Membrane-proximal Site Research Articles

Structural basis for membrane-proximal proteolysis of substrates by ADAM10

The endopeptidase ADAM10 is a critical catalyst for the regulated proteolysis of key drivers of mammalian development, physiology, and non-amyloidogenic cleavage of APP as the primary α-secretase. ADAM10 function requires the formation of a complex with a C8-tetraspanin protein, but how tetraspanin binding enables positioning of the enzyme active site for membrane-proximal cleavage remains unknown. We present here a cryo-EM structure of a vFab-ADAM10-Tspan15 complex, which shows that Tspan15 binding relieves ADAM10 autoinhibition and acts as a molecular measuring stick to position the enzyme active site about 20Å from the plasma membrane for membrane-proximal substrate cleavage. Cell-based assays of N-cadherin shedding establish that the positioning of the active site by the interface between the ADAM10 catalytic domain and the bound tetraspanin influences selection of the preferred cleavage site. Together, these studies reveal the molecular mechanism underlying ADAM10 proteolysis at membrane-proximal sites and offer a roadmap for its modulation in disease.

Human VH-based chimeric antigen receptor T cells targeting glypican 3 eliminate tumors in preclinical models of HCC.

Efficacy of chimeric antigen receptor (CAR) T cells for treating solid tumors, including HCC, remains a challenge. Nanobodies are emerging building blocks of CAR T cells due to their small size and high expression. Membrane proximal sites have been shown as attractive epitopes of CAR T cells. However, current CAR formats are not tailored toward nanobodies or targeting membrane distal epitopes. Using hYP7 Fv (membrane proximal) and HN3 VH nanobody (membrane distal) as GPC3 targeting elements, we sought to determine how hinges and transmembrane portions of varying structures and sizes affect CAR T-cell function. We generated multiple permutations of CAR T cells containing CD8, CD28, IgG4, and Fc domains. We show that engineered HN3 CAR T cells can be improved by 2 independent, synergistic changes in the hinge and transmembrane domains. The T cells expressing the HN3 CAR which contains the hinge region of IgG4 and the CD28 transmembrane domain (HN3-IgG4H-CD28TM) exhibited high cytotoxic activity and caused complete HCC tumor eradication in immunodeficient mice. HN3-IgG4H-CD28TM CAR T cells were enriched for cytotoxic-memory CD8+ T cells and NFAT signals, and reduced β catenin levels in HCC cells. Our findings indicate that altering the hinge and transmembrane domains of a nanobody-based CAR targeting a distal GPC3 epitope, in contrast to a membrane proximal epitope, lead to robust T-cell signaling and induce swift and durable eradication of HCC tumors.

Y-Complex Proteins Show RNA-Dependent Binding Events at the Cell Membrane and Distinct Single-Molecule Dynamics.

Bacteria are dependent on rapid alterations in gene expression. A prerequisite for rapid adaptations is efficient RNA turnover, with endonuclease RNase Y playing a crucial role in mRNA stability as well as in maturation. In Bacillus subtilis, RNase Y in turn interacts with the so-called “Y-complex” consisting of three proteins, which play important functions in sporulation, natural transformation and biofilm formation. It is thought that the Y-complex acts as an accessory factor in RNase Y regulation but might also have independent functions. Using single-molecule tracking, we show that all three Y-complex proteins exhibit three distinct mobilities, including movement through the cytosol and confined motion, predominantly at membrane-proximal sites but also within the cell center. A transcriptional arrest leads to a strong change in localization and dynamics of YmcA, YlbF and YaaT, supporting their involvement in global RNA degradation. However, Y-complex proteins show distinguishable protein dynamics, and the deletion of yaaT or ylbF shows a minor effect on the dynamics of YmcA. Cell fractionation reveals that YaaT displays a mixture of membrane association and presence in the cytosol, while YlbF and YmcA do not show direct membrane attachment. Taken together, our experiments reveal membrane-associated and membrane-independent activities of Y-complex proteins and a dynamic interplay between them with indirect membrane association of YmcA and YlbF via YaaT.

Glycoforms of human prostate-specific membrane antigen (PSMA) in human cells and prostate tissue.

N-glycosylation is a ubiquitous and variable posttranslational modification that regulates physiological functions of secretory and membrane-associated proteins and the dysregulation of glycosylation pathways is often associated with cancer growth and metastasis. Prostate-specific membrane antigen (PSMA) is an established biomarker for prostate cancer imaging and therapy. Mass spectrometry was used to analyze the distribution of the site-specific glycoforms of PSMA in insect, human embryonic kidney, and prostate cancer cells, and in prostate tissue upon immunoaffinity enrichment. While recombinant PSMA expressed in insect cells was decorated mainly by paucimannose and high mannose glycans, complex, hybrid, and high mannose glycans were detected in samples from human cells and tissue. We noted an interesting spatial distribution of the glycoforms on the PSMA surface-high mannose glycans were the dominant glycoforms at the N459, N476, and N638 sequons facing the plasma membrane, while the N121, N195, and N336 sites, located at the exposed apical PSMA domain, carried primarily complex glycans. The presence of high mannose glycoforms at the former sequons likely results from the limited access of enzymes of the glycosynthetic pathway required for the synthesis of the complex structures. In line with the limited accessibility of membrane-proximal sites, no glycosylation was observed at the N51 site positioned closest to the membrane. Our study presents initial descriptive analysis of the glycoforms of PSMA observed in cell lines and in prostate tissue. It will hopefully stimulate further research into PSMA glycoforms in the context of tumor staging, noninvasive detection of prostate tumors, and the impact of glycoforms on physicochemical and enzymatic characteristics of PSMA in a tissue-specific manner.

A vulnerable, membrane-proximal site in human respiratory syncytial virus F revealed by a prefusion-specific single-domain antibody.

Human respiratory syncytial virus (RSV) is a major cause of lower respiratory tract disease, especially in young children and the elderly. The fusion protein (F) exists in a pre- and postfusion conformation and is the main target of RSV-neutralizing antibodies. Highly potent RSV-neutralizing antibodies typically bind sites that are unique to the prefusion conformation of F. In this study we screened a single-domain antibody (VHH) library derived from a llama immunized with prefusion-stabilized F and identified a prefusion F-specific VHH that can neutralize RSV A at subnanomolar concentrations. Structural analysis revealed that this VHH primarily binds to antigenic site I while also making contacts with residues in antigenic site III and IV. This new VHH reveals a previously underappreciated membrane-proximal region sensitive for neutralization.ImportanceRSV is an important respiratory pathogen. This study describes a prefusion F-specific VHH that primarily binds to antigenic site I of RSV F. This is the first time that a prefusion F-specific antibody that binds this site is reported. In general, antibodies that bind to site I are poorly neutralizing, whereas the VHH described here neutralizes RSV A at subnanomolar concentrations. Our findings contribute to insights into the RSV F antigenic map.

Structural Basis of Paxillin Recruitment by Kindlin-2 in Regulating Cell Adhesion

Activation of cell surface receptor integrin has been extensively studied as the first key step to trigger cell adhesion, but the subsequent events, widely regarded as integrin "outside-in" signaling to form supramolecular complexes (focal adhesions [FAs]) to promote dynamic cell adhesion, remain poorly elucidated. Integrin activator kindlin-2 was recently found to associate with paxillin in nascent FAs, implicating an early yet undefined integrin outside-in signaling event. Here we show structurally that kindlin-2 recognizes paxillin via a distinct interface involving the ubiquitin-like kindlin-2 F0 domain and the paxillin LIM4 domain. The interface is adjacent to the membrane binding site of kindlin-2 F0, suggesting a mechanism for kindlin-2 to recruit paxillin to the membrane-proximal site where FA assembly is initiated. Disruption of the interface impaired the localization of paxillin, causing strong defects in FA assembly and cell migration. These data unveil a structural basis of the kindlin-2/paxillin interaction in controlling dynamic cell adhesion.

Hyaluronan-CD44v3 Interaction with Oct4-Sox2-Nanog Promotes miR-302 Expression Leading to Self-renewal, Clonal Formation, and Cisplatin Resistance in Cancer Stem Cells from Head and Neck Squamous Cell Carcinoma

Human head and neck squamous cell carcinoma (HNSCC) is a highly malignant cancer associated with major morbidity and mortality. In this study, we determined that human HNSCC-derived HSC-3 cells contain a subpopulation of cancer stem cells (CSCs) characterized by high levels of CD44v3 and aldehyde dehydrogenase-1 (ALDH1) expression. These tumor cells also express several stem cell markers (the transcription factors Oct4, Sox2, and Nanog) and display the hallmark CSC properties of self-renewal/clonal formation and the ability to generate heterogeneous cell populations. Importantly, hyaluronan (HA) stimulates the CD44v3 (an HA receptor) interaction with Oct4-Sox2-Nanog leading to both a complex formation and the nuclear translocation of three CSC transcription factors. Further analysis reveals that microRNA-302 (miR-302) is controlled by an upstream promoter containing Oct4-Sox2-Nanog-binding sites, whereas chromatin immunoprecipitation (ChIP) assays demonstrate that stimulation of miR-302 expression by HA-CD44 is Oct4-Sox2-Nanog-dependent in HNSCC-specific CSCs. This process results in suppression of several epigenetic regulators (AOF1/AOF2 and DNMT1) and the up-regulation of several survival proteins (cIAP-1, cIAP-2, and XIAP) leading to self-renewal, clonal formation, and cisplatin resistance. These CSCs were transfected with a specific anti-miR-302 inhibitor to silence miR-302 expression and block its target functions. Our results demonstrate that the anti-miR-302 inhibitor not only enhances the expression of AOF1/AOF2 and DNMT1 but also abrogates the production of cIAP-1, cIAP-2, and XIAP and HA-CD44v3-mediated cancer stem cell functions. Taken together, these findings strongly support the contention that the HA-induced CD44v3 interaction with Oct4-Sox2-Nanog signaling plays a pivotal role in miR-302 production leading to AOF1/AOF2/DNMT1 down-regulation and survival of protein activation. All of these events are critically important for the acquisition of cancer stem cell properties, including self-renewal, clonal formation, and chemotherapy resistance in HA-CD44v3-activated head and neck cancer.

Targeting HIV-1 gp41 in Close Proximity to the Membrane Using Antibody and Other Molecules

HIV-1 envelope glycoprotein (Env) spikes are supported at the viral membrane interface by a highly conserved and hydrophobic region of gp41, designated the membrane-proximal external region (MPER). The MPER is mandatory for infection of host cells by HIV-1, and is the target of some of the most broadly neutralizing antibodies described to date. As such, the MPER is also of considerable interest for HIV vaccine design. However, structural models indicate that the MPER assumes distinct conformations prior to and leading up to Env-mediated fusion. Thus, the more of these distinct conformations that antibodies and inhibitors can recognize will likely be the better for antiviral potency. In addition to its flexibility, the MPER is lipophilic and its accessibility to bulky macromolecules is limited by steric and kinetic blocks that present particular challenges for eliciting HIV-1 neutralizing antibodies. Moreover, the ability of the MPER and viral membrane to combine as a complex has critical mechanistic implications for molecules that target lipid-bound and/or unbound states. Interestingly, membrane affinity frequently appears to enhance the potency of both fusion inhibitors and antibodies to different sites on gp41. We therefore highlight mechanisms to be harnessed in targeting membraneproximal sites on HIV gp41 for both vaccine and fusion inhibitor design. Such design efforts will likely need to draw upon knowledge of MPER structure and function, and may in turn inform analogous approaches to MPERs of other enveloped viruses and systems.

N-Terminally Myristoylated Feline Foamy Virus Gag Allows Env-Independent Budding of Sub-Viral Particles

Foamy viruses (FVs) are distinct retroviruses classified as Spumaretrovirinae in contrast to the other retroviruses, the Orthoretrovirinae. As a unique feature of FVs, Gag is not sufficient for sub-viral particle (SVP) release. In primate and feline FVs (PFV and FFV), particle budding completely depends on the cognate FV Env glycoproteins. It was recently shown that an artificially added N-terminal Gag myristoylation signal (myr-signal) overcomes this restriction in PFV inducing an Orthoretrovirus-like budding phenotype. Here we show that engineered, heterologous N-terminal myr-signals also induce budding of the distantly related FFV Gag. The budding efficiency depends on the myr-signal and its location relative to the N-terminus of Gag. When the first nine amino acid residues of FFV Gag were replaced by known myr-signals, the budding efficiency as determined by the detection of extracellular SVPs was low. In contrast, adding myr-signals to the intact N-terminus of FFV Gag resulted in a more efficient SVP release. Importantly, budding of myr-Gag proteins was sensitive towards inhibition of cellular N-myristoyltransferases. As expected, the addition or insertion of myr-signals that allowed Env-independent budding of FFV SVPs also retargeted Gag to plasma membrane-proximal sites and other intracellular membrane compartments. The data confirm that membrane-targeted FV Gag has the capacity of SVP formation.

Hyaluronan-CD44 Interaction with Protein Kinase Cϵ Promotes Oncogenic Signaling by the Stem Cell Marker Nanog and the Production of MicroRNA-21, Leading to Down-regulation of the Tumor Suppressor Protein PDCD4, Anti-apoptosis, and Chemotherapy Resistance in Breast Tumor Cells

Multidrug resistance and disease relapse is a challenging clinical problem in the treatment of breast cancer. In this study, we investigated the hyaluronan (HA)-induced interaction between CD44 (a primary HA receptor) and protein kinase Cepsilon (PKCepsilon), which regulates a number of human breast tumor cell functions. Our results indicate that HA binding to CD44 promotes PKCepsilon activation, which, in turn, increases the phosphorylation of the stem cell marker, Nanog, in the breast tumor cell line MCF-7. Phosphorylated Nanog is then translocated from the cytosol to the nucleus and becomes associated with RNase III DROSHA and the RNA helicase p68. This process leads to microRNA-21 (miR-21) production and a tumor suppressor protein (e.g. PDCD4 (program cell death 4)) reduction. All of these events contribute to up-regulation of inhibitors of apoptosis proteins (IAPs) and MDR1 (multidrug-resistant protein), resulting in anti-apoptosis and chemotherapy resistance. Transfection of MCF-7 cells with PKCepsilon or Nanog-specific small interfering RNAs effectively blocks HA-mediated PKCepsilon-Nanog signaling events, abrogates miR-21 production, and increases PDCD4 expression/eIF4A binding. Subsequently, this PKCepsilon-Nanog signaling inhibition causes IAP/MDR1 down-regulation, apoptosis, and chemosensitivity. To further evaluate the role of miR-21 in oncogenesis and chemoresistance, MCF-7 cells were also transfected with a specific anti-miR-21 inhibitor in order to silence miR-21 expression and inhibit its target functions. Our results indicate that anti-miR-21 inhibitor not only enhances PDCD4 expression/eIF4A binding but also blocks HA-CD44-mediated tumor cell behaviors. Thus, this newly discovered HA-CD44 signaling pathway should provide important drug targets for sensitizing tumor cell apoptosis and overcoming chemotherapy resistance in breast cancer cells.

Tyrosylprotein Sulfotransferase-2 Expression Is Required for Sulfation of RNase 9 and Mfge8 in Vivo

Protein-tyrosine sulfation is mediated by two Golgi tyrosyl-protein sulfotransferases (TPST-1 and TPST-2) that are widely expressed in vivo. However, the full substrate repertoire of this enzyme system is unknown and thus, our understanding of the biological role(s) of tyrosine sulfation is limited. We reported that whereas Tpst1(-/-) male mice have normal fertility, Tpst2(-/-) males are infertile despite normal spermatogenesis. However, Tpst2(-/-) sperm are severely defective in their motility in viscous media and in their ability to fertilize eggs. These findings suggest that sulfation of unidentified substrate(s) is crucial for normal sperm function. We therefore sought to identify tyrosine-sulfated proteins in the male genital tract using affinity chromatography on PSG2, an anti-sulfotyrosine monoclonal antibody, followed by mass spectrometry. Among the several candidate tyrosine-sulfated proteins identified, RNase 9 and Mfge8 were examined in detail. RNase 9, a catalytically inactive RNase A family member of unknown function, is expressed only in the epididymis after onset of sexual maturity. Mfge8 is expressed on mouse sperm and Mfge8(-/-) male mice are subfertile. Metabolic labeling coupled with sulfoamino acid analysis confirmed that both proteins are tyrosine-sulfated and both proteins are expressed at comparable levels in wild type, Tpst1(-/-), and Tpst2(-/-) epididymides. However, we demonstrate that RNase 9 and Mfge8 are tyrosine-sulfated in wild type and Tpst1(-/-), but not in Tpst2(-/-) mice. These findings suggest that lack of sulfation of one or both of these proteins may contribute mechanistically to the infertility of Tpst2(-/-) males.

The Proprotein Convertase PC5A and a Metalloprotease Are Involved in the Proteolytic Processing of the Neural Adhesion Molecule L1

The transmembrane and multidomain neural adhesion molecule L1 plays important functional roles in the developing and adult nervous system. L1 is proteolytically processed at two distinct sites within the extracellular domain, leading to the generation of different fragments. In this report, we present evidence that the proprotein convertase PC5A is the protease that cleaves L1 in the third fibronectin type III domain, whereas the proprotein convertases furin, PC1, PC2, PACE4, and PC7 are not effective in cleaving L1. Analysis of mutations revealed Arg(845) to be the site of cleavage generating the N-terminal 140-kDa fragment. This fragment was present in the hippocampus, which expresses PC5A, but was not detectable in the cerebellum, which does not express PC5A. The 140-kDa L1 fragment was found to be tightly associated with the full-length 200-kDa L1 molecule. The complex dissociated from the membrane upon cleavage by a protease acting at a more membrane-proximal site of full-length L1. This proteolytic cleavage was inhibited by the metalloprotease inhibitor GM 6001 and enhanced by a calmodulin inhibitor. L1-dependent neurite outgrowth of cerebellar neurons was inhibited by GM 6001, suggesting that proteolytic processing of L1 by a metalloprotease is involved in neurite outgrowth.

Proteolytic Processing and Primary Structure ofPlasmodium falciparum Apical Membrane Antigen-1

Plasmodium falciparum apical membrane antigen-1 (PfAMA-1) is a malaria merozoite integral membrane protein that plays an essential but poorly understood role in invasion of host erythrocytes. The PfAMA-1 ectodomain comprises three disulfide-constrained domains, the first of which (domain I) is preceded by an N-terminal prosequence. PfAMA-1 is initially routed to secretory organelles at the apical end of the merozoite, where the 83-kDa precursor (PfAMA-1(83)) is converted to a 66-kDa form (PfAMA-1(66)). At about the time of erythrocyte invasion, PfAMA-1(66) selectively translocates onto the merozoite surface. Here we use direct microsequencing and mass spectrometric peptide mass fingerprinting to characterize in detail the primary structure and proteolytic processing of PfAMA-1. We have determined the site at which processing takes place to convert PfAMA-1(83) to PfAMA-1(66) and have shown that both species possess a completely intact and unmodified transmembrane and cytoplasmic domain. Following relocation to the merozoite surface, PfAMA-1(66) is further proteolytically cleaved at one of two alternative sites, either between domains II and III, or at a membrane-proximal site following domain III. As a result, the bulk of the ectodomain is shed from the parasite surface in the form of two soluble fragments of 44 and 48 kDa. PfAMA-1 is not detectably modified by the addition of N-linked oligosaccharides.

Calmodulin Regulates L-Selectin Adhesion Molecule Expression and Function through a Protease-Dependent Mechanism

Expression of the L-selectin adhesion molecule is rapidly down-regulated upon cell activation through proteolysis at a membrane-proximal site. Here we demonstrate that calmodulin, an intracellular calcium regulatory protein, specifically coprecipitates with L-selectin through a direct association with the cytoplasmic domain of L-selectin. Furthermore, calmodulin inhibitors disrupt L-selectin-dependent adhesion by inducing proteolytic release of L-selectin from the cell surface. The effects of the calmodulin inhibitors on L-selectin expression and function can be prevented by cotreatment with a hydroxamic acid–based metalloprotease inhibitor. Our results suggest a novel role for calmodulin in regulating the expression and function of an integral membrane protein through a protease-dependent mechanism. These findings may have broader implications for other cell surface proteins that also undergo regulated proteolysis.

Induction of cell death by chimeric L-selectin-Fas receptors.

In the present study, we have established a system where engagement of an adhesion molecule triggers a death signal into cells. L-selectin, which is a well characterized adhesion receptor involved in the initial adhesion between lymphocyte and endothelium, was fused to the intracellular domain of an apoptosis-inducing molecule, Fas. Ligation of the chimeric receptors with a carbohydrate ligand for L-selectin, fucoidin or a mAb that recognizes the lectin domain of L-selectin, induced apoptosis in receptor-expressing cells. However, ligation with an anti-L-selectin mAb reactive with a non-ligand binding site did not induce apoptosis, indicating that stimulation through the lectin domain of L-selectin in the chimeric receptor leads to signal delivery. Upon activation L-selectin shows a unique proteolytic cleavage at the membrane proximal site on the extracellular (EC) domain, of which the significance is also unclear. We found that truncations in the EC domain which abrogate the proteolytic cleavage of L-selectin did not influence induction of apoptosis, suggesting that the cleavage on the EC domain itself is not important for the signaling function of the chimeric receptor. This is the first demonstration that an adhesion signal can be converted to a signal that leads to apoptotic cell death.

Neutrophil rolling altered by inhibition of L-selectin shedding in vitro.

The L-selectin adhesion molecule is involved in guiding leukocytes to sites of inflammation. L-selectin is cleaved by an unusual proteolytic activity at a membrane-proximal site resulting in rapid shedding from the cell surface. Although it has been demonstrated that L-selectin mediates, in part, the early event of leukocyte rolling under hydrodynamic flow, the contribution of shedding to L-selectin function has remained unknown. Here we show that hydroxamic acid-based metalloprotease inhibitors block L-selectin downregulation from the cell surface of stimulated neutrophils, without affecting Mac-1 mobilization or general neutrophil activation, and inhibit cleavage of L-selectin in a cell-free system. Unexpectedly, the hydroxamic acid-based inhibitors reduced neutrophil rolling velocity under hydrodynamic flow, resulting in increased neutrophil accumulation. These results suggest that L-selectin is cleaved in seconds--much faster than previously suspected--during the process of rolling under hydrodynamic flow, and that shedding of L-selectin may contribute significantly to the velocity of leukocyte rolling. L-selectin shedding during rolling interactions may be physiologically important for limiting leukocyte aggregation and accumulation at sites of inflammation.

Shedding of the Lymphocyte L-Selectin Adhesion Molecule Is Inhibited by a Hydroxamic Acid-based Protease Inhibitor: IDENTIFICATION WITH AN L-SELECTIN-ALKALINE PHOSPHATASE REPORTER

Expression of the L-selectin adhesion molecule can be rapidly down-modulated by regulated proteolysis at a membrane-proximal site. The L-selectin secretase has remained undefined, and the secretase activity is resistant to a broad panel of common protease inhibitors. We have developed an L-selectin-alkaline phosphatase reporter, consisting of the ectodomain of human placental alkaline phosphatase fused to the membrane-proximal cleavage, transmembrane, and cytoplasmic domains of L-selectin, to aid in the screening for L-selectin secretase inhibitors. A hydroxamic acid-based metalloprotease inhibitor, KD-IX-73-4, inhibited release of the L-selectin-alkaline phosphatase reporter in a dose-dependent manner. The hydroxamic acid-based peptide was also found to inhibit wild type L-selectin down-regulation from the surfaces of phorbol myristate acetate-activated peripheral blood lymphocytes and phytohemagglutinin-stimulated lymphoblasts. Analysis of the proteolytic cleavage fragments of L-selectin confirmed that KD-IX-73-4 inhibited L-selectin proteolysis. Lymphocyte L-selectin was not down-regulated when co-cultured with formylmethionylleucylphenylalanine-stimulated neutrophils, suggesting that the putative secretase acts in cis with the membrane-bound L-selectin. These results suggest that the L-selectin secretase activity may involve a cell surface, zinc-dependent metalloprotease, although L-selectin shedding is not affected by EDTA and may be related to the recently described activity involved in processing of membrane-bound TNF-alpha.

Two proteolytic pathways for down-regulation of the barrier molecule CD43 of human neutrophils.

CD43, an anionic rod-like mucin molecule on white blood cells, is thought to provide a barrier that prevents interactions of other surface molecules and acts as negative regulator of cell function. As a correlate, CD43 is expected to be altered or down-regulated when blood cells are functionally activated. This study examines CD43 of blood neutrophils before and after treatment with known activating agents. Flow cytometry indicated that PMA and A23187, and to a much lesser extent, FMLP and IL-8, decrease neutrophil expression of CD43. Two separate mechanisms were identified for CD43 down-regulation. Both are proteolytic processes. PMA-induced down-regulation is a rapid process involving proteolysis at a minimum of two sites, one within the N-terminal distal region recognized by mAbs and the other at a membrane-proximal site. The PMA-induced protease, cd43' ase, is characterized by insensitivity to DFP, TLCK, leupeptin, pepstatin, and 1,10 phenanthroline (< 5 mM). PMA-induced CD43 down-regulation is extensive but never complete, terminating at approximately 10 min after down-regulating 65 to 85% of molecules, and thereby converting neutrophils from dense to sparse CD43 expression. The second CD43 down-regulation mechanism, although likely a regulated event in vivo, occurred slowly in this study in neutrophils incubated without additives; the process is not affected by PMA, involves the action of a DFP-sensitive protease, releases N-terminal mAb-reactive fragments of 52 kDa or 40 kDa and can be mimicked by exogenous neutrophil elastase. The complexity and apparent tight regulation described here for the two down-regulatory mechanisms are consistent with an important role for CD43 in preventing or dampening cell surface interactions of blood neutrophils.