Importance of the propeptide sequence of human preproparathyroid hormone for signal sequence function

Strategies for improving expression of recombinant human chorionic gonadotropin in Chinese Hamster Ovary (CHO) cells

Optimizations of the gene expression cassette combined with the selection of an appropriate signal peptide are important factors that must be considered to enhance heterologous protein expression in Chinese Hamster Ovary (CHO) cells. In this study, we investigated the effectiveness of different signal peptides on the production of recombinant human chorionic gonadotropin (r-hCG) in CHO-K1 cells. Four optimized expression constructs containing four promising signal peptides were stably transfected into CHO-K1 cells. The generated CHO-K1 stable pool was then evaluated for r-hCG protein production. Interestingly, human serum albumin and human interleukin-2 signal peptides exhibited relatively greater extracellular secretion of the r-hCG with an average yield of (16.59 ± 0.02 μg/ml) and (14.80 ± 0.13 μg/ml) respectively compared to the native and murine IgGκ light chain signal peptides. The stably transfected CHO pool was further used as the cell substrate to develop an optimized upstream process followed by a downstream phase of the r-hCG. Finally, the biological activity of the purified r-hCG was assessed using in vitro bioassays. The combined data highlight that the choice of signal peptide can be imperative to ensure an optimal secretion of a recombinant protein in CHO cells. In addition, the stable pool technology was a viable approach for the production of biologically active r-hCG at a research scale with acceptable bioprocess performances and consistent product quality.

Development of stable HEK293T cell pools expressing CSFV E2 protein: A potential antigen expression platform

Large quantities of antigens are required since protective antigens, such as classical swine fever virus (CSFV) E2 protein, are widely used in diagnostic reagents and subunit vaccines. Compared to clonal cell lines and transient gene expression, stable cell pools provide a potential alternative platform to rapidly produce large amounts of antigens. In this work, firstly, Human embryonic kidney 293 T (HEK293T) cell pools expressing E2 protein were developed by transduction of lentiviral vectors. On the one hand, the SP7 was selected from 7 well-performing signal peptides to remarkably increase the production of E2 protein. On the other hand, it was found that high MOI could improve the expression of E2 protein by increasing gene copy numbers. Moreover, the HEK293T cell pools were evaluated for stability by passages and batch cultures, demonstrating that the cell pools were stable for at least 90 days. And then, the performance of the cell pools in batch, fed-batch, and semi-perfusion was studied. Among them, the titer of E2 protein was up to 2 g/L in semi-perfusion, which is currently the highest to the authors' knowledge. Finally, the aggregations and immunogenicity of the E2 protein were analyzed by SDS-PAGE and immunization of mice, respectively. There was no significant difference in aggregations and antibody titers of E2 protein in three culture methods. These results suggest that stable HEK293T cell pools are a promising and robust platform for rapid and efficient production of recombinant proteins.

The pro-sequence of parathyroid hormone prevents premature amyloid fibril formation

The parathyroid hormone (PTH) regulates the calcium and phosphate level in blood after secretion from parathyroid chief cells. The pre- and pro-sequences of precursor preproPTH get cleaved during PTH maturation. In secretory granules, PTH forms functional amyloids. Using thioflavin T fibrillation assays, circular dichroism, NMR spectroscopy, and cellular cAMP activation, we show that the pro-sequence prevents premature fibrillation by impairing primary nucleation because of Coulomb repulsion of positively charged residues. Under seeding or high salt conditions or in the presence of heparin at pH 5.5, proPTH fibril formation is delayed, but the monomer release properties are conserved. ProPTH can still activate in cellulo PTH receptor 1 but with impaired potency. These findings give some perspectives on medical applications of PTH in hormone therapy.

Modulating the Fibrillization of Parathyroid-Hormone (PTH) Peptides: Azo-Switches as Reversible and Catalytic Entities

We here report a novel strategy to control the bioavailability of the fibrillizing parathyroid hormone (PTH)-derived peptides, where the concentration of the bioactive form is controlled by an reversible, photoswitchable peptide. PTH1–84, a human hormone secreted by the parathyroid glands, is important for the maintenance of extracellular fluid calcium and phosphorus homeostasis. Controlling fibrillization of PTH1–84 represents an important approach for in vivo applications, in view of the pharmaceutical applications for this protein. We embed the azobenzene derivate 3-{[(4-aminomethyl)phenyl]diazenyl}benzoic acid (3,4′-AMPB) into the PTH-derived peptide PTH25–37 to generate the artificial peptide AzoPTH25–37 via solid-phase synthesis. AzoPTH25–37 shows excellent photostability (more than 20 h in the dark) and can be reversibly photoswitched between its cis/trans forms. As investigated by ThT-monitored fibrillization assays, the trans-form of AzoPTH25–37 fibrillizes similar to PTH25–37, while the cis-form of AzoPTH25–37 generates only amorphous aggregates. Additionally, cis-AzoPTH25–37 catalytically inhibits the fibrillization of PTH25–37 in ratios of up to one-fifth. The approach reported here is designed to control the concentration of PTH-peptides, where the bioactive form can be catalytically controlled by an added photoswitchable peptide.

A wild-derived inbred mouse strain, MSM/Ms, provides insights into novel skin tumor susceptibility genes

Cancer is one of the most catastrophic human genetic diseases. Experimental animal cancer models are essential for gaining insights into the complex interactions of different cells and genes in tumor initiation, promotion, and progression. Mouse models have been extensively used to analyze the genetic basis of cancer susceptibility. They have led to the identification of multiple loci that confer, either alone or in specific combinations, an increased susceptibility to cancer, some of which have direct translatability to human cancer. Additionally, wild-derived inbred mouse strains are an advantageous reservoir of novel genetic polymorphisms of cancer susceptibility genes, because of the evolutionary divergence between wild and classical inbred strains. Here, we review mapped Stmm (skintumor modifier of MSM) loci using a Japanese wild-derived inbred mouse strain, MSM/Ms, and describe recent advances in our knowledge of the genes responsible for Stmm loci in the 7,12-dimethylbenz(a)anthracene (DMBA)/12-O-tetradecanoylphorbol-13-acetate (TPA) two-stage skin carcinogenesis model.

The parathyroid hormone regulates skin tumour susceptibility in mice

Using a forward genetics approach to map loci in a mouse skin cancer model, we previously identified a genetic locus, Skin tumour modifier of MSM 1 (Stmm1) on chromosome 7, conferring strong tumour resistance. Sub-congenic mapping localized Parathyroid hormone (Pth) in Stmm1b. Here, we report that serum intact-PTH (iPTH) and a genetic polymorphism in Pth are important for skin tumour resistance. We identified higher iPTH levels in sera from cancer-resistant MSM/Ms mice compared with susceptible FVB/NJ mice. Therefore, we performed skin carcinogenesis experiments with MSM-BAC transgenic mice (Pth^MSM-Tg) and Pth knockout heterozygous mice (Pth^+/−). As a result, the higher amounts of iPTH in sera conferred stronger resistance to skin tumours. Furthermore, we found that the coding SNP (rs51104087, Val28Met) localizes in the mouse Pro-PTH encoding region, which is linked to processing efficacy and increased PTH secretion. Finally, we report that PTH increases intracellular calcium in keratinocytes and promotes their terminal differentiation. Taken together, our data suggest that Pth is one of the genes responsible for Stmm1, and serum iPTH could serve as a prevention marker of skin cancer and a target for new therapies.

Production and sorting of transgenic, modified human parathyroid hormone in vivo in rat salivary glands

Polarized salivary epithelial cells can sort secretory proteins towards either the basolateral or apical pole. Transgenic human parathyroid hormone (hPTH) exclusively sorts apically in rat submandibular glands. To help understand this specific process we modified the hPTH cDNA sequence and delivered the cDNAs to glands in vivo using adenoviral (Ad) vectors. The Ad vectors encoded: (1) the native form of hPTH (Ad.pre-pro-hPTH1-84), (2) the native sequence, but with the pro-segment deleted (Ad.pre-hPTH1-84), and (3) a sequence containing the pre-segment followed by the first 34 amino acids of hPTH (Ad.pre-hPTH1-34). hPTH production and sorting were studied after two days. All constructs were effectively transcribed in targeted glands. However, the pre-hPTH1-84 modification led to reduced hPTH secretion and production, while no immunoreactive hPTH resulted from pre-hPTH1-34 cDNA infusion. The pre-hPTH1-84 modification had no effect on apical sorting. These in vivo results show that the signal responsible for hPTH's apical sorting does not reside in the pro-segment and that deleting both the pro-segment and the carboxyl-terminal region severely impairs post-translational processing of hPTH.

Parathyroid hormone secretion and action: evidence for discrete receptors for the carboxyl-terminal region and related biological actions of carboxyl- terminal ligands

PTH is a major systemic regulator of the concentrations of calcium, phosphate, and active vitamin D metabolites in blood and of cellular activity in bone. Intermittently administered PTH and amino-terminal PTH peptide fragments or analogs also augment bone mass and currently are being introduced into clinical practice as therapies for osteoporosis. The amino-terminal region of PTH is known to be both necessary and sufficient for full activity at PTH/PTHrP receptors (PTH1Rs), which mediate the classical biological actions of the hormone. It is well known that multiple carboxyl-terminal fragments of PTH are present in blood, where they comprise the major form(s) of circulating hormone, but these fragments have long been regarded as inert by-products of PTH metabolism because they neither bind to nor activate PTH1Rs. New in vitro and in vivo evidence, together with older observations extending over the past 20 yr, now points strongly to the existence of novel large carboxyl-terminal PTH fragments in blood and to receptors for these fragments that appear to mediate unique biological actions in bone. This review traces the development of this field in the context of the evolution of our understanding of the "classical" receptor for amino-terminal PTH and the now convincing evidence for these receptors for carboxyl-terminal PTH. The review summarizes current knowledge of the structure, secretion, and metabolism of PTH and its circulating fragments, details available information concerning the pharmacology and actions of carboxyl-terminal PTH receptors, and frames their likely biological and clinical significance. It seems likely that physiological parathyroid regulation of calcium and bone metabolism may involve receptors for circulating carboxy-terminal PTH ligands as well as the action of amino-terminal determinants within the PTH molecule on the classical PTH1R.

Identification and characterization of the murine and human gene encoding the tuberoinfundibular peptide of 39 residues

By screening public databases, we identified human and mouse genomic DNA clones that encode the tuberoinfundibular peptide of 39 residues (TIP39). The TIP39 precursor is encoded by at least three exons; a noncoding exon U1, exon 1 encoding residues -61 (initiator methionine) to -19 of the leader sequence, and exon 2 encoding residues -18 to -1 and residues +1 to +39. Secreted human TIP39 is identical to the previously isolated bovine TIP39, whereas mouse TIP39 differs by four amino acids. Phylogenetic analyses suggested that TIP39, PTH, and PTHrP may have evolved from a common ancestor. Synthetic human and mouse TIP39 showed indistinguishable potencies [EC(50): 0.54 (human) vs. 0.74 nM (mouse)] at the human PTH2-receptor stably expressed in LLCPK(1) cells; furthermore, TIP-(9-39) was an inhibitor of cAMP accumulation stimulated by either [Tyr(34)]PTH(1-34)amide or human/bovine TIP39. In the mouse, an approximately 4.5-kb mRNA encoding TIP39 was identified by Northern blot analysis in testis and, less abundantly, in liver and kidney, whereas other tissues revealed additional smaller transcripts. In situ hybridizations revealed TIP39 expression in seminiferous tubuli and several brain regions, including nucleus ruber, nucleus centralis pontis, and nucleus subparafascicularis thalami. Because PTH2 receptor expression was previously shown to be highest in brain, pancreas, and testis, our findings are consistent with the notion that TIP39 is a neuropeptide which may also have a role in spermatogenesis.

The primary structure and the disulfide links of the bovine relaxin-like factor (RLF)

The relaxin-like factor (RLF), produced by the Leydig cells, is an essential link in the chain of events leading to the proper positioning of the testes during fetal development. The primary structure of RLF, as reported in the literature, is based solely upon cDNA sequences with chain lengths determined according to deduced processing sites and with relaxin-like cross-links. Biochemical characterization of bovine testicular RLF shows clearly that the endogenous hormone does consist of a 26 residue A chain and two forms of B chain, one containing 40 residues, the other 45. In addition, both B chains are 9 residues longer at the C terminus than the cDNA-deduced chain, and about 20% of the B chains have an additional 5 residue extension at the N terminus. Sequence analysis in combination with mass spectrometry and tryptic peptide mapping showed unambiguously that RLF is larger than previously assumed and that it has the relaxin-type disulfide bond distribution that makes it a bona fide member of the relaxin family of hormones.

Retroviral vector-mediated expression of hirudin by human vascular endothelial cells: implications for the design of retroviral vectors expressing biologically active proteins

We constructed a hirudin cDNA cassette, HV-1.1, that encodes mature hirudin variant-1 fused to the signal peptide of human tissue-type plasminogen activator (t-PA). The cassette was subcloned into retroviral vectors and used to transduce human vascular endothelial cells in vitro. Hirudin antigen and activity were measured by ELISA and thrombin inhibition assays, respectively. Transduced cells secreted up to 35 +/- 2 ng/10(6) cells/24 h of biologically active hirudin; expression was stable for at least 7 weeks. Recombinant hirudin, expressed from the HV-1.1 cassette, had a specific activity of 7.1 +/- 0.2 antithrombin units per microgram (ATU/microgram), compared with specific activities of approximately 12 ATU/microgram for both native leech hirudin and recombinant hirudin produced in yeast. Protein sequencing and mass spectroscopic analysis revealed the presence of an extra N-terminal serine residue, indicating aberrant cleavage of the t-PA signal peptide and likely accounting for the diminished activity. We therefore constructed a second cDNA cassette, HV-1.2, in which hirudin secretion was directed by the signal peptide of human growth hormone. Hirudin expressed from the HV-1.2 cassette had a specific activity of 13.5 +/- 0.2 ATU/microgram. Protein sequencing and mass spectroscopic analysis demonstrated proper cleavage of the growth hormone signal peptide. Thus, we achieved high level retrovirus-mediated secretion of biologically active hirudin from endothelial cells in vitro. Use of these vectors may permit sustained local antagonism of thrombin activity in vivo.

The unique proline-rich domain of parotid proline-rich proteins functions in secretory sorting

When expressed in pituitary AtT-20 cells, parotid proline-rich proteins enter the regulated pathway. Because the short N-terminal domain of a basic proline-rich protein is necessary for efficient export from the ER, it has not been possible to evaluate the role of this polypeptide segment as a sorting signal for regulated secretion. We now show that addition of the six-amino acid propeptide of proparathyroid hormone to the proline-rich protein, and especially to a deletion mutant lacking the N-terminal domain, dramatically accelerates intracellular transport of these polypeptides. Under these conditions the chimeric deletion mutant is stored as effectively as the full-length protein in dense core granules. The propeptide does not function as a sorting signal in AtT-20 cells as it does not reroute a constitutively secreted reporter protein to the regulated pathway. During transit, the propeptide is cleaved from the chimeric polypeptides such that the original structures of the full-length and the deletion mutant proline-rich proteins are reestablished. We have also found that the percentage stimulated secretion of the proline-rich proteins increases incrementally (almost twofold) as their level of expression is elevated. The increase reflects an enrichment of these polypeptides in the granule pool and its incremental nature suggests that sorting of proline-rich proteins involves an aggregation-based process. Because we can now rule out contributions to sorting by both N- and C-terminal segments of the proline-rich protein, we deduce that the unique proline-rich domain is responsible for storage. Thus at least some of the determinants of sorting for regulated secretion are protein-specific rather than universal.

Proparathyroid hormone is preferentially cleaved to parathyroid hormone by the prohormone convertase furin. A mass spectrometric study

Parathyroid hormone (PTH), an 84-amino acid peptide, is the major regulator of blood calcium homeostasis. Its mRNA, in addition to encoding the mature peptide, also encodes a "pre" sequence of 25 amino acids and a basic "pro" hexapeptide. To assess which of the subtilisin-like prohormone convertases can process proPTH to PTH we coinfected cells with a vaccinia virus construct expressing human preproPTH and vaccinia virus constructs expressing furin, PC1 or PC2. BSC-40 cells, having a constitutive secretory pathway, and GH4C1 cells, having a regulated secretory pathway, were used. PTH biosynthetic products in cell extracts and media were purified by high performance liquid chromatography, identified by radioimmunoassay, and unambiguously defined as either proPTH or PTH by ion-spray mass spectrometry. In both cell types, furin was the most effective in processing proPTH to PTH. In all cases only PTH was released into the medium. In addition, partially purified furin and PC1 were tested for their ability to appropriately cleave a tridecapeptide spanning the prohormone cleavage site found in proPTH. Here too furin was much more effective at cleaving at the correct site. Northern blot analysis and in situ hybridization showed that furin and preproPTH mRNA are co-expressed in the parathyroid, whereas PC1, PC2, and PC5 are not and PACE4 is expressed only at very low levels. Taken together these studies strongly suggest that furin is the enzyme responsible for the physiological processing of proPTH to PTH.

Inefficient membrane targeting, translocation, and proteolytic processing by signal peptidase of a mutant preproparathyroid hormone protein

A preproparathyroid hormone allele from a patient with familial isolated hypoparathyroidism was shown to have a single point mutation in the hydrophobic core of the signal sequence. This mutation, changing a cysteine to an arginine codon at the -8 position of the signal peptide, was associated with deleterious effects on the processing of preproparathyroid hormone to proparathyroid hormone in vitro. To examine the biochemical consequence(s) of this mutation, proteins produced by cell-free translation of wild-type and mutant cRNAs were used in assays that reconstitute the early steps of the secretory pathway. We find that the mutation impairs interaction of the nascent protein with signal recognition particle and the translocation machinery. Moreover, cleavage of the mutant signal sequence by solubilized signal peptidase is ineffective. The consequence of this mutation on processing and secretion of parathyroid hormone is confirmed in intact cells by pulse-chase experiments following transient expression of the mutant protein in COS-7 cells. The inability of the mutant signal sequence, however, to interfere with the targeting and processing of other secreted proteins does not support obstruction of the translocation apparatus as the mechanism underlying the dominant mode of inheritance of hypoparathyroidism in this family.

A mutant Kex2 enzyme with a C-terminal HDEL sequence releases correctly folded human insulin-like growth factor-1 from a precursor accumulated in the yeast endoplasmic reticulum

Mutations in the pro region of the yeast DNA hybrid of prepro-alpha-factor and human insulin-like growth factor-1 (IGF-1) cause the accumulation, in the yeast Saccharomyces cerevisiae, of an unglycosylated precursor protein where the pre sequence is missing. The prepro sequence of the prepro-alpha-factor consists of a pre or signal sequence and a proregion which possesses three sites for N-glycosylation. Isolation of a precursor, where the pro region is still linked to IGF-1 through a pair of dibasic amino acid residues, implies that the polypeptide may have translocated into the endoplasmic reticulum (ER) but has not been processed by the Golgi membrane-bound Kex2 endoprotease. However, the lack of any N-glycosylation in the translocated polypeptide is surprising. The mutated pro region, can be processed, in vitro, by treatment with a soluble form of the Kex2 enzyme. It is also possible to release the pro region, in vivo, by coexpressing a mutant Kex2 protease which is partially retained in the ER with the help of the C-terminal tetrapeptide sequence, HDEL. The mature IGF-1, which is secreted from the intracellular pool of precursor proteins, is predominantly an active, monomeric molecule, corroborating observations that early removal of the pro region before folding in the ER helps to prevent aberrant intermolecular disulfide-bond formation in IGF-1. These results have revealed the utility of the ER-retained Kex2 enzyme as a novel in vivo biochemical tool.

The pro-region of the yeast prepro-alpha-factor is essential for membrane translocation of human insulin-like growth factor 1 in vivo

Four yeast secretion signals, the 19-amino-acid invertase signal sequence, the 17-amino-acid acid-phosphatase signal sequence, and the pre-sequence and prepro-sequence of prepro-alpha-factor have been used to look for the secretion of recombinant human insulin-like growth factor 1 (IGF1) from Saccharomyces cerevisiae. Only the prepro-sequence, often referred to as the alpha-factor leader and consisting of an N-terminal 19-amino-acid pre-sequence or signal sequence attached to a 66-amino-acid pro-region, permits secretion of IGF1. The signal sequences alone do not allow the translocation of IGF1 into the endoplasmic reticulum. This is evident from the fact that IGF1-like molecules, to which the signal sequences are still attached, accumulate intracellularly in the cytosol. Fusion of the pro-region of the alpha-factor leader to the C-terminus of the acid-phosphatase and invertase signal sequences allows IGF1 to be secreted once again. These results reveal the essential role of the pro-region of the alpha-factor leader in the secretion of IGF1 and indicate that it may have a function in guiding a nascent IGF1 polypeptide to a state in which translocation can occur.

A modified Kex2 enzyme retained in the endoplasmic reticulum prevents disulfide-linked dimerisation of recombinant human insulin-like growth factor-1 secreted from yeast

The majority of the recombinant human insulin-like growth factor-1 (IGF1) molecules, secreted from yeast using the prepro sequence of the prepro-alpha-factor, are not active monomers but inactive, disulfide-linked dimers. The prepro sequence of the prepro-alpha-factor, usually referred to as the alpha-factor leader (alpha FL), consists of a pre or signal sequence and a proregion. After signal sequence removal during translocation into the endoplasmic reticulum (ER) the proregion is still attached to IGF1 when it folds to acquire a tertiary structure. Mature IGF1 is released only in a late Golgi compartment by the membrane-bound endoprotease Kex2p. We find that co-expression of a novel ER-retained Kex2p variant, soluble Kex2pHDEL, can prevent intermolecular disulfide bond formation between two IGF1 molecules, implying that the presence of the proregion during the folding of IGF1 in the ER could be a reason for disulfide-linked dimerisation. This result indicates that the proregion of the alpha FL may have a role in the folding of some heterologous proteins in yeast, and that the ER-retained Kex2p mutant could be used as a convenient tool to study the cellular function of the proregions present naturally in various eucaryotic precursor proteins.

A novel Kex2 enzyme can process the proregion of the yeast alpha-factor leader in the endoplasmic reticulum instead of in the Golgi

The prepro sequence of the yeast prepro-alpha-factor, usually referred to as the alpha-factor leader, has often been used for the efficient secretion of heterologous proteins from the yeast Saccharomyces cerevisiae. The alpha-factor leader consists of a 19-amino acid N-terminal pre or signal sequence followed by a 66-amino acid proregion. After removal of the signal sequence during membrane translocation, the proregion is cleaved from the precursor protein by the Kex2 endoprotease only in a late Golgi compartment. Here we report that a modified Kex2 enzyme, containing at the C-terminus the HDEL tetrapeptide, cleaves the proregion from the alpha-factor leader--human insulin like growth factor-1 fusion protein in the endoplasmic reticulum. The processing of pro-proteins earlier in the secretion pathway could be helpful in defining the cellular function of the proregions present naturally in various eucaryotic precursor proteins.

Proteolysis in protein import and export: signal peptide processing in eu- and prokaryotes

Numerous proteins in pro- and eukaryotes must cross cellular membranes in order to reach their site of function. Many of these proteins carry signal sequences that are removed by specific signal peptidases during, or shortly after, membrane transport. Signal peptidases have been identified in the rough endoplasmic reticulum, the matrix and inner membrane of mitochondria, the stroma and thylakoid membrane of chloroplasts, the bacterial plasma membrane and the thylakoid membrane of cyanobacteria. The composition of these peptidases varies between one and several subunits. No site-specific inhibitors are known for the majority of these enzymes. Accordingly, signal peptidases recognize structural motifs rather than linear amino acid sequences. Such motifs have become evident by employing extensive site-directed mutagenesis to investigate the anatomy of signal sequences. Analysis of the reaction specificities and the primary sequences of several signal peptidases suggests that the enzymes of the endoplasmic reticulum, the inner mitochondrial membrane and the thylakoid membrane of chloroplasts all have evolved from bacterial progenitors.

Lignin peroxidase from the basidiomycete Phanerochaete chrysosporium is synthesized as a preproenzyme

The cDNA clone L18 encoding lignin peroxidase LiP2, the most highly expressed LiP isozyme from Phanerochaete chrysosporium strain OGC101, was isolated and sequenced. Comparison of the cDNA sequence with the N-terminal sequence of the mature LiP2 protein isolated from culture medium suggests that the mature protein contains 343 amino acids (aa) and is preceded by a 28-aa leader sequence. In vitro transcription followed by in vitro translation and processing by signal peptidase resulted in cleavage at a site following the Ala21 (counted from the N-terminal Met1 of the initial translation product). The resultant protein contains a 7-aa propeptide, indicating that LiP is synthesized as a preproenzyme.

The isolation and partial characterization of bovine parathyroid secretory granules

We have developed a procedure which allows the isolation of secretion granules from fresh parathyroid glands. Following collagenase digestion of the tissue, the cells were broken with osmotic shock and a crude granule/mitochondrial pellet was obtained by differential centrifugation. Before loading this fraction onto a metrizamide density gradient it was subjected to brief sonication to disrupt the mitochondria. This procedure was necessary in order to achieve separation of the granules from the mitochondria during ultracentrifugation of the gradient. When the fractionated gradient was analysed for PTH by radioimmunoassay, three bands containing parathyroid hormone were found, at densities of 1.0, 1.05 and 1.18. Upon electron microscopic examination of the gradient fractions, granules were found only in those fractions containing hormone. A typical granule appearance was observed for two of the populations, but the third population (density 1.18), consisted of granules without membranes and which appeared less electron dense than those of populations 1 (density of 1.0) and 2 (density of 1.05). Moreover, the lack of a limiting membrane imparted a fuzzy appearance to the population 3 granules. When fresh tissue sections were examined as control samples, granules with and without membranes were also observed. Standard marker enzyme assays further confirmed that populations 2 and 3 were relatively free of other cellular contaminants, but population 1 contained endoplasmic reticulum and lysosomal material. Because the number of granules contained in this population is very small, we have not been successful in achieving further purification of population 1. Based on radioimmunoassay of extracts of each granule population, PTH was concentrated in population 3, while the other two contained lesser amounts. Interestingly, results obtained with a radioimmunoassay for SP-1 revealed a striking difference in the distribution of SP-1 in the three granule populations. This protein, which is also secreted by the parathyroid gland, was concentrated in population 1 and 2. Only very low levels were found in population 3. Thus, the two major secretory products are localized in different granule populations. The isolated granules were stable to pH changes, cycles of freeze/thaw and sonication. The yields of PTH extracted from each of the granule populations by freezing and thawing in buffer or by Triton containing solutions were low. PTH was completely extracted from each population only by using 8 M urea in HCl. Lower concentrations of urea were less effective. These results indicate that the molecular architecture of the granules is highly resistant to disruption.

Expression of a deletion mutant of the prosegment of human prorenin in Chinese hamster ovary cells

Expression plasmids encoding native human preporenin and a mutant deleted in its entire prosegment were transfected into Chinese hamster ovary cells. The cells transfected with the expression plasmid of native preporenin secreted exclusively inactive prorenin, while the cells transfected with the mutant secreted the active enzyme. The secreted amount of renin from the latter cells was much lower than that of prorenin from the former ones, although these two enzymes had little difference in specific activity after trypsin activation. These results suggest that the prosegment plays an important role in the secretory process of renin, although the fully active enzyme can be formed in its absence.