Isolation and characterization of yeast mutants in the cytoplasm to vacuole protein targeting pathway

In Saccharomyces cerevisiae the vacuolar protein aminopeptidase I (API) is localized to the vacuole independent of the secretory pathway. The alternate targeting mechanism used by this protein has not been characterized. API is synthesized as a 61-kD soluble cytosolic precursor. Upon delivery to the vacuole, the amino-terminal propeptide is removed by proteinase B (PrB) to yield the mature 50-kD hydrolase. We exploited this delivery-dependent maturation event in a mutant screen to identify genes whose products are involved in API targeting. Using antiserum to the API propeptide, we isolated mutants that accumulate precursor API. These mutants, designated cvt, fall into eight complementation groups, five of which define novel genes. These five complementation groups exhibit a specific defect in maturation of API, but do not have a significant effect on vacuolar protein targeting through the secretory pathway. Localization studies show that precursor API accumulates outside of the vacuole in all five groups, indicating that they are blocked in API targeting and/or translocation. Future analysis of these gene products will provide information about the subcellular components involved in this alternate mechanism of vacuolar protein localization.     

Aminopeptidase I of Saccharomyces cerevisiae is localized to the vacuole independent of the secretory pathway

The Saccharomyces cerevisiae APE1 gene product, aminopeptidase I (API), is a soluble hydrolase that has been shown to be localized to the vacuole. API lacks a standard signal sequence and contains an unusual amino-terminal propeptide. We have examined the biosynthesis of API in order to elucidate the mechanism of its delivery to the vacuole. API is synthesized as an inactive precursor that is matured in a PEP4-dependent manner. The half-time for processing is approximately 45 min. The API precursor remains in the cytoplasm after synthesis and does not enter the secretory pathway. The precursor does not receive glycosyl modifications, and removal of its propeptide occurs in a sec-independent manner. Neither the precursor nor mature form of API are secreted into the extracellular fraction in vps mutants or upon overproduction, two additional characteristics of soluble vacuolar proteins that transit through the secretory pathway. Overproduction of API results in both an increase in the half-time of processing and the stable accumulation of precursor protein. These results suggest that API enters the vacuole by a posttranslational process not used by most previously studied resident vacuolar proteins and will be a useful model protein to analyze this alternative mechanism of vacuolar localization.     

The prepropeptide of vacuolar aminopeptidase I is necessary and sufficient to target the fluorescent reporter protein GFP to the vacuole of yeast by the Cvt pathway

We have studied the capacity of the prepro amino extension of vacuolar protease leucine aminopeptidase I (API) to target the fluorescent reporter protein GFP to the vacuole of yeast. The preproGFP chimera constructed by extending the amino end of GFP with the prepro-part of API is rapidly degraded in both wild-type WCG cells and WCG 11/21a cells deficient in the proteasome. In contrast, the chimera expressed in WCG-PP cells deficient in both proteasome activity and vacuolar proteinase A accumulates in the vacuole, where it remains stable. Replacement of Gly by Ile-7, a substitution that prevents folding of the pre-part into an amphipathic helix and inhibits the targeting of the API precursor to the vacuole, inhibits the targeting of preproGFP to the vacuole. The separated pre- and pro-parts of the API precursor do not target GFP to the vacuole. Targeting of preproGFP to the vacuole is independent of its levels of expression, as the fluorescent protein localizes to the vacuole in cells expressing the protein under the control of both the GAL 1/10 or the API promoter. The preproGFP expressed under both promoters is recovered as monomers from cytosolic cell extracts. PreproGFP expressed under the API promoter is packed into cytoplasmic bodies that penetrate into the vacuolar lumen to release the protein. Altogether our results show that the prepro-part of the API precursor is necessary and sufficient to target the green fluorescent reporter protein to the vacuole.     

Vacuolar localization of oligomeric alpha-mannosidase requires the cytoplasm to vacuole targeting and autophagy pathway components in Saccharomyces cerevisiae

One challenge facing eukaryotic cells is the post-translational import of proteins into organelles. This problem is exacerbated when the proteins assemble into large complexes. Aminopeptidase I (API) is a resident hydrolase of the vacuole/lysosome in the yeast Saccharomyces cerevisiae. The precursor form of API assembles into a dodecamer in the cytosol and maintains this oligomeric form during the import process. Vacuolar delivery of the precursor form of API requires a vesicular mechanism termed the cytoplasm to vacuole targeting (Cvt) pathway. Many components of the Cvt pathway are also used in the degradative autophagy pathway. alpha-Mannosidase (Ams1) is another resident hydrolase that enters the vacuole independent of the secretory pathway; however, its mechanism of vacuolar delivery has not been established. We show vacuolar localization of Ams1 is blocked in mutants that are defective in the Cvt and autophagy pathways. We have found that Ams1 forms an oligomer in the cytoplasm. The oligomeric form of Ams1 is also detected in subvacuolar vesicles in strains that are blocked in vesicle breakdown, indicating that it retains its oligomeric form during the import process. These results identify Ams1 as a second biosynthetic cargo protein of the Cvt and autophagy pathways.     

In vitro reconstitution of cytoplasm to vacuole protein targeting in yeast

Although the majority of known vacuolar proteins transit through the secretory pathway, two vacuole-resident proteins have been identified that reach this organelle by an alternate pathway. These polypeptides are targeted to the vacuole directly from the cytoplasm by a novel import mechanism. The best characterized protein that uses this pathway is aminopeptidase I (API). API is synthesized as a cytoplasmic precursor containing an amino-terminal propeptide that is cleaved off when the protein reaches the vacuole. To dissect the biochemistry of this pathway, we have reconstituted the targeting of API in vitro in a permeabilized cell system. Based on several criteria, the in vitro import assay faithfully reconstitutes the in vivo reaction. After incubation under import conditions, API is processed by a vacuolar- resident protease, copurifies with a vacuole-enriched fraction, and becomes inaccessible to the cytoplasm. These observations demonstrate that API has passed from the cytoplasm to the vacuole. The reconstituted import process is dependent on time, temperature, and energy. ATP gamma S inhibits this reaction, indicating that API transport is ATP driven. API import is also inhibited by GTP gamma S, suggesting that this process may be mediated by a GTP-binding protein. In addition, in vitro import requires a functional vacuolar ATPase; import is inhibited both in the presence of the specific V-ATPase inhibitor bafilomycin A1, and in a yeast strain in which one of the genes encoding a V-ATPase subunit has been disrupted.     

Colocalization of barley lectin and sporamin in vacuoles of transgenic tobacco plants.

Various targeting motifs have been identified for plant proteins delivered to the vacuole. For barley (Hordeum vulgare) lectin, a typical Gramineae lectin and defense-related protein, the vacuolar information is contained in a carboxyl-terminal propeptide. In contrast, the vacuolar targeting information of sporamin, a storage protein from the tuberous roots of the sweet potato (Ipomoea batatas), is encoded in an amino-terminal propeptide. Both proteins were expressed simultaneously in transgenic tobacco plants to enable analysis of their posttranslational processing and subcellular localization by pulse-chase labeling and electron-microscopic immunocytochemical methods. The pulse-chase experiments demonstrated that processing and delivery to the vacuole are not impaired by the simultaneous expression of barley lectin and sporamin. Both proteins were targeted quantitatively to the vacuole, indicating that the carboxyl-terminal and amino-terminal propeptides are equally recognized by the vacuolar protein-sorting machinery. Double-labeling experiments showed that barley lectin and sporamin accumulate in the same vacuole of transgenic tobacco (Nicotiana tabacum) leaf and root cells.     

Structure of yeast Ape1 and its role in autophagic vesicle formation

In Saccharomyces cerevisiae, a constitutive biosynthetic transport pathway, termed the cytoplasm-to-vacuole targeting (Cvt) pathway, sequesters precursor aminopeptidase I (prApe1) dodecamers in the form of a large complex into a Cvt vesicle using autophagic machinery, targeting it into the vacuole (the yeast lysosome) where it is proteolytically processed into its mature form, Ape1, by removal of an amino-terminal 45-amino acid propeptide. prApe1 is thought to serve as a scaffolding cargo critical for the assembly of the Cvt vesicle by presenting the propeptide to mediate higher-ordered complex formation and autophagic receptor recognition. Here we report the X-ray crystal structure of Ape1 at 2.5 Å resolution and reveal its dodecameric architecture consisting of dimeric and trimeric units, which associate to form a large tetrahedron. The propeptide of prApe1 exhibits concentration-dependent oligomerization and forms a stable tetramer. Structure-based mutagenesis demonstrates that disruption of the inter-subunit interface prevents dodecameric assembly and vacuolar targeting in vivo despite the presence of the propeptide. Furthermore, by examining the vacuolar import of propeptide-fused exogenous protein assemblies with different quaternary structures, we found that 3-dimensional spatial distribution of propeptides presented by a scaffolding cargo is essential for the assembly of the Cvt vesicle for vacuolar delivery. This study describes a molecular framework for understanding the mechanism of Cvt or autophagosomal biogenesis in selective macroautophagy.     

Yeast carboxypeptidase Y vacuolar targeting signal is defined by four propeptide amino acids

The amino-terminal propeptide of carboxypeptidase Y (CPY) is necessary and sufficient for targeting this glycoprotein to the vacuole of Saccharomyces cerevisiae. A 16 amino acid stretch of the propeptide was subjected to region-directed mutagenesis using randomized oligonucleotides. Mutations altering any of four contiguous amino acids, Gln-Arg-Pro-Leu, resulted in secretion of the encoded CPY precursor (proCPY), demonstrating that these residues form the core of the vacuolar targeting signal. Cells that simultaneously synthesize both wild-type and sorting-defective forms of proCPY efficiently sort and deliver only the wild-type molecule to the vacuole. These results indicate that the PRC1 missorting mutations are cis-dominant, implying that the mutant forms of proCPY are secreted as a consequence of failing to interact with the sorting apparatus, rather than a general poisoning of the vacuolar protein targeting system.     

Transport of a Large Oligomeric Protein by the Cytoplasm to Vacuole Protein Targeting Pathway

Aminopeptidase I (API) is transported into the yeast vacuole by the cytoplasm to vacuole targeting (Cvt) pathway. Genetic evidence suggests that autophagy, a major degradative pathway in eukaryotes, and the Cvt pathway share largely the same cellular machinery. To understand the mechanism of the Cvt import process, we examined the native state of API. Dodecameric assembly of precursor API in the cytoplasm and membrane binding were rapid events, whereas subsequent vacuolar import appeared to be rate limiting. A unique temperature-sensitive API-targeting mutant allowed us to kinetically monitor its oligomeric state during translocation. Our findings indicate that API is maintained as a dodecamer throughout its import and will be useful to study the posttranslational movement of folded proteins across biological membranes.     

Identification and characterization of a prevacuolar compartment in stigmas of nicotiana alata

The stigmas of the ornamental tobacco plant Nicotiana alata accumulate large quantities of a series of 6-kD proteinase inhibitors (PIs) in the central vacuole that are derived from a 40-kD precursor protein, Na-PI. The sorting information that directs Na-PI to the vacuole is likely to reside in a C-terminal propeptide domain of 25 amino acids that forms an amphipathic alpha helix. Using cell fractionation techniques, we have examined transit of Na-PI through the endomembrane system and have identified a prevacuolar compartment that contains Na-PI with an intact targeting signal. In contrast, the targeting signal is not present on the predominant form of Na-PI in the vacuole. The prevacuolar compartment is marked by the presence of homologs of both the t-SNARE, PEP12p, and the putative vacuolar sorting receptor BP-80. Cross-linking and affinity precipitation studies revealed that Na-PI associates with BP-80 within this compartment, providing in vivo evidence for the function of BP-80 as a sorting receptor for a protein with a C-terminal vacuolar targeting signal.     

Protein sorting in yeast: the localization determinant of yeast vacuolar carboxypeptidase Y resides in the propeptide

We have isolated cis-acting mutations in the gene encoding the yeast vacuolar protein carboxypeptidase Y (CPY) that result in missorting and aberrant secretion of up to 95% of newly synthesized CPY. The CPY polypeptides synthesized by these mutants use the late secretory pathway to exit the cell, since the late-acting sec1 mutation prevents their secretion. The mutant versions of CPY are secreted as the proCPY zymogen and are enzymatically activatable in vivo and in vitro. All the mutations, including small deletions and an amino acid substitution, map to the amino-terminal propeptide region and define a discrete yeast vacuolar localization domain whose integrity is required for efficient sorting of the CPY zymogen. Thus, the N-terminal propeptide of CPY carries out at least three functions: it mediates translocation across the endoplasmic reticulum, renders the enzyme inactive during transit, and targets the molecule to the vacuole.     

Transport of proteins to the yeast vacuole: autophagy, cytoplasm-to-vacuole targeting, and role of the vacuole in degradation

The vacuole/lysosome performs a central role in degradation. Proteins and organelles are transported to the vacuole by selective and non-selective pathways. Transport to the vacuole by autophagy is the primary mode for degradation of cytoplasmic constituents under starvation conditions. Autophagy overlaps mechanistically and genetically with a biosynthetic pathway termed Cvt (Cytoplasm-to-vacuole targeting) that operates under vegetative conditions to transport the resident vacuolar hydrolase aminopeptidase I (API). API import has been dissected to reveal the action of a novel mechanism that transports cargo within double-membrane vesicles. Recent work has uncovered molecular components involved in autophagy and the Cvt pathway.     

Cytoplasm to vacuole trafficking of aminopeptidase I requires a t-SNARE-Sec1p complex composed of Tlg2p and Vps45p.

Aminopeptidase I (API) is imported into the yeast vacuole/lysosome by a constitutive non-classical vesicular transport mechanism, the cytoplasm to vacuole targeting (Cvt) pathway. Newly synthesized precursor API is sequestered in double-membrane cytoplasmic Cvt vesicles. The Cvt vesicles fuse with the vacuole, releasing single-membrane Cvt bodies containing proAPI into the vacuolar lumen, and maturation of API occurs when the Cvt body is degraded, releasing mature API. Under starvation conditions, API is transported to the vacuole by macroautophagy, an inducible, non-selective mechanism that shares many similarities with the Cvt pathway. Here we show that Tlg2p, a member of the syntaxin family of t-SNARE proteins, and Vps45p, a Sec1p homologue, are required in the constitutive Cvt pathway, but not in inducible macroautophagy. Fractionation and protease protection experiments indicate that Tlg2p is required prior to or at the step of API segregation into the Cvt vesicle. Thus, the early Vps45-Tlg2p-dependent step of the Cvt pathway appears to be mechanistically distinct from the comparable stage in macroautophagy. Vps45p associates with both the Tlg2p and Pep12p t-SNAREs, but API maturation is not blocked in a pep12(ts) mutant, indicating that Vps45p independently regulates the function of multiple t-SNARES at distinct trafficking steps.     

Proaleurain vacuolar targeting is mediated by short contiguous peptide interactions.

Targeting of soluble proteins to the plant vacuole is mediated by determinants that reside in the polypeptide. We identified the vacuolar targeting determinant of aleurain, a plant vacuolar thiol protease, by incorporating different sequences from proaleurain into the secreted thiol protease, proendoproteinase B (proEP-B), and vice versa. The targeting fates of the chimeric proteins were analyzed by transient expression in electroporated tobacco protoplasts. The targeting determinant SSSSFADSNPIR is positioned at the N terminus of the aleurain propeptide, and its substitution into the propeptide of EP-B caused vacuolar targeting of the resulting chimeric protein. This determinant can be divided into two smaller determinants, SSSSFADS and SNPIR, each of which is sufficient to target proEP-B chimeras to the vacuole, but with lower efficiency. These smaller determinants interact in a positive manner because the combined determinant SSSSFADSNPIR targeted proEP-B with an efficiency greater than each of the smaller determinants alone. Accordingly, the efficiency of aleurain targeting was decreased when either of the smaller determinants was disrupted by replacement with similarly positioned proEP-B sequences. Further experiments on proaleurain identified an additional determinant, VTDRAAST, adjacent to the SSSSFADSNPIR determinant that is also necessary for efficient vacuolar targeting. Our results provide evidence that efficient vacuolar targeting of this thiol protease in plant cells is mediated by the combined action of smaller contiguous determinants; two of these alone are sufficient for vacuolar targeting.     

Determination of the functional elements within the vacuolar targeting signal of barley lectin.

We have previously demonstrated that the carboxyl-terminal propeptide of barley lectin is both necessary and sufficient for protein sorting to the plant vacuole. Specific mutations were constructed to determine which amino acid residues or secondary structural determinants of the carboxyl-terminal propeptide affect proper protein sorting. We have found that no consensus sequence or common structural determinants are required for proper sorting of barley lectin to the vacuole. However, our analysis demonstrated the importance of hydrophobic residues in vacuolar targeting. In addition, at least three exposed amino acid residues are necessary for efficient sorting. Sorting was disrupted by the addition of two glycine residues at the carboxyl-terminal end of the targeting signal or by the translocation of the glycan to the carboxy terminus of the propeptide. These results suggest that some components of the sorting apparatus interact with the carboxy terminus of the propeptide.     

Aspartyl aminopeptidase is imported from the cytoplasm to the vacuole by selective autophagy in Saccharomyces cerevisiae

Macroautophagy is a catabolic process by which cytosolic components are sequestered by double membrane vesicles called autophagosomes and sorted to the lysosomes/vacuoles to be degraded. Saccharomyces cerevisiae has adapted this mechanism for constitutive transport of the specific vacuolar hydrolases aminopeptidase I (Ape1) and α-mannosidase (Ams1); this process is called the cytoplasm to vacuole targeting (Cvt) pathway. The precursor form of Ape1 self-assembles into an aggregate-like structure in the cytosol that is then recognized by Atg19 in a propeptide-dependent manner. The interaction between Atg19 and autophagosome-forming machineries allows selective packaging of the Ape1-Atg19 complex by the autophagosome-like Cvt vesicle. Ams1 also forms oligomers and utilizes the Ape1 transport system by interacting with Atg19. Although the mechanism of selective transport of the Cvt cargoes has been well studied, it is unclear whether proteins other than Ape1 and Ams1 are transported via the Cvt pathway. We describe here that aspartyl aminopeptidase (Yhr113w/Ape4) is the third Cvt cargo, which is similar in primary structure and subunit organization to Ape1. Ape4 has no propeptide, and it does not self-assemble into aggregates. However, it binds to Atg19 in a site distinct from the Ape1- and Ams1-binding sites, allowing it to "piggyback" on the Ape1 transport system. In growing conditions, a small portion of Ape4 localizes in the vacuole, but its vacuolar transport is accelerated by nutrient starvation, and it stably resides in the vacuole lumen. We propose that the cytosolic Ape4 is redistributed to the vacuole when yeast cells need more active vacuolar degradation.     

Yol082p, a novel CVT protein involved in the selective targeting of aminopeptidase I to the yeast vacuole

The yeast vacuolar enzyme aminopeptidase I (API) is synthesized in the cytoplasm as a precursor (pAPI). Upon its assembly into dodecamers, pAPI is wrapped by double-membrane saccular structures for its further transport within vesicles that fuse with the vacuolar membrane and release their content in the vacuolar lumen. Targeting of API to the vacuole occurs by two alternative transport routes, the cvt and the autophagy pathways, which although mechanistically similar specifically operate under vegetative growth or nitrogen starvation conditions, respectively. We have studied the role of Yol082p, a protein identified by its ability to interact with API, in the transport of its precursor to the vacuole. We show that Yol082p interacts with mature API, an interaction that is strengthened by the amino extension of the API protein. Yol082p is required for targeting of pAPI to the vacuole, both under growing and short term nitrogen starvation conditions. Absence of Yol082p does not impede the assembly of pAPI into dodecamers, but precludes the enclosure of pAPI within transport vesicles. Microscopy studies show that during vegetative growth Yol082p is distributed between a cytoplasmic pool and a variable number of 0.13--0.27-microm round, mobile structures, which are no longer observed under conditions of nitrogen starvation, and become larger in cells expressing the inactive Yol082 Delta C32p, or lacking Apg12p. In contrast to the autophagy mutants involved in API transport, a Delta yol082 strain does not lose viability under nitrogen starvation conditions, indicating normal function of the autophagy pathway. The data are consistent with a role of Yol082p in an early step of the API transport, after its assembly into dodecamers. Because Yol082p fulfills the functional requisites that define the CVT proteins, we propose to name it Cvt19.     

Prevacuolar compartment morphology in vps mutants of Saccharomyces cerevisiae

Over 60 genes have been identified that affect protein sorting to the lysosome-like vacuole in Saccharomyces cerevisiae. Cells with mutations in these vacuolar protein sorting (vps) genes fall into seven general classes based upon their vacuolar morphology. Class A mutants have a morphologically wild type vacuole, while Class B mutants have a fragmented vacuole. There is no discernable vacuolar structure in Class C mutants. Class D mutants have a slightly enlarged vacuole, but Class E mutants have a normal looking vacuole with an enlarged prevacuolar compartment (PVC), which is analogous to the mammalian late endosome. Class F mutants have a wild type appearing vacuole as well as fragmented vacuolar structures. vps mutants have also been found with a tubulo-vesicular vacuole structure. vps mutant morphology is pertinent, as mutants of the same class may work together and/or have a block in the same general step in the vacuolar protein sorting pathway. We probed PVC morphology and location microscopically in live cells of several null vps mutants using a GFP fusion protein of Nhx1p, an Na(+)/H(+) exchanger normally localized to the PVC. We show that cell strains deleted for VPS proteins that have been previously shown to work together, regardless of VPS Class, have the same PVC morphology. Cell strains lacking VPS genes that have not been implicated in the same pathway show different PVC morphologies, even if the mutant strains are in the same VPS Class. These new studies indicate that PVC morphology is another tier of classification that may more accurately identify proteins that function together in vacuolar protein sorting than the original vps mutation classes.     

The Membrane Protein Alkaline Phosphatase Is Delivered to the Vacuole by a Route That Is Distinct from the VPS-dependent Pathway

Membrane trafficking intermediates involved in the transport of proteins between the TGN and the lysosome-like vacuole in the yeast Saccharomyces cerevisiae can be accumulated in various vps mutants. Loss of function of Vps45p, an Sec1p-like protein required for the fusion of Golgi-derived transport vesicles with the prevacuolar/endosomal compartment (PVC), results in an accumulation of post-Golgi transport vesicles. Similarly, loss of VPS27 function results in an accumulation of the PVC since this gene is required for traffic out of this compartment.

The vacuolar ATPase subunit Vph1p transits to the vacuole in the Golgi-derived transport vesicles, as defined by mutations in VPS45, and through the PVC, as defined by mutations in VPS27. In this study we demonstrate that, whereas VPS45 and VPS27 are required for the vacuolar delivery of several membrane proteins, the vacuolar membrane protein alkaline phosphatase (ALP) reaches its final destination without the function of these two genes. Using a series of ALP derivatives, we find that the information to specify the entry of ALP into this alternative pathway to the vacuole is contained within its cytosolic tail, in the 13 residues adjacent to the transmembrane domain, and loss of this sorting determinant results in a protein that follows the VPS-dependent pathway to the vacuole.

Using a combination of immunofluorescence localization and pulse/chase immunoprecipitation analysis, we demonstrate that, in addition to ALP, the vacuolar syntaxin Vam3p also follows this VPS45/27-independent pathway to the vacuole. In addition, the function of Vam3p is required for membrane traffic along the VPS-independent pathway.

     

The role of the cathepsin D propeptide in sorting to the lysosome.

The propeptides of lysosomal enzymes have been implicated in membrane association and mannose 6-phosphate-independent sorting to the lysosome (Rijnboutt, S., Aerts, H., Geuze, H. J., Tager, J. M., and Strous, G. J. (1991) J. Biol. Chem. 266, 4862-4868; McIntyre, G. F., and Erickson, A. H. (1991) J. Biol. Chem. 266, 15438-15445). In this report, the function of the propeptide of procathepsin D in sorting to the lysosome was directly assessed using a cathepsin D deletion mutant lacking the propeptide, and using a chimeric cDNA encoding the cathepsin D propeptide fused to the secretory protein alpha-lactalbumin. Proteins encoded by these cDNAs were expressed in mouse Ltk- cells and in human hepatoma Hep G2 cells, and then immunoprecipitated and analyzed by SDS-polyacrylamide gel electrophoresis. The deletion mutant was glycosylated but was rapidly degraded in a chloroquine-independent fashion and did not assume an active conformation. Thus the propeptide appeared to be necessary for correct folding. The chimeric protein was glycosylated and secreted. The coincidence of complex oligosaccharide modification and secretion of the chimeric protein suggested that it was slowly released from the endoplasmic reticulum and rapidly passed through the cell to the extracellular compartment. This was confirmed by immunofluorescent localization of the proteins. The data indicated that the propeptide appeared to be necessary for folding of cathepsin D but, unlike the yeast vacuolar propeptides, was not sufficient to direct a secretory protein to the lysosome in fibroblasts or in epithelial cells.