese hamster ovary cells were able to convert the protoxin to aerolysin in the absence of exogenous proteases and that activation did not require internalization of the toxin. The furin inhibitor ␣ 1 -antitrypsin Portland reduced the rate of proaerolysin activation in vivo, and proaerolysin processing was even further reduced in furin-deficient FD11 Chinese hamster ovary cells. The cells were also less sensitive to proaerolysin than wild type cells; however, transient transfection of FD11 cells with the cDNA encoding furin conferred normal sensitivity to the protoxin. Together these findings argue that furin catalyzes the cell-surface activation of proaerolysin in vivo.Many toxins are secreted by pathogenic organisms as inactive precursors, presumably to protect the producing cells from self-destruction or to increase the efficiency of delivery to the target cells. Activation of toxin precursors often involves proteolytic processing by enzymes produced either by the pathogen itself or by the host organism. The identification of these proteases may be crucial to our understanding of the pathogenesis of the organism.Aerolysin is a virulence factor secreted by the human pathogen Aeromonas hydrophila (Refs. 1-3, for review see Refs. 4 and 5). The protein is released as a soluble dimeric precursor (6, 7) that can bind to specific receptors on target cells (8 -12). Proaerolysin must be activated by proteolytic cleavage (13), which releases a C-terminal peptide (14) and leads to a change in secondary structure (15). This enables the next step in channel formation, which is the generation of a heptameric oligomer (16,17). Being amphipathic (18), the heptamer can insert into the membrane thereby producing well defined channels (19). In the case of erythrocytes, channel formation leads to cell lysis; however, depending upon the toxin concentration, nucleated cells may undergo a number of changes before death occurs. These include loss of small molecules and ions through the aerolysin channels, vacuolation of the endoplasmic reticulum (12), or even apoptosis. 1 We have shown that activation of proaerolysin with trypsin is due to cleavage at the carboxyl side of Lys-427 2 (20), which is located in an 18-amino acid surface-exposed flexible loop (21). This loop also contains the sequence K 427 VRRAR 432 which corresponds to one of the motifs recognized by furin-like endoproteases, also called proprotein convertases (PC), suggesting that proaero...

Most monoclonal antibodies (mAbs) generated from humans infected or vaccinated with the 2009 pandemic H1N1 (pdmH1N1) influenza virus targeted the hemagglutinin (HA) stem. These anti-HA stem mAbs mostly used IGHV1-69 and bound readily to epitopes on the conventional seasonal influenza and pdmH1N1 vaccines. The anti-HA stem mAbs neutralized pdmH1N1, seasonal influenza H1N1 and avian H5N1 influenza viruses by inhibiting HA-mediated fusion of membranes and protected against and treated heterologous lethal infections in mice with H5N1 influenza virus. This demonstrated that therapeutic mAbs could be generated a few months after the new virus emerged. Human immunization with the pdmH1N1 vaccine induced circulating antibodies that when passively transferred, protected mice from lethal, heterologous H5N1 influenza infections. We observed that the dominant heterosubtypic antibody response against the HA stem correlated with the relative absence of memory B cells against the HA head of pdmH1N1, thus enabling the rare heterosubtypic memory B cells induced by seasonal influenza and specific for conserved sites on the HA stem to compete for T-cell help. These results support the notion that broadly protective antibodies against influenza would be induced by successive vaccination with conventional influenza vaccines based on subtypes of HA in viruses not circulating in humans.