Two differentially regulated mRNAs with different 5′ ends encode secreted and intracellular forms of yeast invertase
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A number of studies have introduced mutations into the yeast invertase signal peptide, using it as a model system to elucidate features for targeting, translocation and intracellular transport Using molecular modelling of the invertase signal peptide we have analysed the hydrophobicity potential and the change in the dielectric constant of the energy transfer, when the molecule moves from a hydrophobic to a hydrophilic phase at the simulated hydrophobic-hydrophilic interface. This modelling has been carried out on wild type and mutant invertase signal peptides of altered function, previously reported in the literature. While the predicted angle of insertion correlates with the measured extent of invertase secretion, with an optimum angle of 45°, mutations that change the angle of orientation reduce the extent of invertase secretion. We have applied these same molecular modelling principles to the naturally occurring variants of the human apolipo-protein B (apoB) signal peptide, that confer a secretion defective phenotype when fused to yeast invertase and expressed in yeast. Our modelling thus identifies a strong correlation between the predicted angle of insertion of the signal peptide into the membrane and its ability to direct secretion.
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The regulation of cell growth can be achieved at many levels but ultimately the regulatory factors must alter protein synthesis since growing cells always exhibit an increased rate of protein synthesis compared to resting cells. Some studies using growing and nongrowing mammalian cells have shown that the rate of protein synthesis is directly dependent on mRNA content. Other studies have shown that growing the resting cells have similar amounts of mRNA and that protein synthesis is regulated by the proportion of mRNA in polysomes. We have analyzed mRNA content in growing and resting epithelial cells of Xenopus laevis. Quantitation of poly(A)+ mRNA by uniform labeling with 3H-uridine and by 3H-poly(U) hybridization demonstrated a direct relationship between mRNA content and the relative rate of protein synthesis in growing and resting cells. Likewise, after serum stimulation of resting cells the increase in mRNA content closely paralleled the increase in protein synthesis. Our results suggest that control of protein synthesis in growing and nongrowing cells is exerted before the translational level.
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Abstract Numerous reports have demonstrated that specific protein synthesis in response to specific inducers is markedly stimulated by a simultaneous brief exposure to protein synthesis inhibitors such as cycloheximide. This phenomenon is known as “superinduction” and is most often attributed to the accumulation of cytoplasmic messenger RNA during the inhibition period. Messenger RNA, as defined by rapid labeling, oligo (dt)‐cellulose binding, and cell free protein synthesis stimulation was measured in cycloheximide treated human fibroblasts. In spite of a consistent 40% decrease in total polysomal 3 H‐uridine labeled RNA, a 1.5‐ to 2‐fold increase in extractable mRNA was observed. These data provide direct evidence that protein synthesis inhibition stimulates the appearance of cytoplasmic mRNA and/or completely blocks its degradation and, are consistent with the hypothesis that mRNA accumulation partly underlies the superinduction phenomena.
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Insertion mutations previously constructed within the proximal region of the yeast invertase signal sequence did not interfere with secretion or glycosylation of the enzyme. We now describe deletion mutations within the same signal sequence. Large deletions truncating the hydrophobic core of the signal peptide prevented both secretion and glycosylation of the enzyme and increased the intracellular concentration of nonglycosylated invertase. This increase was coupled with the appearance of a new invertase polypeptide, 2 kilodaltons larger than cytoplasmic invertase. The new polypeptide was consistent in size with uncleaved (signal peptide intact) pre-secretory invertase previously identified by using in vitro translation (apparent molecular mass, 62 kilodaltons). The data on enzyme activity indicate that invertase whose secretion is aborted by large deletion mutations augments the normal pool of cytoplasmic invertase found in sucrose-fermenting yeast cells.
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