Translocation of proteins across the endoplasmic reticulum III. Signal recognition protein (SRP) causes signal sequence-dependent and site-specific arrest of chain elongation that is released by microsomal membranes

The previously observed (Walter, et al. 1981 J. Cell Biol. 91:545-550) inhibitory effect of SRP selectively on the cell-free translation of mRNA for secretory protein (preprolactin) was shown here to be caused by a signal sequence-induced and site-specific arrest in polypeptide chain elongation. The Mr of the SRP-arrested nascent preprolactin chain was estimated to be 8,000 corresponding to approximately 70 amino acid residues. Because the signal sequence of preprolactin comprises 30 residues and because approximately 40 residues of the nascent chain are buried (protected from protease) in the large ribosomal subunit, we conclude that it is the interaction of SRP with the amino-terminal signal peptide of the nascent chain (emerged from the large ribosomal subunit) that modulates translation and thereby causes an arrest in chain elongation. This arrest is released upon SRP-mediated binding of the elongation-arrested ribosomes to the microsomal membrane, resulting in chain completion and translocation into the microsomal vesicle.

Translocation of proteins across the endoplasmic reticulum. II. Signal recognition protein (SRP) mediates the selective binding to microsomal membranes of in-vitro-assembled polysomes synthesizing secretory protein

Translocation-competent microsomal membrane vesicles of dog pancreas were shown to selectively bind nascent, in vitro assembled polysomes synthesizing secretory protein (bovine prolactin) but not those synthesizing cytoplasmic protein (alpha and beta chain of rabbit globin). This selective polysome binding capacity was abolished when the microsomal vesicles were salt-extracted but was restored by an 11S protein (SRP, Signal Recognition Protein) previously purified from the salt-extract of microsomal vesicles (Walter and Blobel, 1980. Proc. Natl. Acad. Sci. U. S. A. 77:7112-7116). SRP-dependent polysome recognition and binding to the microsomal membrane was shown to be a prerequisite for chain translocation. Modification of SRP by N-ethyl maleimide abolished its ability to mediate nascent polysome binding to the microsomal vesicles. Likewise, polysome binding to the microsomal membrane was largely abolished when beta-hydroxy leucine, a Leu analogue, was incorporated into nascent secretory polypeptides. The data in this and the preceding paper provide conclusive experimental evidence that chain translocation across the endoplasmic reticulum membrane is a receptor-mediated event and thus rule out proposals that chain translocation occurs spontaneously and without the mediation by proteins. Moreover, our data here demonstrate conclusively that the initial events that lead to translocation and provide for its specificity are protein-protein (signal sequence plus ribosome with SRP) and not protein-lipid (signal sequence with lipid bilayer) interactions.

Human growth hormone: complementary DNA cloning and expression in bacteria

The nucleotide sequence of a DNA complementary to human growth hormone messenger RNA was cloned; it contains 29 nucleotides in its 5' untranslated region, the 651 nucleotides coding for the prehormone, and the entire 3' untranslated region (108 nucleotides). The data reported predict the previously unknown sequence of the signal peptide of human growth hormone and, by comparison with the previously determined sequences of rat growth hormone and human chorionic somatomammotropin, strengthens the hypothesis that these genes evolved by gene duplication from a common ancestral sequence. The human growth hormone gene sequences have been linked in phase to a fragment of the trp D gene of Escherichia coli in a plasmid vehicle, and a fusion protein is synthesized at high level (approximately 3 percent of bacterial protein) under the control of the regulatory region of the trp operon. This fusion protein (70 percent of whose amino acids are coded for by the human growth hormone gene) reacts specifically with antibodies to human growth hormone and is stable in E. coli.

Ribosome pausing and stacking during translation of a eukaryotic mRNA

We have devised a sensitive assay to determine the distribution of translating ribosomes on a mRNA. Using this assay to monitor ribosome transit on bovine preprolactin mRNA, we have detected four major positions of ribosome pausing in both wheat-germ and rabbit reticulocyte extracts. Two of these rate-limiting steps represent initiation and termination. One pause occurs after approximately 75 amino acids have been polymerized; signal recognition particle arrests preprolactin synthesis at this position. The other internal pause occurs at 160 amino acids. In these latter two cases ribosomes stop at a GGC glycine codon; however, two other GGC codons are translated without apparent pausing. Surprisingly, we find that up to nine ribosomes are tightly stacked behind each pausing ribosome, such that the ribosome centers are only 27-29 nucleotides apart. The assay should prove useful for probing mechanisms of translational regulation.

Photocrosslinking of the signal sequence of nascent preprolactin to the 54-kilodalton polypeptide of the signal recognition particle

Photoreactive moieties were incorporated into nascent polypeptides in a wheat germ protein-synthesizing system by using a plasmid-derived preprolactin mRNA and a Lys-tRNA analog, N epsilon-(5-azido-2-nitrobenzoyl)-Lys-tRNA (epsilon ANB-Lys-tRNA). The presence of the abnormally large amino acid side chains in the nascent chains did not impair function: complete preprolactin chains were synthesized in the absence of the signal recognition particle (SRP), elongation was arrested in the presence of SRP, and SRP-dependent translocation across the membrane of the endoplasmic reticulum and signal peptidase cleavage were observed in the presence of salt-extracted microsomes. Photolysis of elongation-arrested ribosomes resulted in several light- and epsilon ANB-Lys-tRNA-dependent crosslinks. By using antibodies specific for each of the proteins, one covalent complex was shown to be a photocrosslink between the preprolactin nascent chain and the 54-kDa protein subunit of SRP. This demonstrates that the N-terminal end of a secretory protein is located adjacent to the SRP in elongation-arrested ribosomes and strongly suggests that the signal sequence is recognized by and binds to the 54-kDa subunit of SRP. The other photocrosslinks involve as-yet-unidentified proteins in the large ribosomal subunit, indicating that this method of incorporating probes provides a powerful approach to examining the environment and interactions of the nascent chain during translation and translocation across the membrane of the endoplasmic reticulum. The Lys-tRNA analog also successfully photoaffinity-labeled the Escherichia coli elongation factor Tu (EF-Tu) in the epsilon ANB-Lys-tRNA.EF-Tu.GTP ternary complex.

Production of both prolactin and growth hormone by clonal strains of rat pituitary tumor cells. Differential effects of hydrocortisone and tissue extracts

Several established clonal strains of rat pituitary cells which produce growth hormone in culture have been shown to secrete a second protein hormone, prolactin. Prolactin was measured immunologically in culture medium and within cells by complement fixation. Rates of prolactin production varied from 6.6 to 12 microg/mg cell protein per 24 hr in four different cell strains. In these cultures ratios of production of prolactin to growth hormone varied from 1.0 to 4.1. A fifth clonal strain produced growth hormone but no detectable prolactin. Intracellular prolactin was equivalent to the amount secreted into medium in a period of about 1-2 hr. Both cycloheximide and puromycin suppressed prolactin production by at least 94%. Hydrocortisone (3 x 10(-6)M), which stimulated the production of growth hormone 4- to 8-fold in most of the cell strains, reduced the rate of prolactin production to less than 25% of that in control cultures. Conversely, addition of simple acid extracts of several tissues, including hypothalamus, to the medium of all strains increased the rate of production of prolactin six to nine times and decreased growth hormone production by about 50%. We conclude that multifunctional rat pituitary cells in culture show unusual promise for further studies of the control of expression of organ-specific activities in mammalian cells.

A signal sequence receptor in the endoplasmic reticulum membrane

Protein translocation across the endoplasmic reticulum (ER) membrane is triggered at several stages by information contained in the signal sequence. Initially, the signal sequence of a nascent secretory protein upon emergence from the ribosome is recognized by a polypeptide of relative molecular mass 54,000 (Mr54K) which is part of the signal recognition particle (SRP). Binding of SRP may induce a site-specific elongation arrest of translation in vitro. Attachment of the arrested translation complex to the ER membrane is mediated by the SRP-receptor (docking protein) and is accompanied by displacement of the SRP from both the ribosome and the signal sequence. We have investigated the fate of the signal sequence following the disengagement of SRP and its receptor by a crosslinking approach. We report here that the signal sequence of nascent preprolactin, after its release from the SRP, interacts with a newly discovered component, a signal sequence receptor (SSR), which is an integral, glycosylated protein of the rough ER membrane (Mr approximately 35K).

Recovery of surface membrane in anterior pituitary cells. Variations in traffic detected with anionic and cationic ferritin

Cells dissociated from rat anterior pituitaries were incubated with native or cationized ferritin (CF) to trace the fate of surface membrane. Native ferritin, which did not bind to the cell surface, was taken up in small amounts by bulk-phase endocytosis and was found increasingly (over 1-2 h) concentrated in lysosomes. CF at 100-fold less concentrations bound rapidly to the cell membrane, was taken up by endocytosis in far greater amounts, and within 15-60 min was found increasingly within multiple stacked Golgi cisternae, around forming secretion granules, and within elements of GERL, as well as within lysosomes. The findings demonstrate that the fate of the tracer--and presumably also that of the surface membrane--varies with the same molecule differing only in net charge: vesicles carrying anionic ferritin (net negative charge) fuse only with elements of the lysosomal system whereas those carrying CF (net positive charge) can fuse not only with elements of the lysosomal system, but also with elements along the secretory pathway (Golgi cisternae and condensing granules) as well.

A posttargeting signal sequence recognition event in the endoplasmic reticulum membrane

We have analyzed early phases of the cotranslational transport of the secretory protein preprolactin through the mammalian endoplasmic reticulum (ER) membrane. Following recognition of the signal sequence of the nascent polypeptide chain in the cytosol by the SRP, the chain is transferred into the membrane, where a second signal sequence recognition step takes place for which the presence in the lipid bilayer of the Sec61p complex is essential and sufficient. This step leads to a tight junction between the ribosomenascent chain complex and the Sec61p complex, and to the productive insertion of the nascent chain into the translocation site. These results show that a translocation substrate is subjected to two recognition events before being allowed to cross the ER membrane.

Each of the activities of signal recognition particle (SRP) is contained within a distinct domain: analysis of biochemical mutants of SRP

Signal recognition particle (SRP), a small ribonucleoprotein required for targeting secretory proteins to the ER, has three known functions: signal recognition, elongation arrest, and translocation promotion. Because SRP is inactivated by the sulfhydryl alkylating reagent N-ethylmaleimide (NEM), we have attempted to establish structure-function relationships within SRP by assembling particles in which a single protein is modified. Alkylation of the 68/72 kd protein of SRP yields a particle that arrests elongation but fails to promote translocation and no longer interacts with SRP receptor. Alkylation of the 54 kd protein yields a particle that fails to recognize signal sequences. This approach has allowed us to map activities to specific protein domains on SRP, and should be generally useful for analyzing other ribonucleoproteins.

Escherichia coli trigger factor is a prolyl isomerase that associates with nascent polypeptide chains

Correct folding of newly synthesized proteins is proposed to be assisted by molecular chaperones and folding catalysts. To identify cellular factors involved in the initial stages of this process we searched for proteins associated with nascent polypeptide chains. In an Escherichia coli transcription/translation system synthesizing beta-galactosidase we identified a 58-kDa protein which associated with translating ribosomes but dissociated from these ribosomes upon release of nascent beta-galactosidase. N-terminal sequencing identified it as trigger factor, previously implicated in protein secretion. Direct evidence for association of trigger factor with nascent polypeptide chains was obtained by crosslinking. In a wheat germ translation system complemented with E. coli lysates, epsilon-4-(3-trifluoromethyldiazirino)benzoic acid-lysine residues were incorporated into nascent secretory preprolactin and a nonsecretory preprolactin mutant. Trigger factor crosslinked to both types of nascent chains, provided they were ribosome bound. Trigger factor contains key residues of the substrate-binding pocket of FK506-binding protein-type peptidyl-prolyl-cis/trans-isomerases and has prolyl isomerase activity in vitro. We propose that trigger factor is a folding catalyst acting cotranslationally.

Specification of the anteroposterior neural axis through synergistic interaction of the Wnt signaling cascade with noggin and follistatin

Embryological data and the activities of the neural-inducing factors noggin and follistatin are consistent with the hypothesis that the nervous system is initially induced with an anterior character, with subsequent signals imparting posterior pattern. We report that Xwnt3a is a candidate for involvement in anteroposterior neural patterning, as it synergizes with the neural-inducing factors noggin and follistatin to increase the expression of posterior neural genes. Furthermore we show that beta-catenin, an intracellular protein implicated in the Wnt signal transduction cascade, mimics the activity of Xwnt3a. These data suggest that the generation of pattern within the vertebrate nervous system may rely on synergism between a Wnt signaling pathway and multiple neural-inducing factors.

Brca2 is coordinately regulated with Brca1 during proliferation and differentiation in mammary epithelial cells

We have analyzed the expression of the breast cancer susceptibility gene, Brca2, in mammary epithelial cells as a function of proliferation and differentiation. Our results demonstrate that Brca2 mRNA expression is tightly regulated during mammary epithelial proliferation and differentiation, and that this regulation occurs coordinately with Brca1. Specifically, Brca2 mRNA expression is up-regulated in rapidly proliferating cells; is down-regulated in response to serum deprivation; is expressed in a cell cycle-dependent manner, peaking at the G1/S boundary; and is up-regulated in differentiating mammary epithelial cells in response to glucocorticoids. In each case, an identical pattern of expression was observed for Brca1. These results indicate that proliferative stimuli modulate the mRNA expression of these two breast cancer susceptibility genes. In addition, the coordinate regulation of Brca1 and Brca2 revealed by these experiments suggests that these genes are induced by, and may function in, overlapping regulatory pathways involved in the control of cell proliferation and differentiation.

Hormonal regulation of human endometrial stromal cells in culture: an in vitro model for decidualization

Stromal cells derived from proliferative or secretory human endometria, cultured in the absence of steroid hormones, grew as monolayers that showed only occasional areas of immunoreactive fibronectin and did not produce detectable levels of prolactin (PRL) or laminin. Treatment with physiological doses of estradiol and progesterone induced PRL production and stimulated cell proliferation, resulting in multilayering with an increase of the saturation density. Electron microscopy showed the development of gap junctions, whereas immunofluorescence revealed a dense pericellular matrix containing fibronectin and laminin. These findings show that human endometrial stromal cells in culture respond to physiological doses of ovarian hormones with ultrastructural, proliferative, and biochemical changes that are characteristic of decidualization in vivo. This culture system thus provides an in vitro model for human decidualization.

Epidermal growth factor and thyrotropin-releasing hormone act similarly on a clonal pituitary cell strain. Modulation of hormone production and inhbition of cell proliferation

GH(4)C(1) cells are a clonal strain of rat pituitary cells that synthesize and secrete prolactin and growth hormone. Chronic treatment (longer than 24 h) of GH(4)C(1) cells with epidermal growth factor (EGF) (10(-8) M) decreased by 30-40 percent both the rate of cell proliferation and the plateau density reached by cultures. Inhibition of cell proliferation was accompanied by a change in cellular morphology from a spherical appearance to an elongated flattened shape and by a 40-60 percent increase in cell volume. These actions of EGF were qualitatively similar to those of the hypothalamic tripeptide thyrotropin-releasing hormone (TRH) (10(-7) M) which decreased the rate of cell proliferation by 10-20 percent and caused a 15 percent increase in cell volume. The presence of supramaximal concentrations of both EGF (10(-8)M) and TRH (10(-7)M) resulted in greater effects on cell volume and cell multiplication than either peptide alone. EGF also altered hormone production by GH(4)C(1) cells in the same manner as TRH. Treatment of cultures with 10(-8) M EGF for 2-6 d increased prolactin synthesis five- to ninefold compared to a two- to threefold stimulation by 10(-7) M TRH. Growth hormone production by the same cultures was inhibited 40 percent by EGF and 15 percent by TRH. The half- maximal effect of EGF to increase prolactin synthesis, decrease growth hormone production, and inhibit cell proliferation occurred at a concentration of 5 x 10 (-11) M. Insulin and multiplication stimulating activity, two other growth factors tested, did not alter cell proliferation, cell morphology, or hormone production by GH(4)C(1) cells, indicating the specificity of the EGF effect. Fibroblast growth factor, however, had effects similar to those of EGF and TRH. Of five pituitary cell strains tested, all but one responded to chronic EGF treatment with specifically altered hormone production. Acute chronic EGF treatment with specifically altered hormone production. Acute treatment (30 min) of GH(4)C(1) cells with 10(-8) M EGF caused a 30 percent enhancement of prolactin release compared to a greater than twofold increase caused by 10(-7) M TRH. Therefore, although EGF and TRH have qualitatively similar effects on GH(4)C(1) cells, their powers to affect hormone release acutely or hormone synthesis and cell proliferation chronically are distinct.

Epidermal growth factor rapidly stimulates prolactin gene transcription

Epidermal growth factor (EGF) was originally characterized as a growth factor for various cell types1,2 and was subsequently shown to affect a number of cellular and molecular processes3,4, of which many might be considered as a part of the pleiotropic growth response (enhanced uptake of glucose, uridine and amino acids and stimulated synthesis of protein, RNA and DNA). Very early responses to EGF, such as increased sodium fluxes5 and stimulation of tyrosine phosphorylation6, have been proposed to mediate some or all of EGF's effects. In a number of tissues, EGF has been shown to increase the synthesis of specific proteins7-10. We have investigated the effects of EGF on prolactin synthesis in the GH4 rat pituitary cell line to gain further insight into the mechanism of EGF's actions on cellular functions. Addition of EGF to GH4 cells results in a three- to sixfold stimulation of prolactin synthesis, as well as a partial inhibition of cell growth7,8. In this report, we demonstrate that the increased prolactin synthesis appears to be the result of a rapid stimulation of prolactin gene transcription by EGF. It is tempting to speculate that very early transcription by EGF. It is tempting to speculate that very early transcriptional effects on specific genes, such as reported here in te case of the prolactin gene in GH4 cells, may mediate some or all of the later effects of EGF on cell cycle regulation in those cells for which it serves as a growth factor.

Nascent prehormones are intermediates in the biosynthesis of authentic bovine pituitary growth hormone and prolactin

Major translation products of bovine pituitary RNA in a wheat germ cell-free system were identified as larger forms (prehormones) of growth hormone and prolactin containing amino-terminal extensions of 26 or 27 and 30 amino acid residues, respectively. However, translation of bovine pituitary RNA in the wheat germ cell-free system in the presence of microsomal membranes prepared from canine pancreas or bovine pituitary yielded products that were of the same size as authentic growth hormone and prolactin; by partial amino-terminal sequence analysis they were shown to contain the correct unique amino-terminal sequence of prolactin and the two correct amino termini of authentic growth hormone; moreover, they were found to be segregated within the microsomal vesicles in that they were largely inaccessible to degradation by proteolytic enzymes. When microsomal membranes were present after rather than during translation, prehormones were neither cleaved nor segregated. These results strongly suggest that the synthesis and segregation of the authentic hormone observed in the presence of membranes proceeds via a nascent prehormone rather than a completed prehormone.

Binding of ribosomes to the rough endoplasmic reticulum mediated by the Sec61p-complex

The cotranslational translocation of proteins across the ER membrane involves the tight binding of translating ribosomes to the membrane, presumably to ribosome receptors. The identity of the latter has been controversial. One putative receptor candidate is Sec61 alpha, a multi-spanning membrane protein that is associated with two additional membrane proteins (Sec61 beta and gamma) to form the Sec61p-complex. Other receptors of 34 and 180 kD have also been proposed on the basis of their ability to bind at low salt concentration ribosomes lacking nascent chains. We now show that the Sec61p-complex has also binding activity but that, at low salt conditions, it accounts for only one third of the total binding sites in proteoliposomes reconstituted from a detergent extract of ER membranes. Under these conditions, the assay has also limited specificity with respect to ribosomes. However, if the ribosome-binding assay is performed at physiological salt concentration, most of the unspecific binding is lost; the Sec61p-complex then accounts for the majority of specific ribosome-binding sites in reconstituted ER membranes. To study the membrane interaction of ribosomes participating in protein translocation, native rough microsomes were treated with proteases. The amount of membrane-bound ribosomes is only slightly reduced by protease treatment, consistent with the protease-resistance of Sec61 alpha which is shielded by these ribosomes. In contrast, p34 and p180 can be readily degraded, indicating that they are not essential for the membrane anchoring of ribosomes in protease-treated microsomes. These data provide further evidence that the Sec61p-complex is responsible for the membrane-anchoring of ribosomes during translocation and make it unlikely that p34 or p180 are essential for this process.

Prolactin synthesis by human chorion-decidual tissue: a possible source of prolactin in the amniotic fluid

Explants of human chorion-decidual tissue obtained at delivery from normal, full-term pregnancies synthesize and secrete prolactin. This hormone is indistinguishable from pituitary prolactin by chromatographic, electrophoretic, immunologic, and receptor assay techniques. These results suggest that chorion-decidua may be the source of the large quantities of prolactin in amniotic fluid.

Prolactin synthesized and secreted by human peripheral blood mononuclear cells: an autocrine growth factor for lymphoproliferation

Prolactin has been shown to have an immunoregulatory role in the rodent immune response. A prolactin-like molecule has also been found in mouse splenocytes and a human B-lymphoblastoid cell line. We have evaluated whether human peripheral blood mononuclear cells (PBMCs) synthesize and/or secrete prolactin. We used the polymerase chain reaction (PCR) to generate a 276-base-pair prolactin product from human PBMCs, and Southern blot analysis confirmed that it was related to prolactin. Western blotting using a polyclonal antibody to prolactin indicated that cell extracts prepared from human PBMCs contained a high molecular mass (60-kDa) immunoreactive prolactin. To determine whether this PBMC prolactin was being secreted, we developed a highly sensitive and specific hormonal enzyme-linked immunoplaque assay. With this assay, we were able to detect human prolactin secretion from concanavalin A (Con A)- or phytohemagglutinin-stimulated PBMCs but not from unstimulated PBMCs. We next sought to determine whether this secreted prolactin could function as an autocrine growth factor in lymphoproliferation. We observed that anti-human prolactin antiserum significantly inhibited human PBMC proliferation in response to Con A or phytohemagglutinin. We conclude that a prolactin-like molecule is synthesized and secreted by human PBMCs and that it functions in an autocrine manner as a growth factor for lymphoproliferation.

Immunomodulation during sublingual therapy in allergic children

The clinical efficacy of sublingual immunotherapy (SLIT) has been demonstrated, but its mechanism of action is still controversial. The most recent experimental observations suggest that a critical role in the modulation of immune response is sustained by Th2 cytokines, such as interleukin-4 (IL-4), IL-5 and IL-13, by co-stimulatory molecules, such as CD40 on B cells, and by hormones and neuropeptides. To better understand whether SLIT affects immune responses we used a double-blind placebo-controlled design. Eighty-six children with mild asthma due to allergy to Dermatophagoides pteronyssinus (33 of whom also had rhinoconjunctivitis) were randomly assigned SLIT (n = 47) or placebo (n = 39). We assessed symptom scores using diary cards of each patient and determined the expression of CD40 on B cells and the serum concentration of ECP, IL-13, prolactin (PRL) and ACTH at enrolment and after 6 months of therapy. We observed a significant reduction in asthma and rhinitis scores in the immunotherapy group compared with the placebo group, no variation in CD40 and ACTH, but a significant decrease in ECP, IL-13 and PRL after 6 months of therapy (p <0.01). Our results confirm the efficacy and safety of SLIT, and lead us to believe that it could modulate the synthesis of Th2 cytokines, as revealed from the decrease of IL-13. In addition, the reduction of PRL might be a signal of reduced activation of T lymphocytes.

Cell-specific expression of the prolactin gene in transgenic mice is controlled by synergistic interactions between promoter and enhancer elements

Prolactin gene expression is restricted to the lactotrophic and somatomammotrophic cells of the anterior pituitary. In transgenic mice, a fusion gene consisting of 3 kb of prolactin 5'-flanking region fused to a firefly luciferase or human growth hormone (hGH) reporter gene is expressed at high levels with the strict tissue and cell-type specificity that is characteristic of the endogenous prolactin gene. High levels of expression require two cis-acting regions: a distal enhancer (-1.8 to -1.5 kb) and a proximal region (-422 to +33 bp). Each of these regions alone can direct low levels of fusion gene expression to prolactin-producing cell types in transgenic mice, but a synergistic interaction between these regions is necessary for high levels of expression. The ontogeny of the prolactin transgene expression closely follows the appearance of high levels of a POU homeo-domain transcription factor, Pit-1, that has been shown previously to bind structurally related sequences in both the distal enhancer and proximal regions and to activate the expression of the prolactin gene in vitro. Unexpectedly, transgenes containing the distal enhancer removed from its normal context are expressed in both the prolactin-producing lactotrophs and the thyroid-stimulating hormone (TSH)-producing thyrotrophs, thereby suggesting that sequences flanking this enhancer are necessary to restrict expression to the correct cell type within the pituitary. These data indicate that distinct processes of gene activation and restriction are necessary for the fidelity of cell-type-specific expression within an organ.