Cell-free synthesis of mouse corticotropin. Evidence for two high molecular weight gene products

Analysis of brain and pituitary RNA metabolism: a review of recent methodologies

Recent characterization of brain and pituitary RNA metabolism is reviewed. Relative to other tissues, the brain transcribes more of the unique, single-copy DNA. This transcriptional diversity reflects the inherent heterogeneity in organization and development of the brain. The end product of transcriptional regulation in the brain is a population of functional cytoplasmic mRNAs with multiple components, differing in complexity and abundance. Analysis of nuclear and cytoplasmic RNA provides evidence that both brain-specific synthesis and processing may determine the final mRNA population. Both polyadenylated and non-polyadenylated RNA classes contribute significantly to the total brain polysomal mRNA fraction. Characterizations of individual species of mRNA from both brain and pituitary are described. One possible transcriptional modulator in both the pituitary and brain is the presence of steroid hormone at responsive sites. Functional consequences of steroid accumulation within the brain may be (1) interactions with neurotransmitter, especially catecholamine, metabolism and function, (2) developmental interactions with neuronal systems, and (3) differentiation of glial cell function. The pleiotropic nature of steroid hormone effects (both transcriptional and non-transcriptional) within one brain region is considered by examining the biochemical effects of glucocorticoids in the hippocampus.

Biosynthesis of two structurally different pro-opiomelanocortins in the pars intermedia of the amphibian pituitary gland

This study reports the biosynthesis of two forms of pro-opiomelanocortin in the pars intermedia of the pituitary gland of the African clawed toad Xenopus laevis. The two forms could be resolved by dodecyl sulphate gel electrophoresis on a 9-16% acrylamide gradient and their molecular weights were 38200 and 37300. Incubation of neurointermediate lobes with [3H]glucosamine followed by tryptic digestion of the newly synthesized glycoproteins, revealed that both prohormones have only one glycosylated site, namely within the region corresponding to gamma 3-melanotropin. Biosynthesis of proteins in lobes treated with tunicamycin to prevent glycosylation again resulted in the production of two pro-opiomelanocortins (Mr 35000 and 34200), indicating that the two forms differ in their primary structure. This notion was corroborated by the results of tryptic mapping of the newly synthesized prohormones. The maps showed that the primary structures of the two forms of pro-opiomelanocortin differ in at least two parts of the molecules, one part concerning the endorphin region.

Biosynthesis of pairs of peptides related to melanotropin, corticotropin and endorphin in the pars intermedia of the amphibian pituitary gland

This study concerns the biosynthesis of a number of peptides in the neurointermediate lobe of the pituitary gland of the aquatic toad, Xenopus laevis. Using pulse-chase incubations in vitro and high-performance liquid chromatographic analysis, it could be shown that these peptides are synthesized through processing of a prohormone, pro-opiomelanocortin; all peptides were released into the incubation medium. On the basis of electrophoretic analysis, selective amino acid incorporation and immunoprecipitation, as well as peptide mapping by high-performance liquid chromatography, the peptides were classified into three distinct groups: two related to melanocyte-stimulating hormone (melanotropin), two related to adrenocorticotropic hormone (corticotropin) and two endorphin-like peptides. Using tryptic and chymotryptic maps of synthetic alpha-melanotropin and des-Ac alpha N-alpha-melanotropin as references, one of the melanotropin-like peptides was identified as des-Ac alpha N-alpha-melanotropin; the other one represents neither alpha-melanotropin nor any other known melanotropic peptide. The two peptides that were immunologically related to corticotropin had characteristics consistent with a structures resembling a peptide previously named 'corticotropin-like intermediate lobe peptide', corticotropin-(18-39). The two endorphin-like peptides, although highly related, do not have the same primary structure. In view of the apparent structural differences between the two peptides in each group, the possible occurrence of two prohormones is discussed.

Cell-free synthesis of putative type V procollagen chains programmed by Chinese hamster lung cell mRNA

A messenger RNA fraction isolated from cultured Chinese hamster lung (CHL) cells programs in a cell-free system prepared from wheat germ the efficient incorporation of [14C] proline into newly synthesized protein with a significant fraction of the incorporated substrate being digestible with bacterial collagenase. This reaction requires both subcellular fractions, an energy source, and is inhibited by the antibiotic puromycin. The relative amount of collagenase-digestible to non-digestible cell-free product depends upon the ratio of CHL mRNA to wheat germ lysate, is not affected by either the Mg2+ or K+ concentrations employed, and under optimal condition, approximately 38% of the total incorporated substrate is collagenase-sensitive. Electrophoresis on SDS-polyacrylamide gels of the products programmed by CHL mRNA indicates that the collagenase-digestible material corresponds in size to a procollagen chain with an apparent molecular mass of approximately 170,000 daltons. These studies suggest that the collagen alpha 1 (V) chain is initially synthesized as a precursor procollagen chain and demonstrate that a significant amount of the mRNA in Chinese hamster lung cells codes for this protein.

Evidence that glycosylation of pro-opiocortin and ACTH influences their proteolysis by trypsin and blood proteases

The role of the carbohydrate in the stabilizaion and protection of the glycoprotein, pro-opiocortin, from non-specific proteolysis by trypsin and blood proteases was studied in vitro. [3H]Arginine-labeled, glycosylated and non-glycosylated forms of pro-opiocortin were isolated from frog neurointermediate lobes and subjected to proteolysis by trypsin. The non-glycosylated form was degraded by trypsin more rapidly than the glycosylated form. Analysis of the tryptic products after trypsin treatment, showed that the non-glycosylated pro-opiocortin was cleaved to unidentified peptides within 1 min, whereas the glycosylated prohormone yielded 2 products, mol. wt. 23 000 ACTH and mol. wt. 21 000 ACTH, synthesized by the intact neurointermediate lobe. These data provide direct evidence in support of the hypothesis, derived from studies on the intact lobe (Loh and Gainer, 1978, 1979) that the glycosylation of pro-opiocortin is important: (1) to protect it against non-specific proteolysis in situ, and (2) to direct processing by limiting proteolysis. In addition, we demonstrate that glycosylated forms of ACTH are much more stable in blood than non-glycosylated forms.