Conversion of pre-proparathyroid hormone to proparathyroid hormone by dog pancreatic microsomes

Quantitative and three-dimensional aspects of the rat parathyroid gland in normo-, hypo-, and hypercalcemia

The ultrastructure of the rat parathyroid has been under study for more than 35 years, but controversies still exist, especially regarding structure-function relationships. The present review focuses on recent morphological parathyroid research on rats under normal conditions and in various states of disturbed calcium metabolism. To facilitate discussions on functional aspects, current biochemical data, particularly those dealing with the regulation of parathyroid hormone synthesis and release, are also considered. Our results from quantitative studies and from investigations employing serial sectioning form the basis for the discussions. A central issue is whether the parathyroid secretory cells undergo secretory cycles. Prompted by results obtained from improved fixation procedures and serial sectioning, we question the basis for the theory of secretory cycles. Since the rat parathyroid secretory cell is polar, a single section is not an appropriate sample for estimating functional activity and for comparing the structure and distribution of intracellular components of adjacent cells. The heterogeneity in ultrastructural appearance of intracellular vesicles calls for the use of specific markers in relating the structure of the vesicular compartment to intracellular processing of hormone. The importance of unbiased quantitative techniques is illustrated in discussions on cell number and size for estimating the response of the parathyroid gland to different functional states or disorders demanding changes in secretion of parathyroid hormone, e.g., hyper- and hypocalcemia.

The selection of the first AUG as the initiator of eucaryotic mRNAs translation is favored by a 5'-terminal cap group and a purine in the -3' position

We have constructed a series of plasmids containing variations in the DNA that codes for the translation initiation region of preproparathyroid hormone. Gel electrophoretic analyses of the character and extent of synthesis of various modified preproparathyroid hormone like proteins derived from the mRNAs transcribed from these plasmids reveal that the presence of a 5'-terminal cap group on mRNA's facilitates recognition of the most 5'-terminal AUG sequence on a mRNA and that AUG sequences within the consensus sequence PuXXAUGPu are favored sites for the initiation of mRNA translation.

Molecular biology of parathyroid hormone

The entire biosynthetic pathway of PTH has been elucidated from the determination of the chromosomal location to the eventual secretion of the hormone from the cell. The human gene is present on the short arm of chromosome 11, and restriction site polymorphisms near the gene have been detected. The PTH genes and cDNAs have been isolated and characterized in the bovine, human, and rat species. The gene contains two introns, which are in the same position in each species, and dissect the gene into 3 exons that code, respectively, for the 5' untranslated region, the signal peptide, and PTH plus the 3' untranslated region. The mRNAs are about twice as long as necessary to code for preProPTH and contain a 7-methylquanosine cap at the 5' terminus and polyadenylic acid at the 3' terminus. The 5' termini of the bovine and human mRNAs are heterogeneous at the 5' terminus, the basis of which is two TATA sequences in the 5' flanking regions of the gene. In contrast, the rat gene contains a single TATA sequence and the mRNA has a single 5' terminus. The initial translational product of the mRNA is preProPTH, and the pre-peptide of 25 amino acids is equivalent to signal peptides of other secreted and membrane proteins. The genes of the three species are very homologous in the region that codes for preProPTH. Substantial homology is also retained in the gene flanking regions, introns, and mRNA untranslated regions. Silent sites are also conserved more than would be expected, particularly between the human and bovine sequences. The bovine and human sequences are more closely related than the rat is to either the human or bovine. These studies of the basic molecular biology of PTH will provide the framework for future analysis of significant biological and medical questions. In vitro mutagenesis techniques should soon provide information about the elements of the gene involved in regulating transcription and about functional elements of the signal peptide. Eventually, signals involved in directing the ProPTH molecule to secretory granules as well as the biologically active regions of PTH, itself, will be examined by these methods. The molecular biological studies, combined with the development of dispersed cell cultures, provide the opportunity to study the effects of chronic changes in calcium on gene transcription and mRNA metabolism. The restriction site polymorphisms associated with the human PTH gene will allow a search for correlations between PTH gene structure and parathyroid disease.(ABSTRACT TRUNCATED AT 400 WORDS)

Are prohormones converted to hormones during secretion?

A number of lines of evidence suggest that the polypeptide prohormone converting enzyme is a trypsin-like serine protease. A model is proposed in which the converting enzyme is activated from an inactive zymogen during secretion. Converting enzyme then activates co-secreted prohormone proteolytically. An important feature of the model lies in the geometry of the secretory granule immediately after exocytosis. It is suggested that initially diffusion of the granule contents is limited enough to allow extensive proteolysis to occur. Conversion of prohormone to hormone is terminated by diffusion of converting enzyme and prohormone from the site of release.

In vitro translation of poliovirus RNA: utilization of internal initiation sites in reticulocyte lysate

The translation of poliovirus RNA in rabbit reticulocyte lysate was examined. Translation of poliovirus RNA in this cell-free system resulted in an electrophoretic profile of poliovirus-specific proteins distinct from that observed in vivo or after translation in poliovirus-infected HeLa cell extract. A group of proteins derived from the P3 region of the polyprotein was identified by immunoprecipitation, time course, and N-formyl-[35S]methionine labeling studies to be the product of the initiation of protein synthesis at an internal site(s) located within the 3'-proximal RNA sequences. Utilization of this internal initiation site(s) on poliovirus RNA was abolished when reticulocyte lysate was supplemented with poliovirus-infected HeLa cell extract. Authentic P1-1a was also synthesized in reticulocyte lysate, indicating that correct 5'-proximal initiation of translation occurs in that system. We conclude that the deficiency of a component(s) of the reticulocyte lysate necessary for 5'-proximal initiation of poliovirus protein synthesis resulted in the ability of ribosomes to initiate translation on internal sequences. This aberrant initiation could be corrected by factors present in the HeLa cell extract. Apparently, under certain conditions, ribosomes are capable of recognizing internal sequences as authentic initiation sites.

Translation of mRNA for Limulus polyphemus haemocyanin polypeptides in vitro: studies on subunit heterogeneity

The haemocyanin of Limulus polyphemus is composed of a number (possibly 10-15) of polypeptides and is believed to be synthesised in cells called cyanoblasts. In vitro translation in the rabbit reticulocyte haemolysate system and in Xenopus oocytes, of mRNA isolated from cyanoblast-containing tissue, allowed the detection of several haemocyanin polypeptides amongst the products of translation. At least seven polypeptides with molecular weights in the range 68 000-71 000 were identified by an immunological method followed by electrophoretic characterisation on two-dimensional polyacrylamide gels. Comparison of the polypeptide patterns of authentic haemocyanin, reticulocyte lysate translation products and Xenopus oocyte translation products led to the conclusion that the polypeptides are unlikely to undergo significant post-translational modification or to possess cleavable signal sequences. It is proposed that release of haemocyanin into the haemolymph in vivo may involve bursting of the cyanoblasts.

Cleavage of human placental lactogen precursor by an enzyme from microbial membranes

When membranes from Escherichia coli MRE 600 are added to an in vitro translation system, they are able to cleave correctly prehuman placental lactogen to yield the mature hormone. The protein was identified by SDS/polyacrylamide-gel electrophoresis and by determining its amino acid sequence. These studies were aided by the development of a new method for the separation of human placental lactogen from its precursor.

Mechanism of compartmentation of secretory proteins: transport of exocrine pancreatic proteins across the microsomal membrane

The mechanism by which secretory proteins are segregated within the cisternal space of microsomal vesicles was studied using dog pancreas mRNA which directs the synthesis of 14 well-characterized nonglycosylated pancreatic exocrine proteins. In the absence of microsomal membranes, each of the proteins was synthesized as larger polypeptide chains (presecretory proteins). 1,000-2,000 daltons larger than their authentic counterparts as judged by polyacrylamide gel electrophoresis in SDS. Conditions optimal for the study of reconstituted rough microsomes in the reticulocyte lysate system were examined in detail using mRNA and microsomal membranes isolated from dog pancreas. Functional reconstitution of rough microsomes was considerably more efficient in the presence of micrococcal nuclease- treated membranes than in the presence of EDTA-treated membranes. Analysis for segregation of nascent secretory proteins by microsomal vesicles, using post-translational incubation in the presence of trypsin and chymotrypsin, 50 mug/ml each, was shown to be inadequate, because of the disruption of vesicles by protease activity. Addition of 1-3 mM tetracaine or 1 mM dibucaine stabilized microsomal membranes incubated in the presence of trypsin and chymotrypsin at either 0 degrees or 22 degrees C. Each of the pancreatic presecretory proteins studied was correctly processed to authentic secretory proteins by nuclease-treated microsomal membranes, as judged by both one-dimensional and two-dimensional gel electophoresis. Post-translational addition of membranes did not result in either segregation or processing of nascent polypeptide chains. Post- translational proteolysis, carried out in the presence of 3 mM tetracaine, indicated that each of the 14 characterized dog pancreas secretory proteins was quantitatively segregated by nuclease-treated microsomal vesicles. Segregation of nascent secretory proteins was irreversible, since radioactive amylase, as well as the other labeled secretory proteins, remained quantitatively sequestered in microsomal vesicles during a 90-min incubation at 22 degrees C after the cessation of protein synthesis. Studies employing synchronized protein synthesis and delayed addition of membranes indicated that all pancreatic presecretory proteins contain amino terminal peptide extensions. These peptide extensions are shown to mediate the cotranslational binding of presecretory proteins to microsomal membranes and the transport of nascent secretory proteins to the vesicular space. The maximum chain lengths which, during synthesis, allow segregation of nascent polypeptide chains varied between 61 (pretrypsinogen 2 + 3) and 88 (preprocarboxypeptidase A1) amino acid residues among dog pancreas presecretory proteins. Reconstitution studies using homologous and heterologous mixtures of mRNA (dog, guinea pig, and rat pancreas; rat liver) and micrococcal nuclease-treated microsomal membranes (dog, guinea pig, and rat liver; dog pancreas), in the presence of placental ribonuclease inhibitor, suggest that the translocation mechanism described is common to the rough endoplasmic reticulum of all mammalian tissues.

Processing mechanisms in the biosynthesis of proteins

Limited proteolysis is a widely occurring mechanism in protein biosynthesis. Protein precursors can be classified according to their functions, localization within cell compartments, and enzymic cleavage mechanisms. The presecretory proteins represent an important class of very rapidly turning over precursors which play an early role in the sequestration of secretory products and whose cleavage appears to be intimately associated with structures formed at the ribosome-membrane junction during protein synthesis. A model is proposed which predicts that the prepeptide forms a beta-pleated sheet structure with other components of the membrane which results in the transfer of a loop of peptide across the microsomal membrane. Proinsulin is representative of the general class of proproteins that are processed post-translationally within their secretory cells either during the formation and maturation of secretory granules (peptides hormones and neurotransmitters, serum albumins) or during the assembly of macromolecular structures (virus capsules, membrane-associated enzyme complexes). The former group are cleaved by Golgi-associated proteases having tryptic and carboxypeptidase B-like specificity. Some precursors are secreted as such and processed extracellularly either in the circulation or at special sites (procollagens, zymogens, provenoms, vitellogenins).

Inhibition of preprotein processing in ascites tumor lysates by incorporation of a leucine analog

Leucine analogs were tested in the Krebs II ascites cell-free translation system for the ability to inhibit preprotein cleavage by replacing leucine in nascent chains of bovine preprolactin, rat preprolactin, human placental prelactogen (pre-hPL), and pre-alpha subunit of human chorionic gonadotropin (alpha-hCG). In the absence of analog, ascites microsomal membranes cleaved these preproteins to their mature forms and sequestered the processed products. Also, two asparagine residues in alpha-hCG were glycosylated. When 4 mM beta-DL-hydroxyleucine was added to the lysate instead of L-leucine, cotranslational processing and sequestration of both species of preprolactin and pre-hPL were inhibited. Sequential Edman degradation confirmed that pre-hPL was not cleaved. The inhibition of processing by beta-hydroxyleucine resulted from its incorporation into protein. This was shown by reversal of the effect by addition of leucine and by inhibition of [(3)H]leucine incorporation into protein. Of significance, the processing of pre-alpha-hCG was less sensitive to beta-hydroxyleucine because its prepeptide contains only four scattered leucine residues, whereas the presegments of hPL and the prolactins contain six to eight clustered leucine residues. These experiments demonstrate that translocation and processing of secretory proteins require structural features determined by the primary amino acid sequence.