In vitro evolution driven by epistasis reveals alternative cholesterol-specific binding motifs of perfringolysin O.

The crucial molecular factors that shape the interfaces of lipid-binding proteins with their target ligands and surfaces remain unknown due to the complex makeup of biological membranes. Cholesterol, the major modulator of bilayer structure in mammalian cell membranes, is recognized by various proteins, including the well-studied cholesterol-dependent cytolysins. Here, we use in vitro evolution to investigate the molecular adaptations that preserve the cholesterol specificity of perfringolysin O, the prototypical cholesterol-dependent cytolysin from Clostridium perfringens. We identify variants with altered membrane-binding interfaces whose cholesterol-specific activity exceeds that of the wild-type perfringolysin O. These novel variants represent alternative evolutionary outcomes and have mutations at conserved positions that can only accumulate when epistatic constraints are alleviated. Our results improve the current understanding of the biochemical malleability of the surface of a lipid-binding protein.

Sequestration of membrane cholesterol by cholesterol-binding proteins inhibits SARS-CoV-2 entry into Vero E6 cells.

Membrane lipids and proteins form dynamic domains crucial for physiological and pathophysiological processes, including viral infection. Many plasma membrane proteins, residing within membrane domains enriched with cholesterol (CHOL) and sphingomyelin (SM), serve as receptors for attachment and entry of viruses into the host cell. Among these, human coronaviruses, including severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), use proteins associated with membrane domains for initial binding and internalization. We hypothesized that the interaction of lipid-binding proteins with CHOL in plasma membrane could sequestrate lipids and thus affect the efficiency of virus entry into host cells, preventing the initial steps of viral infection. We have prepared CHOL-binding proteins with high affinities for lipids in the plasma membrane of mammalian cells. Binding of the perfringolysin O domain four (D4) and its variant D4E458L to membrane CHOL impaired the internalization of the receptor-binding domain of the SARS-CoV-2 spike protein and the pseudovirus complemented with the SARS-CoV-2 spike protein. SARS-CoV-2 replication in Vero E6 cells was also decreased. Overall, our results demonstrate that the integrity of CHOL-rich membrane domains and the accessibility of CHOL in the membrane play an essential role in SARS-CoV-2 cell entry.

Expansion and Neofunctionalization of Actinoporin-like Genes in Mediterranean Mussel (Mytilus galloprovincialis).

Pore-forming toxins are an important component of the venom of many animals. Actinoporins are potent cytolysins that were first detected in the venom of sea anemones; however, they are occasionally found in animals other than cnidarians and are expanded in a few predatory gastropods. Here, we report the presence of 27 unique actinoporin-like genes with monophyletic origin in Mytilus galloprovincialis, which we have termed mytiporins. These mytiporins exhibited a remarkable level of molecular diversity and gene presence-absence variation, which warranted further studies aimed at elucidating their functional role. We structurally and functionally characterized mytiporin-1 and found significant differences from the archetypal actinoporin fragaceatoxin C. Mytiporin-1 showed weaker permeabilization activity, no specificity towards sphingomyelin, and weak activity in model lipid systems with negatively charged lipids. In contrast to fragaceatoxin C, which forms octameric pores, functional mytiporin-1 pores on negatively charged lipid membranes were hexameric. Similar hexameric pores were observed for coluporin-26 from Cumia reticulata and a conoporin from Conus andremenezi. This indicates that also other molluscan actinoporin-like proteins differ from fragaceatoxin C. Although the functional role of mytiporins in the context of molluscan physiology remains to be elucidated, the lineage-specific gene family expansion event that characterizes mytiporins indicates that strong selective forces acted on their molecular diversification. Given the tissue distribution of mytiporins, this process may have broadened the taxonomic breadth of their biological targets, which would have important implications for digestive processes or mucosal immunity.

Nanoscale transformations of amphiboles within human alveolar epithelial cells.

Amphibole asbestos is related to lung fibrosis and several types of lung tumors. The disease-triggering mechanisms still challenge our diagnostic capabilities and are still far from being fully understood. The literature focuses primarily on the role and formation of asbestos bodies in lung tissues, but there is a distinct lack of studies on amphibole particles that have been internalized by alveolar epithelial cells (AECs). These internalized particles may directly interact with the cell nucleus and the organelles, exerting a synergistic action with asbestos bodies (AB) from a different location. Here we document the near-atomic- to nano-scale transformations induced by, and taking place within, AECs of three distinct amphiboles (anthophyllite, grunerite, "amosite") with different Fe-content and morphologic features. We show that: (i) an Fe-rich layer is formed on the internalized particles, (ii) particle grain boundaries are transformed abiotically by the internal chemical environment of AECs and/or by a biologically induced mineralization mechanism, (iii) the Fe-rich material produced on the particle surface does not contain large amounts of P, in stark contrast to extracellular ABs, and (iv) the iron in the Fe-rich layer is derived from the particle itself. Internalized particles and ABs follow two distinct formation mechanisms reaching different physicochemical end-states.

Nep1-like proteins as a target for plant pathogen control.

The lack of efficient methods to control the major diseases of crops most important to agriculture leads to huge economic losses and seriously threatens global food security. Many of the most important microbial plant pathogens, including bacteria, fungi, and oomycetes, secrete necrosis- and ethylene-inducing peptide 1 (Nep1)-like proteins (NLPs), which critically contribute to the virulence and spread of the disease. NLPs are cytotoxic to eudicot plants, as they disturb the plant plasma membrane by binding to specific plant membrane sphingolipid receptors. Their pivotal role in plant infection and broad taxonomic distribution makes NLPs a promising target for the development of novel phytopharmaceutical compounds. To identify compounds that bind to NLPs from the oomycetes Pythium aphanidermatum and Phytophthora parasitica, a library of 587 small molecules, most of which are commercially unavailable, was screened by surface plasmon resonance. Importantly, compounds that exhibited the highest affinity to NLPs were also found to inhibit NLP-mediated necrosis in tobacco leaves and Phytophthora infestans growth on potato leaves. Saturation transfer difference-nuclear magnetic resonance and molecular modelling of the most promising compound, anthranilic acid derivative, confirmed stable binding to the NLP protein, which resulted in decreased necrotic activity and reduced ion leakage from tobacco leaves. We, therefore, confirmed that NLPs are an appealing target for the development of novel phytopharmaceutical agents and strategies, which aim to directly interfere with the function of these major microbial virulence factors. The compounds identified in this study represent lead structures for further optimization and antimicrobial product development.

In-line detection of monoclonal antibodies in the effluent of protein A chromatography with QCM sensor.

A major drawback of the IgG capture step is the high cost of the protein A resin. For a better utilization of the resin, a continuous multi-column operation was recently proposed. In this method, accurate detection of leaking IgG is crucial to divert the breakthrough fluid from the waste to the next column and prolong the loading step without product loss. The detection of a breakthrough point as a change in UV absorption is based on a relatively small signal addition of IgGs to the bulk signal of host cell proteins. To achieve specificity, we used a quartz crystal microbalance and immobilized protein A as specific ligand on the sensor surface. We integrated the quartz crystal microbalance sensor in-line after the protein A column for real-time detection of IgGs in the breakthrough fluid. We show that this specific IgG detection in the breakthrough fluid can be more sensitive than with the UV detector. The use of the same product-specific ligand in the affinity column and in the sensor allows simultaneous in-line regeneration of column and sensor in a single step. Such a sensor could support cost-efficient load control during the entire continuous multi-column capture step in downstream processing.

Protein A affinity chromatography of Chinese hamster ovary (CHO) cell culture broths containing biopharmaceutical monoclonal antibody (mAb): Experiments and mechanistic transport, binding and equilibrium modeling.

Protein A-based affinity chromatography is a highly-efficient separation method to capture, purify and isolate biosimilar monoclonal antibodies (mAb) - an important medical product of biopharmaceutical industrial manufacturing. It is considered the most expensive step in purification downstream operations; therefore, its performance optimization offers a great cost saving in the overall production expenditure. The biochemical mixture-separating specific interaction experiments with Chinese hamster ovary (CHO) cell culture harvest, containing glycosylated extracellular immunoglobulins (Ig), were made using five different state-of-the-art commercial resins. Packing breakthrough curves were recorded at an array of prolonged residence times. A mathematical simulation model was developed, applied and validated in combination with non-linear regression algorithms on bed effluent concentrations to determine the previously-unknown binding properties of stationary phase materials. Apart from the columns' differential partitioning, the whole external system was also integrated. It was confirmed that internal pore diffusion is the global rate-limiting resistance of the compound retention process. Immobilizing substrate characteristics, obtained in this engineering study, are indispensable for the scale-up of the periodic counter-current control with mechanistic load, elution and wash reduction. Furthermore, unit's volumetric flow screening measurements revealed dynamic effect correlation to eluate quality parameters, like the presence of aggregates, the host cell-related impurities at supernatant's extended feeding, and titre. Numerical sensitivity outputs demonstrated the impacts of fluidics (e.g. axial dispersion coefficient), thermodynamics (Langmuir adsorption) and mass transfer fluxes.

Engineering a pH responsive pore forming protein.

Listeriolysin O (LLO) is a cytolysin capable of forming pores in cholesterol-rich lipid membranes of host cells. It is conveniently suited for engineering a pH-governed responsiveness, due to a pH sensor identified in its structure that was shown before to affect its stability. Here we introduced a new level of control of its hemolytic activity by making a variant with hemolytic activity that was pH-dependent. Based on detailed structural analysis coupled with molecular dynamics and mutational analysis, we found that the bulky side chain of Tyr406 allosterically affects the pH sensor. Molecular dynamics simulation further suggested which other amino acid residues may also allosterically influence the pH-sensor. LLO was engineered to the point where it can, in a pH-regulated manner, perforate artificial and cellular membranes. The single mutant Tyr406Ala bound to membranes and oligomerized similarly to the wild-type LLO, however, the final membrane insertion step was pH-affected by the introduced mutation. We show that the mutant toxin can be activated at the surface of artificial membranes or living cells by a single wash with slightly acidic pH buffer. Y406A mutant has a high potential in development of novel nanobiotechnological applications such as controlled release of substances or as a sensor of environmental pH.

How to Study Protein-protein Interactions.

Physical and functional interactions between molecules in living systems are central to all biological processes. Identification of protein complexes therefore is becoming increasingly important to gain a molecular understanding of cells and organisms. Several powerful methodologies and techniques have been developed to study molecular interactions and thus help elucidate their nature and role in biology as well as potential ways how to interfere with them. All different techniques used in these studies have their strengths and weaknesses and since they are mostly employed in in vitro conditions, a single approach can hardly accurately reproduce interactions that happen under physiological conditions. However, complementary usage of as many as possible available techniques can lead to relatively realistic picture of the biological process. Here we describe several proteomic, biophysical and structural tools that help us understand the nature and mechanism of these interactions.

A Cytolethal Distending Toxin Variant from Aggregatibacter actinomycetemcomitans with an Aberrant CdtB That Lacks the Conserved Catalytic Histidine 160.

The periodontopathogen Aggregatibacter actinomycetemcomitans synthesizes several virulence factors, including cytolethal distending toxin (CDT). The active CDT holoenzyme is an AB-type tripartite genotoxin that affects eukaryotic cells. Subunits CdtA and CdtC (B-components) allow binding and intracellular translocation of the active CdtB (A-component), which elicits nuclear DNA damage. Different strains of A. actinomycetemcomitans have diverse virulence genotypes, which results in varied pathogenic potential and disease progression. Here, we identified an A. actinomycetemcomitans strain isolated from two patients with advance chronic periodontitis that has a regular cdtABC operon, which, however, codes for a unique, shorter, variant of the CdtB subunit. We describe the characteristics of this CdtBΔ116-188, which lacks the intact nuclear localisation signal and the catalytic histidine 160. We show that the A. actinomycetemcomitans DO15 isolate secretes CdtBΔ116-188, and that this subunit cannot form a holotoxin and is also not genotoxic if expressed ectopically in HeLa cells. Furthermore, the A. actinomycetemcomitans DO15 isolate is not toxic, nor does it induce cellular distention upon infection of co-cultivated HeLa cells. Biological significance of this deletion in the cdtB remains to be explained.

Listeriolysin O Affects the Permeability of Caco-2 Monolayer in a Pore-Dependent and Ca2+-Independent Manner.

Listeria monocytogenes is a food and soil-borne pathogen that secretes a pore-forming toxin listeriolysin O (LLO) as its major virulence factor. We tested the effects of LLO on an intestinal epithelial cell line Caco-2 and compared them to an unrelated pore-forming toxin equinatoxin II (EqtII). Results showed that apical application of both toxins causes a significant drop in transepithelial electrical resistance (TEER), with higher LLO concentrations or prolonged exposure time needed to achieve the same magnitude of response than with EqtII. The drop in TEER was due to pore formation and coincided with rearrangement of claudin-1 within tight junctions and associated actin cytoskeleton; however, no significant increase in permeability to fluorescein or 3 kDa FITC-dextran was observed. Influx of calcium after pore formation affected the magnitude of the drop in TEER. Both toxins exhibit similar effects on epithelium morphology and physiology. Importantly, LLO action upon the membrane is much slower and results in compromised epithelium on a longer time scale at lower concentrations than EqtII. This could favor listerial invasion in hosts resistant to E-cadherin related infection.

Morphological impact of zinc oxide particles on the antibacterial activity and human epithelia toxicity.

ZnO nanoparticles are utilized in an ever growing number of products and can, therefore, be readily encountered in our everyday life. Human beings' outermost tissues consist of different epithelia and are, therefore, the most exposed to materials from the environment. In this paper, Caco-2 and Calu-3 cell lines were used, having been previously broadly applied for in vitro modelling of intestinal and respiratory epithelia, respectively. The toxicity of synthesized micro-, submicro- and nanoparticulate ZnO on these epithelia was measured and compared to the efficacy of the same ZnO particles as antibacterial agents. An approximately four-fold excess in antibacterial activity of ZnO nanoparticles over ZnO granulate was observed. The results of this paper reveal a sharp distinction between toxic nanoparticulate ZnO and safe ZnO particles of larger sizes in intestinal and airway in vitro epithelial models. In contrast, ZnO of larger particle sizes had only modestly lower antibacterial activity, which can be compensated for with higher dosing. These results show that nanoparticulate ZnO requires critical in vivo assessment before application.

Calu-3 model under AIC and LCC conditions and application for protein permeability studies.

Broad area of respiratory epithelium with mild surface conditions is an attractive possibility when trans-mucosal delivery of protein drugs is considered. A mucus and cellular barrier of respiratory epithelium can be modelled in vitro by Calu-3 cell line. We have monitored morphology and barrier properties of Calu-3 culture on permeable supports while developing into liquid covered or air interfaced and mucus lined cellular barrier. Besides morphological differences, cultures differed in electrical resistance and permeability to proteins as well. The accelerated permeability to proteins in these models, due to permeability modulator MP C16, was examined. The effect on electrical resistance of cellular layer was rapid in both cultures suggesting easy access of MP C16 to cells even though its overall impact on cell permeability was strongly reduced in mucus covered culture. Differences in properties of the two models enable better understanding of protein transmucosal permeability, suggesting route of transport and MP C16 modulator action.

The Caco-2 cell culture model enables sensitive detection of enhanced protein permeability in the presence of N-decyl-ß-d-maltopyranoside.

Appropriate assessment of transepithelial permeability in vitro is needed to estimate and model trans-mucosal bioavailability to achieve oral delivery of protein biopharmaceuticals. The Caco-2 cell-based intestinal epithelium model is widely used for this purpose for low molecular mass drugs. The aim of this study was to test the suitability of the Caco-2 model for assessing enhanced transepithelial permeability to proteins. Four unrelated proteins were chosen to test the permeability of Caco-2 monolayers. It was found that proteins could cross the epithelium model, in spite of their size. All tested proteins had similar very low apparent permeability coefficients (Papp) of around 4×10(-10)cm/s. Protein stability over three-hour exposure to Caco-2 was also confirmed. Their crossing rate in a cell-free setup was also measured, to determine the upper limit of permeability to proteins. An epithelium permeability enhancer N-decyl-ß-d-maltopyranoside (MP C10) was used to demonstrate accelerated permeability conditions. Papp values could be increased dose dependently up to about 1×10(-7)cm/s, close to the level in the cell-free setup, indicating distinctive potential of the model. This along with enhancing effect of known specific route permeators suggests involvement of the paracellular route in protein transport. Our results thus indicate that the Caco-2 model is a suitable tool for in vitro assessment of enhanced permeability to proteins.

Cysteine-specific PEGylation of rhG-CSF via selenylsulfide bond.

A new PEGylation reagent enabling selective modification of free thiol groups is described in this article. The reagent was synthesized by attaching linear polyethylene glycol (PEG) N-hydroxysuccinimide to selenocystamine. The reaction was very fast, resulting in over 95% conversion yield. The active group of this new PEG-Se reagent is a diselenide, reacting with thiols via thiol/diselenide exchange reaction. Recombinant human granulocyte colony-stimulating factor (rhG-CSF) with an unpaired cysteine at the position 18 (Cys18) was used as a model protein. It was comparatively PEGylated with the new PEG-Se reagent, as well as with commercially available maleimide (PEG-Mal) and ortho-pyridyl disulfide (PEG-OPSS) PEG reagents. The highest PEGylation yield was obtained with PEG-Mal, followed by PEG-OPSS and PEG-Se. The reaction rates of PEG-Mal and PEG-Se were comparable, while the reaction rate of PEG-OPSS was lower. Purified monoPEGylated rhG-CSF conjugates were characterized and compared. Differences in activity, stability, and in vivo performance were observed, although all conjugates contained a 20 kDa PEG attached to the Cys18. Minor conformational changes were observed in the conjugate prepared with PEG-Mal. These changes were also reflected in low in vitro biological activity and aggregate formation of the maleimide conjugate. The conjugate prepared with PEG-Se had the highest in vitro biological activity, while the conjugate prepared with PEG-OPSS had the best in vivo performance.

The Characterization and Potential use of G-CSF Dimers and their Pegylated Conjugates.

G-CSF successfully prevents chemotherapy-induced neutropenia. Two second-generation drugs with improved therapeutic properties are already available and the development of new forms is still ongoing. For an efficient receptor dimerization two G-CSF molecules have to bind. Development of G-CSF dimers acting as receptor dimerizers was explored and their potential use evaluated. The in vitro biological activities of the prepared dimers were lower than G-CSF monomer activity, presumably due to non-optimal spatial orientation of the molecules. Most likely two dimers had to bind to trigger receptor dimerization instead of one dimer acting as a dimerizer. Although significantly lower in the residual in vitro biological activity, the diPEG-Fdim conjugate exhibited pharmacokinetical (PK) and pharmacodynamical (PD) properties comparable to pegfilgrastim or even better. An interesting PD profile with the second maximum in absolute neutrophil count (ANC) and a balanced elevated ANC profile over the longer time interval was namely observed.

Correlations between in vitro potency of polyethylene glycol-protein conjugates and their chromatographic behavior.

Pegylation is the most widely used and accepted methodology for half-life extension of biopharmaceutical drugs that also improves physicochemical and biological characteristics of proteins considerably. Most of the positive pharmacological effects of pegylated proteins are believed to be related to an increased hydrodynamic volume and molecular size. To explore the size impact of polyethylene glycol (PEG) on in vitro potency, a series of well-defined conjugates of interferon alpha-2b (IFN) were prepared with PEGs of different lengths and shapes specifically attached to the N-terminal amino group of the protein. Specificity of the attachment was confirmed by peptide mapping and mass spectroscopy. When potency values determined by reporter gene assay were correlated with methods for molecular weight and size characterization, such as size exclusion chromatography and dynamic light scattering, rough parallels were found. Unexpectedly, the retention times on cation exchange chromatography showed much higher correlation with experimentally determined in vitro potency. It appears that in a series of N-terminally pegylated IFNs, their in vitro potency could be predicted from the retention times on the cation exchange chromatography columns, probably because both methods reflect not only the influence of molecular size but also the impact of protein masking exerted by attached PEG moiety.

Thermal stability of the WHO international standard of interferon alpha 2b (IFN-alpha 2b): application of new reporter gene assay for IFN-alpha 2b potency determinations.

A World Health Organization requirement for biological standards is that they should exhibit long-term stability at their recommended storage temperature. Thermal stability is usually predicted in accelerated thermal degradation studies, where ampoules of the lyophilized standard are stored at elevated temperatures for relatively short times before testing. To confirm the predicted thermal stability of the 2nd international standard of human interferon alpha 2b (IFN-alpha2b; 95/566), we tested the potency of the ampouled contents of this standard after 9 years storage at the customary storage temperature of -20 degrees C in comparison with ampoules of the IS which had been stored continuously at temperatures ranging from -150 degrees C to 56 degrees C. Since IFN-alpha2b potency estimates derived from the results of antiviral assays (AVA) showed high within-assay variability, we investigated a novel reporter gene assay (RGA) based on induction of secreted alkaline phosphatase (SEAP) for comparability and precision of such estimations. We show that this RGA generated comparable estimates with overall lower variation. Additionally, the SEAP conversion of p-nitrophenyl phosphate to yellow product could be followed kinetically. Absorbance readings were shown to increase with time in proportion with increasing concentration of IFN-alpha2b. When the time-dependent increments of absorbance were plotted graphically, the slopes of lines corresponded to concentration. This approach enabled single dilutions of IFN samples, identical in molecular structure to an IFN-alpha2b standard, to be used for potency estimates by interpolation of slope value against those of the standard at fixed concentrations. It appears attractive for high through-put potency testing of various R&D IFN-alpha2b samples.

Hereditary medullary thyroid cancer in Slovenia--genotype-phenotype correlations.

BACKGROUND: Medullary thyroid cancer (MTC) is a rare endocrine tumor that may be sporadic or inherited in settings of MEN2A, MEN2B and FMTC. Germline point mutations in the RET proto-oncogene are responsible for tumor occurrence, inheritance and great clinical variability. The aim of this study was to correlate the genotype and phenotype of patients with hereditary MTC (age at diagnosis, sex, TNM classification and clinical features). PATIENTS: Between 1997 and 2003 genetic testing was performed in 69 out of 98 patients with "sporadic" MTC. Carriage of mutation was found in 14 (20.2%) patients (index patients) and in 16 out of 31 (51.6%) of their relatives. One patient with MEN2B and codon 918 mutation was excluded from further analysis. METHODS: Genomic DNA was isolated from peripheral blood leukocytes. Exons 10, 11, 13, 14, 15 and 16 of the RET proto-oncogene were amplified in polymerase chain reactions. Point mutations of the RET gene were detected with single-strand conformation analysis and DNA sequencing. Detected mutations were confirmed with restriction enzyme analysis. RESULTS: Codon 634 mutations were detected in 15 patients (50%; aged 18-76 years; 6 families), codon 618 in nine patients (30%; aged 12-65 years; 4 families) and codon 790 in five patients (16.6%; aged 16-74 years; 3 families). The median age at diagnosis was 31 +/- 17.3, 33 +/- 15.9 and 36 +/- 23.8 years for patients with codon 618, 634 and 790 mutations. Selected by sex, females with codon mutations 618 and 634 versus 790 had median age at diagnosis of 34.5 +/- 15.6 years and 43.5 +/- 22.9 years, whereas the inverse result was observed in males (26.5 +/- 18.0 versus 16 years). The male/female ratio was 1:2 for patients with codon 618 and 634 mutations and 1:4 for patients with codon 790 mutations. Some of the data suggested correlation between specific genotypes, tumor size, stage of MTC and age at diagnosis. Pheochromocytoma (12 out of 15 patients) and primary hyperparathyroidism (6 out of 15 patients) were diagnosed solely in patients with codon 634 mutations. One patient with FMTC and Hirschprung disease was found in a family with codon 618 mutations. CONCLUSION: Correlation between tumor size, stage of MTC at diagnosis in view of patient's age, and specific genotype were indicated in our limited series and were more evident in female patients with codon 790 mutations. Later onset and a probably less aggressive course of MTC in these patients than in patients with other mutations should be considered in planning prophylactic thyroid surgery. MEN2A syndrome was related solely to codon 634 mutations.

Cathepsin L splice variants in human breast cell lines.

Transcripts of the lysosomal cysteine proteinase cathepsin L are spliced into five variants (L-A, L-A1, LA-2, LA-3 and L-B), which have similar stabilities but different translation efficiencies, thus potentially yielding diverse amounts of the protein. The aim of this study was to investigate whether the abnormally elevated expression of cathepsin L in invasive tumours is due to overexpression of L-A3, the splice variant translated most efficiently. The expression pattern of cathepsin L mRNA variants was measured by quantitative polymerase chain reaction (qPCR) in two panels of cell lines obtained from precancerous and cancerous breast tissue. In both panels, the cell line exhibiting the highest in vitro invasiveness also expressed the highest amount of L-A3. Although the pattern of expression varied, the expression of the L-B variant was always remarkably lower than for other variants. We propose that posttranscriptional regulation of cathepsin L expression is altered during breast tumour progression.