Loss of ectodomain shedding due to mutations in the metalloprotease and cysteine-rich/disintegrin domains of the tumor necrosis factor-alpha converting enzyme (TACE)

Tumor necrosis factor-alpha converting enzyme (TACE), a multidomain protease essential for development and disease, releases the ectodomains from many transmembrane proteins in a regulated fashion. To understand the mechanism underlying the regulation of TACE activity, we sought to identify the cause of ectodomain shedding deficiencies in two mutated CHO sublines designated M1 and M2. Transfection of expression vectors for human and mouse TACE restored ectodomain shedding of TNF-alpha and TGF-alpha, suggesting that defects in the TACE gene contribute to the phenotype of M1 and M2 cells. The overall levels of endogenous TACE forms in M1 cells were significantly lower than those found in their parental cells, whereas only TACE zymogen, but not its mature form, was detectable in M2 cells. Molecular analyses suggested that M1 cells contained only one expressible TACE allele encoding an M435I point mutation in the catalytic center of the protease, and M2 cells produced two TACE variants with distinct point mutations in the catalytic domain (C225Y) and the cysteinerich/disintegrin domain (C600Y). Overexpression of the C225Y and C600Y TACE by transient transfection largely compensated for maturation defects in the variants but failed to restore TNF-alpha and TGF-alpha release in the shedding-defective CHO cell lines and fibroblasts derived from TACE-null mouse embryo. Further mutagenesis and functional analyses demonstrated that Cys(600) was absolutely essential for ectodomain shedding, suggesting that Cys(600), similar to Cys(225), participates in disulfide bonding, which is critical for both the processing and catalysis of TACE.     

Characterization of the ectodomain shedding of the beta-site amyloid precursor protein-cleaving enzyme 1 (BACE1)

Generation of the amyloid peptide through proteolytic processing of the amyloid precursor protein by beta- and gamma-secretases is central to the etiology of Alzheimer's disease. beta-secretase, known more widely as the beta-site amyloid precursor protein cleaving enzyme 1 (BACE1), has been identified as a transmembrane aspartic proteinase, and its ectodomain has been reported to be cleaved and secreted from cells in a soluble form. The extracellular domains of many diverse proteins are known to be cleaved and secreted from cells by a process known as ectodomain shedding. Here we confirm that the ectodomain of BACE1 is secreted from cells and that this processing is up-regulated by agents that activate protein kinase C. A metalloproteinase is involved in the cleavage of BACE1 as hydroxamic acid-based metalloproteinase inhibitors abolish the release of shed BACE1. Using potent and selective inhibitors, we demonstrate that ADAM10 is a strong candidate for the BACE1 sheddase. In addition, we show that the BACE1 sheddase is distinct from alpha-secretase and, importantly, that inhibition of BACE1 shedding does not influence amyloid precursor protein processing at the beta-site.     

Calmodulin interacts with angiotensin-converting enzyme-2 (ACE2) and inhibits shedding of its ectodomain

Angiotensin-converting enzyme-2 (ACE2) is a regulatory protein of the renin-angiotensin system (RAS) and a receptor for the causative agent of severe-acute respiratory syndrome (SARS), the SARS-coronavirus. We have previously shown that ACE2 can be shed from the cell surface in response to phorbol esters by a process involving TNF-α converting enzyme (TACE; ADAM17). In this study, we demonstrate that inhibitors of calmodulin also stimulate shedding of the ACE2 ectodomain, a process at least partially mediated by a metalloproteinase. We also show that calmodulin associates with ACE2 and that this interaction is decreased by calmodulin inhibitors.     

Homophilic interactions of the amyloid precursor protein (APP) ectodomain are regulated by the loop region and affect beta-secretase cleavage of APP

We found previously by fluorescence resonance energy transfer experiments that amyloid precursor protein (APP) homodimerizes in living cells. APP homodimerization is likely to be mediated by two sites of the ectodomain and a third site within the transmembrane sequence of APP. We have now investigated the role of the N-terminal growth factor-like domain in APP dimerization by NMR, biochemical, and cell biological approaches. Under nonreducing conditions, the N-terminal domain of APP formed SDS-labile and SDS-stable complexes. The presence of SDS was sufficient to convert native APP dimers entirely into monomers. Addition of an excess of a synthetic peptide (APP residues 91-116) containing the disulfide bridge-stabilized loop inhibited cross-linking of pre-existing SDS-labile APP ectodomain dimers. Surface plasmon resonance analysis revealed that this peptide specifically bound to the N-terminal domain of APP and that binding was entirely dependent on the oxidation of the thiol groups. By solution-state NMR we detected small chemical shift changes indicating that the loop peptide interacted with a large protein surface rather than binding to a defined pocket. Finally, we studied the effect of the loop peptide added to the medium of living cells. Whereas the levels of alpha-secretory APP increased, soluble beta-cleaved APP levels decreased. Because Abeta40 and Abeta42 decreased to similar levels as soluble beta-cleaved APP, we conclude either that beta-secretase binding to APP was impaired or that the peptide allosterically affected APP processing. We suggest that APP acquires a loop-mediated homodimeric state that is further stabilized by interactions of hydrophobic residues of neighboring domains.     

Constraints imposed by transmembrane domains affect enzymatic activity of membrane-associated human CD39/NTPDase1 mutants

Human CD39/NTPDase1 is an endothelial cell membrane-associated nucleotidase. Its large extracellular domain rapidly metabolizes nucleotides, especially ADP released from activated platelets, inhibiting further platelet activation/recruitment. Previous studies using our recombinant soluble CD39 demonstrated the importance of residues S57, D54, and D213 for enzymatic/biological activity. We now report effects of S57A, D54A, and D213A mutations on full-length (FL)CD39 function. Enzymatic activity of alanine modified FLCD39s was less than wild-type, contrasting the enhanced activity of their soluble counterparts. Furthermore, conservative substitutions D54E and D213E led to enzymes with activities greater than the alanine modified FLCD39s, but less than wild-type. Reductions in mutant activities were primarily associated with reduced catalytic rates. Differences in enzymatic activity were not attributable to gross changes in the nucleotide binding pocket or the enzyme's ability to multimerize. Thus, composition of the active site of wild-type CD39 appears optimized for ADPase function in the context of the transmembrane domains.     

The cytosolic and transmembrane domains of the beta 1,6 N-acetylglucosaminyltransferase (C2GnT) function as a cis to medial/Golgi-targeting determinant

The beta 1,6 N-acetylglucosaminyltransferase (C2GnT) has been recently mapped to the cis/medial-Golgi compartment. To analyze the Golgi-targeting determinants of C2GnT, we constructed various deletion mutants of the enzyme fused to the enhanced green fluorescent protein (EGFP) and localized these proteins by fluorescence microscopy in living cells. We found that the N-terminal peptide encompassing amino acids 1 to 32 represents the minimal Golgi-targeting signal sufficient to localize EGFP to the same compartment as the full-length C2GnT. This peptide makes up the cytoplasmic and the transmembrane domains of the enzyme and was referred to as CTd (cytoplasmic and transmembrane domains). We compared the Golgi-targeting efficiency of the C2GnT-derived CTd with its homologous domains from other glycosyltransferases, including the H-type alpha(1,2)-fucosyltransferase (FucTI), the polypeptide N-acetylgalactosaminyltransferase-I (GalNAcT-I), the alpha(1,3)-fucosyltransferase VII (FucTVII), and the alpha(2,6)-sialyltransferase (ST6Gal-I) and found that the Golgi-targeting determinants of these glycosyltransferases were also composed of their cytosolic and transmembrane domains. To investigate whether the CTd of C2GnT could serve as a cis to medial Golgi-specific signal, we tested its ability to mislocalize two late-Golgi acting glycosyltransferases FucTI and FucTVII. We show that fusing the C2GnT-derived CTd with the catalytic domain of FucTVII resulted in a complete mislocalization of the enzyme to the C2GnT compartment, with a parallel alteration of sialyl-Lewis x synthesis and P-selectin binding. The intracellular distribution and activity of FucTI, however, were not affected. Thus, CTds of either early or late-Golgi acting glycosyltransferases represent the Golgi-targeting domains of these enzymes. In addition, we show that C2GnT-derived CTd can function as a cis/medial Golgi-targeting determinant.     

A disintegrin and metalloproteinase (ADAM)-mediated ectodomain shedding of ADAM10

A disintegrin and metalloproteinase (ADAM) 10 is a type I transmembrane glycoprotein responsible for the ectodomain shedding of a range of proteins including the amyloid precursor protein implicated in Alzheimer's disease. In this study we demonstrate that ADAM10 itself is subject to shedding by one or more ADAMs. Expression of epitope-tagged wild-type ADAM10 in SH-SY5Y cells enabled the detection of a soluble ectodomain in conditioned medium. Shedding of the ADAM10 ectodomain was inhibited by a known ADAM inhibitor with a reciprocal accumulation of the full-length mature protein in both cell lysates and extracellular membrane vesicles. Shedding was also stimulated by phorbol ester treatment of cells. A glycosylphosphatidylinositol-anchored form of ADAM10 lacking the cytosolic, transmembrane and α-helical juxtamembrane regions of the wild-type protein was shed in a similar manner. Furthermore, a truncated soluble ADAM10 construct, although correctly post-translationally processed and catalytically active against a synthetic peptide substrate, was incapable of shedding cell-associated amyloid precursor protein. Finally, we show that ADAM9 is, at least in part, responsible for the ectodomain shedding of ADAM10. In conclusion, this is a new mechanism by which levels of ADAM10 are regulated and may have implications in a range of human diseases including Alzheimer's disease.     

Inhibitors bound to Ca(2+)-free sarcoplasmic reticulum Ca(2+)-ATPase lock its transmembrane region but not necessarily its cytosolic region, revealing the flexibility of the loops connecting transmembrane and cytosolic domains

Ca2+-free crystals of sarcoplasmic reticulum Ca2+-ATPase have, up until now, been obtained in the presence of inhibitors such as thapsigargin (TG), bound to the transmembrane region of this protein. Here, we examined the consequences of such binding for the protein. We found that, after TG binding, an active site ligand such as beryllium fluoride can still bind to the ATPase and change the conformation or dynamics of the cytosolic domains (as revealed by the protection afforded against proteolysis), but it becomes unable to induce any change in the transmembrane domain (as revealed by the intrinsic fluorescence of the membranous tryptophan residues). TG also obliterates the Trp fluorescence changes normally induced by binding of MgATP or metal-free ATP, as well as those induced by binding of Mg2+ alone. In the nucleotide binding domain, the environment of Lys515 (as revealed by fluorescein isothiocyanate fluorescence after specific labeling of this residue) is significantly different in the ATPase complex with aluminum fluoride and in the ATPase complex with beryllium fluoride, and in the latter case it is modified by TG. All these facts document the flexibility of the loops connecting the transmembrane and cytosolic domains in the ATPase. In the absence of active site ligands, TG protects the ATPase from cleavage by proteinase K at Thr242-Glu243, suggesting TG-induced reduction in the mobility of these loops. 2,5-Di-tert-butyl-1,4-dihydroxybenzene or cyclopiazonic acid, inhibitors which also bind in or near the transmembrane region, also produce similar overall effects on Ca2+-free ATPase.     

Substitution of methionine 435 with leucine, isoleucine, and serine in tumor necrosis factor alpha converting enzyme inactivates ectodomain shedding activity.

Tumor necrosis factor alpha (TNF-alpha) converting enzyme (TACE) is a zinc metalloprotease that has emerged as a general sheddase, which is responsible for ectodomain release of numerous membrane proteins, including the proinflammatory cytokine TNF-alpha, the leukocyte adhesin L-selectin and epidermal growth factor receptor ligand-transforming growth factor alpha (TGF-alpha), and related family members. Structurally, TACE belongs to a large clan of proteases, designated the metzincins, because TACE possesses a conserved methionine (Met435), frequently referred to as the met-turn residue, in its active site. A vital role of this residue in the function of TACE is supported by the fact that cells expressing the M435I TACE variant are defective in ectodomain shedding. However, the importance of Met435 in TACE appears to be uncertain, since another metzincin, matrix metalloprotease-2, has been found to be enzymatically fully active with either leucine or serine in place of its met-turn residue. We constructed TACE mutants with leucine or serine in place of Met435 to further examine the role of the met-turn residue in TACE-mediated ectodomain cleavage. Similar to the M435I TACE mutant, both the M435L and M435S constructs are defective in cleaving transmembrane TNF-alpha, TGF-alpha, and L-selectin. Comparative modeling and dynamic computation detected structural perturbations, which resulted in higher energy, in the catalytic zinc complexes of the Met435 TACE mutants compared with that in the wild-type enzyme. Thus, Met435 serves to maintain the stability of the catalytic center of TACE for the hydrolysis of peptide bonds in substrates.     

The role of ADAM10 and ADAM17 in the ectodomain shedding of angiotensin converting enzyme and the amyloid precursor protein.

Numerous transmembrane proteins, including the blood pressure regulating angiotensin converting enzyme (ACE) and the Alzheimer's disease amyloid precursor protein (APP), are proteolytically shed from the plasma membrane by metalloproteases. We have used an antisense oligonucleotide (ASO) approach to delineate the role of ADAM10 and tumour necrosis factor-alpha converting enzyme (TACE; ADAM17) in the ectodomain shedding of ACE and APP from human SH-SY5Y cells. Although the ADAM10 ASO and TACE ASO significantly reduced (> 81%) their respective mRNA levels and reduced the alpha-secretase shedding of APP by 60% and 30%, respectively, neither ASO reduced the shedding of ACE. The mercurial compound 4-aminophenylmercuric acetate (APMA) stimulated the shedding of ACE but not of APP. The APMA-stimulated secretase cleaved ACE at the same Arg-Ser bond in the juxtamembrane stalk as the constitutive secretase but was more sensitive to inhibition by a hydroxamate-based compound. The APMA-activated shedding of ACE was not reduced by the ADAM10 or TACE ASOs. These results indicate that neither ADAM10 nor TACE are involved in the shedding of ACE and that APMA, which activates a distinct ACE secretase, is the first pharmacological agent to distinguish between the shedding of ACE and APP.     

Functional dissection of the transmembrane domains of the transporter associated with antigen processing (TAP)

The transporter associated with antigen processing (TAP1/2) translocates cytosolic peptides of proteasomal degradation into the endoplasmic reticulum (ER) lumen. A peptide-loading complex of tapasin, major histocompatibility complex class I, and several auxiliary factors is assembled at the transporter to optimize antigen display to cytotoxic T-lymphocytes at the cell surface. The heterodimeric TAP complex has unique N-terminal domains in addition to a 6 + 6-transmembrane segment core common to most ABC transporters. Here we provide direct evidence that this core TAP complex is sufficient for (i) ER targeting, (ii) heterodimeric assembly within the ER membrane, (iii) peptide binding, (iv) peptide transport, and (v) specific inhibition by the herpes simplex virus protein ICP47 and the human cytomegalovirus protein US6. We show for the first time that the translocation pore of the transporter is composed of the predicted TM-(5-10) of TAP1 and TM-(4-9) of TAP2. Moreover, we demonstrate that the N-terminal domains of TAP1 and TAP2 are essential for recruitment of tapasin, consequently mediating assembly of the macromolecular peptide-loading complex.     

Modulation of cystic fibrosis transmembrane conductance regulator (CFTR) activity and genistein binding by cytosolic pH

Potentiators are molecules that increase the activity of the cystic fibrosis transmembrane conductance regulator (CFTR). Some potentiators can also inhibit CFTR at higher concentrations. The activating binding site is thought to be located at the interface of the dimer formed by the two nucleotide-binding domains. We have hypothesized that if binding of potentiators involves titratable residues forming salt bridges, then modifications of cytosolic pH (pH(i)) would alter the binding affinity. Here, we analyzed the effect of pH(i) on CFTR activation and on the binding of genistein, a well known CFTR potentiator. We found that pH(i) does modify CFTR maximum current (I(m)) and half-activation concentration (K(d)): I(m) = 127.7, 185.5, and 231.8 μA/cm(2) and K(d) = 32.7, 56.6 and 71.9 μm at pH 6, 7.35, and 8, respectively. We also found that the genistein apparent dissociation constant for activation (K(a)) increased at alkaline pH(i), near cysteine pK (K(a) = 1.83, 1.81 and 4.99 μm at pH(i) 6, 7.35, and 8, respectively), suggesting the involvement of cysteines in the binding site. Mutations of cysteine residues predicted to be within (Cys-491) or outside (Cys-1344) the potentiator-binding site showed that Cys-491 is responsible for the sensitivity of potentiator binding to alkaline pH(i). Effects of pH(i) on inhibition by high genistein doses were also analyzed. Our results extend previous data about multiple effects of pH(i) on CFTR activity and demonstrate that binding of potentiators involves salt bridge formation with amino acids of nucleotide-binding domain 1.     

The transmembrane domain of the Alzheimer's beta-secretase (BACE1) determines its late Golgi localization and access to beta -amyloid precursor protein (APP) substrate

Release of Abeta peptides from beta-amyloid precursor protein (APP) requires sequential cleavage by two endopeptidases, beta- and gamma-secretases. beta-Secretase was recently identified as a novel membrane-bound aspartyl protease, named BACE1, Asp2, or memapsin 2. Employing confocal microscopy and subcellular fractionation, we have found that BACE1 is largely situated in the distal Golgi membrane with a minor presence in the endoplasmic reticulum, endosomes, and plasma membrane in human neuroblastoma SHEP cells and in mouse Neuro-2a cell lines expressing either endogenous mouse BACE1 or additional exogenous human BACE1. The major cellular beta-secretase activity is located in the late Golgi apparatus, consistent with its cellular localization. Furthermore, we demonstrate that the single transmembrane domain of BACE1 alone determines the retention of BACE1 to the Golgi compartments, through examination of recombinant proteins of various BACE1 fragments fused to a reporter green fluorescence protein. In addition, we show that the transmembrane domain of BACE1 is required for the access of BACE1 enzymatic activity to the cellular APP substrate and hence for the optimal generation of the C-terminal fragment of APP (CTF99). The results suggest a molecular and cell biological mechanism for the regulation of beta-secretase activity in vivo.     

Endoproteolysis of beta-secretase (beta-site amyloid precursor protein-cleaving enzyme) within its catalytic domain. A potential mechanism for regulation

Sequential proteolysis of the amyloid precursor protein (APP) by beta- and gamma-secretase activities yields the amyloid beta peptide that is widely deposited in the brains of individuals with Alzheimer's disease. The membrane-anchored aspartyl protease beta-site APP-cleaving enzyme (BACE) exhibits all of the characteristics of a beta-secretase and has been shown to cleave APP at its beta-site in vitro and in vivo. We found that BACE undergoes cleavage on a surface-exposed alpha-helix between amino acid residues Leu-228 and Ala-229, generating stable N- and C-terminal fragments that remain covalently associated via a disulfide bond. The efficiency of BACE endoproteolysis was observed to depend heavily on cell and tissue type. In contrast to brain where holoprotein was predominant, BACE was found primarily as endoproteolyzed fragments in pancreas, liver, and muscle. In addition, we observed a marked up-regulation of BACE endoproteolysis in C2 myoblasts upon differentiation into multinucleated myotubes, a well established model system of muscle tissue specification. As in liver, BACE exists as endoproteolyzed fragments in the hepatic cell line, HepG2. We found that HepG2 cells are capable of generating amyloid beta peptide, suggesting that endoproteolyzed BACE retains measurable beta-secretase activity. We also found that BACE endoproteolysis occurs only after export from the endoplasmic reticulum, is enhanced in the trans-Golgi network, and is sensitive to inhibitors of vesicular acidification. The membrane-bound proteases tumor necrosis factor alpha-converting enzyme and furin were not found to be responsible for this cleavage nor was BACE observed to mediate its own endoproteolysis by an autocatalytic mechanism. Thus, we characterize a specific processing event that may serve to regulate the enzymatic activity of BACE on a post-translational level.     

Post-translational processing of chicken bone phosphoproteins. Identification of bone (phospho)protein kinase.

We have detected a protein kinase which phosphorylates bone phosphoproteins (BPPs) in the detergent extract of the membranous fractions in the periosteal bone strips of 12-day-embryonic-chick tibia. This enzyme, tentatively named BPP kinase, has a catalytic subunit of Mr approximately 39,000, utilizes GTP as well as ATP as a phospho-group donor, is inhibited by 2,3-bisphosphoglycerate and heparin, and is therefore similar to casein kinase II. The enzyme can phosphorylate dephosphorylated proteins such as casein, phosvitin and chicken BPPs, but the last-named are preferred substrates. The in vitro-phosphorylation-assay products of this enzyme in the extract were indistinguishable on an SDS/polyacrylamide gel from the major [32P]phosphoproteins metabolically labelled in the embryonic-chick bone tissue. The regulatory mechanisms of the phosphorylation process of BPPs by BPP kinase as well as the potential role of this enzyme in mineralization are discussed.     

Memapsin 2 (beta-secretase) cytosolic domain binds to the VHS domains of GGA1 and GGA2: implications on the endocytosis mechanism of memapsin 2

Memapsin 2, or beta-secretase, is a membrane-anchored aspartic protease that initiates the cleavage of beta-amyloid precursor protein (APP) leading to the production of beta-amyloid peptide in the brain and the onset of Alzheimer's disease. Memapsin 2 and APP are both endocytosed into endosomes for cleavage. Here we show that the cytosolic domain of memapsin 2, but not that of memapsin 1, binds the VHS domains of GGA1 and GGA2. Gel-immobilized VHS domains of GGA1 and GGA2 also bound to full-length memapsin 2 from cell mammalian lysates. Mutagenesis studies established that Asp(496), Leu(499) and Leu(500) were essential for the binding. The spacing of these three residues in memapsin 2 is identical to those in the cytosolic domains of mannose-6-phosphate receptors, sortilin and low density lipoprotein receptor-related protein 3. These observations suggest that the endocytosis and intracellular transport of memapsin 2, mediated by its cytosolic domain, may involve the binding of GGA1 and GGA2.     

CD39 reveals novel insights into the role of transmembrane domains in protein processing, apical targeting and activity

Cargo proteins of the biosynthetic secretory pathway are folded in the endoplasmic reticulum (ER) and proceed to the trans Golgi network for sorting and targeting to the apical or basolateral sides of the membrane, where they exert their function. These processes depend on diverse protein domains. Here, we used CD39 (NTPdase1), a modulator of thrombosis and inflammation, which contains an extracellular and two transmembrane domains (TMDs), as a model protein to address comprehensively the role of native TMDs in folding, polarized transport and biological activity. In MDCK cells, CD39 exits Golgi dynamin-dependently and is targeted to the apical side of the membrane. Although the N-terminal TMD possesses an apical targeting signal, the N- and C-terminal TMDs are not required for apical targeting of CD39. Folding and transport to the plasma membrane relies only on the C-terminal TMD, while the N-terminal one is redundant. Nevertheless, both N- and C-terminal anchoring as well as genuine TMDs are critical for optimal enzymatic activity and activation by cholesterol. We conclude therefore that TMDs are not just mechanical linkers between proteins and membranes but are also able to control folding and sorting, as well as biological activity via sensing components of lipid bilayers.     

Regulation and activity of cytosolic 5'-nucleotidase II. A bifunctional allosteric enzyme of the Haloacid Dehalogenase superfamily involved in cellular metabolism

In many vertebrate tissues, cytosolic 5'-nucleotidase II (cN-II) either hydrolyses or phosphorylates a number of purine (monophosphorylated) nucleosides through a scheme common to the Haloacid Dehalogenase superfamily members. It possesses a pivotal role in purine cellular metabolism and it acts on anti-tumoural and antiviral nucleoside analogues, thus being of potential therapeutic importance. cN-II is Mg2+-dependent, regulated and stabilised by several factors such as allosteric effectors ATP and 2,3-DPG, although these are not directly involved in the reaction stoichiometry. We review herein the experimental knowledge currently available about this remarkable enzymatic activity.     

Heparin activates beta-secretase (BACE1) of Alzheimer's disease and increases autocatalysis of the enzyme

BACE1 is an aspartic protease that generates the N-terminus of the beta-amyloid protein (Alphabeta) from the beta-amyloid precursor protein (APP). BACE1 is a key target for Alzheimer drug development. However, little is known about the physiological regulation of the enzyme. Heparin can promote beta-secretase cleavage of APP in neuroblastoma cells. However, heparin has also been reported to directly inhibit BACE1 activity in vitro. To clarify the role of heparin in regulating BACE1, we examined the effect of heparin on the activity of recombinant human BACE1 (rBACE1) in vitro. Low concentrations (1 microg/mL) of heparin were found to stimulate rBACE1, increasing enzyme V(max) and decreasing the K(M). In contrast, higher concentrations of heparin (10 or 100 microg/mL) were inhibitory. Heparin affinity chromatography demonstrated that heparin interacted strongly with the zymogen form of rBACE1 and bound to a peptide homologous to the N-terminal pro sequence of BACE1. Mature (pro sequence cleaved) enzyme lacked the capacity to be stimulated by heparin, indicating that the pro domain was necessary for the stimulation by heparin. Furthermore, in the presence of stimulatory concentrations of heparin, there was an increase in autocatalytic cleavage of the protease domain and a subsequent loss of enzyme activity in vitro. Our results strongly suggest that heparin stimulates the partially active BACE1 zymogen, and we propose that the activation is mediated by high-affinity binding of heparin to the pro domain. Our study provides evidence that heparan sulfate proteoglycans could regulate the rate of Alphabeta production in vivo.