Using reverse transcriptase-PCR and degenerate oligonucleotides derived from the active-site residues of subtilisin͞kexin-like serine proteinases, we have identified a highly conserved and phylogenetically ancestral human, rat, and mouse type I membrane-bound proteinase called subtilisin͞kexin-isozyme-1 (SKI-1). Computer databank searches reveal that human SKI-1 was cloned previously but with no identified function. In situ hybridization demonstrates that SKI-1 mRNA is present in most tissues and cells. Cleavage specificity studies show that SKI-1 generates a 28-kDa product from the 32-kDa brain-derived neurotrophic factor precursor, cleaving at an RGLT2SL bond. In the endoplasmic reticulum of either LoVo or HK293 cells, proSKI-1 is processed into two membrane-bound forms of SKI-1 (120 and 106 kDa) differing by the nature of their N-glycosylation. Late along the secretory pathway some of the membrane-bound enzyme is shed into the medium as a 98-kDa form. Immunocytochemical analysis of stably transfected HK293 cells shows that SKI-1 is present in the Golgi apparatus and within small punctate structures reminiscent of endosomes. In vitro studies suggest that SKI-1 is a Ca 2؉ -dependent serine proteinase exhibiting a wide pH optimum for cleavage of pro-brainderived neurotrophic factor.
All proprotein convertases (PCs) of the subtilisin/ kexin family contain an N-terminal prosegment that is presumed to act both as an intramolecular chaperone and an inhibitor of its parent enzyme. In this work, we examined inhibition by purified, recombinant bacterial prosegments of furin and PC7 on the in vitro processing of either the fluorogenic peptide pERTKR-MCA or the human immunodeficiency virus envelope glycoprotein gp160. These propeptides are potent inhibitors that display measurable selectivity toward specific proprotein convertases. Small, synthetic decapeptides derived from the C termini of the prosegments are also potent inhibitors, albeit less so than the full-length proteins, and the C-terminal P1 arginine is essential for inhibition. The bacterial, recombinant prosegments were also used to generate specific antisera, allowing us to study the intracellular metabolic fate of the prosegments of furin and PC7 expressed via vaccinia virus constructs. These vaccinia virus recombinants, along with transient transfectants of the preprosegments of furin and PC7, efficiently inhibited the ex vivo processing of the neurotrophins nerve growth factor and brain-derived neurotrophic factor. Thus, we have demonstrated for the first time that PC prosegments, expressed ex vivo as independent domains, can act in trans to inhibit precursor maturation by intracellular PCs.
Processing of the -amyloid precursor protein (APP) by -and ␥-secretases generates the amyloidogenic peptide A, a major factor in the etiology of Alzheimer's disease. Following the recent identification of the -secretase -amyloid-converting enzyme (BACE), we herein investigate its zymogen processing, molecular properties, and cellular trafficking. Our data show that among the proprotein convertase family members, furin is the major converting enzyme of pro-BACE into BACE within the trans-Golgi network of HK293 cells. While we demonstrate that the 24-amino acid prosegment is required for the efficient exit of pro-BACE from the endoplasmic reticulum, it may not play a strong inhibitory role since we observe that pro-BACE can produce significant quantities of the Swedish mutant APP sw -secretase product C99. BACE is palmitoylated at three Cys residues within its transmembrane/cytosolic tail and is sulfated at mature N-glycosylated moieties. Data with three different antibodies show that a small fraction of membrane-bound BACE is shed into the medium and that the extent of ectodomain shedding is palmitoylationdependent. Overexpression of full-length BACE causes a significant increase in the production of C99 and a decrease in the ␣-secretase product APPs␣. Although there is little increase in the generation of A by full-length BACE, overexpression of either a soluble form of BACE (equivalent to the shed form) or one lacking the prosegment leads to enhanced A levels. These findings suggest that the shedding of BACE may play a role in the amyloidogenic processing of APP.Alzheimer's disease is a progressive degenerative disorder of the brain characterized by mental deterioration, memory loss, confusion, and disorientation. Among the cellular mechanisms contributing to this pathology are two types of fibrous protein deposition in the brain, intracellular neurofibrillary tangles consisting of polymerized tau protein, and abundant extracellular fibrils largely composed of -amyloid 1 (for reviews see Refs. 1-3). -Amyloid, also known as A, arises from proteolytic processing of the -amyloid precursor protein (APP) at the -and ␥-secretase cleavage sites. The cellular toxicity and amyloid-forming capacity of the two major forms of A (A 40 and especially A 42 ) have been well documented (1-3).An alternative, anti-amyloidogenic cleavage carried out by ␣-secretase(s) is located within the A peptide sequence of APP, thus precluding the formation of intact insoluble A. This cleavage by ␣-secretase within the (His-His-GlnLys2Leu-Val) sequence of APP is the major physiological route of APP maturation. The products of this reaction are a soluble 100 -120-kDa N-terminal fragment (APPs␣) and a C-terminal membrane-bound ϳ9-kDa segment (C83). In several recent reports, metalloproteinases such as ADAM9, -10, and -17 were shown to be involved in the ␣-secretase cleavage
Biochemical and enzymatic characterization of the novel proprotein convertase rat PC7 (rPC7) was carried out using vaccinia virus recombinants overexpressed in mammalian BSC40 cells. Pro-PC7 is synthesized as a glycosylated zymogen (101 kDa) and processed into mature rPC7 (89 kDa) in the endoplasmic reticulum. No endogenously produced soluble forms of this membrane-anchored protein were detected. A deletion mutant (65 kDa), truncated well beyond the expected Cterminal boundary of the P-domain, produced soluble rPC7 in the culture medium. Enzymatic activity assays of rPC7 using fluorogenic peptidyl substrates indicated that the pH optimum, Ca 2؉ dependence, and cleavage specificity of this enzyme are largely similar to those of furin. However, with some substrates, cleavage specificity more closely resembled that of yeast kexin, suggesting differential processing of proprotein substrates by this novel convertase. We examined the rPC7-and human furin-mediated cleavage of synthetic peptides containing the processing sites of three proteins known to colocalize in situ with rPC7. Whereas both enzymes correctly processed the pro-parathyroid hormone tridecapeptide and the pro-PC4 heptadecapeptide, neither enzyme cleaved a pro-epidermal growth factor hexadecapeptide. Thus, this study establishes that rPC7 is an enzymatically functional subtilisin/kexin-like serine proteinase with a cleavage specificity resembling that of hfurin. In addition, we have demonstrated that rPC7 can correctly process peptide precursors that contain the processing sites of at least two potential physiological substrates.
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