Alpha subunit of eukaryotic translational initiation factor-2 is a heat-shock protein

Proteome analysis of Saccharomyces cerevisiae: a methodological outline

Proteome analysis offers a unique means of identifying important proteins, characterizing their modifications and beginning to describe their function. This is achieved through the combination of two technologies: protein separation and selection by two-dimensional gel electrophoresis, and protein identification and characterization by mass spectrometry. This methodological outline sketches the strengths and weaknesses of the two central technologies used, and provides both practical tips and the theoretical background for their utilization. One application of these technologies is illustrated by the characterization of genes, revealed by sequencing, but which have no--or only weak homology--to any other known genes. Other applications, for example the identification of protein markers for particular human diseases, are only referred to. The aim of the article is thus to provide the basis for a sound understanding of the full potential and limitations of proteome analysis.

Cloning and characterization of the promoter region of a gene encoding a 67-kDa glycoprotein

A rat genomic library constructed in lambda-EMBL3 (SP6/T7) vector () was screened using 32P-labeled rat p67 cDNA. A clone containing a segment of 5'-upstream region of p67 genomic DNA was obtained. The DNA (about 1.7 kilobase pairs) was isolated and characterized. Sequence analysis of this DNA fragment showed that the 898 base pairs at the 5'-end of the upstream region was identical to several long interspersed nucleotide sequences. One hundred forty-eight base pairs at the 3'-end contained the beginning of the first exon including the ATG initiator codon. The remaining 652 base pairs in between contained two AT-rich regions and several regulatory sequences. The mRNA initiation site was identified at 89 base pairs upstream from the translation start codon. The DNA fragment was also analyzed by transient transfection. When linked to a firefly luciferase reporter gene, this fragment enhanced transcription in a rat hepatoma cell line (KRC-7). Using a series of deletions in the DNA, the minimum essential promoter region (from -177 to -60) was identified. The promoter activity was also enhanced by treatment with phorbol 13-myristate 12-acetate (PMA). This enhancement required an AP-1 sequence (-298 to -292; 5'-TGACTCA-3') and a similar sequence (-97 to -88; 5'-ATGACATCAT-3'). Deletion of either of these sequences significantly reduced PMA enhancement. Deletion of both of these sequences almost completely eliminated PMA enhancement.

Heat-shock induced protein modifications and modulation of enzyme activities

Upon heat stress, the cell physiology is profoundly altered. The extent of the alterations depends on the severity of the stress and may lead to cell death. The heat shock response is an array of metabolic changes characterized by the impairment of major cellular functions and by an adaptative reprogramming of the cell metabolism. The enhanced synthesis of the HSPs is a spectacular manifestation of this reprogramming. Numerous post translational modifications of proteins occur in response to heat stress and can be related to altered cellular functions. Some proteins are heat-denatured and temporarily inactivated. Heat-denaturation is reversible, chaperones may contribute to the repair. The extent of heat-denaturation depends on the cell metabolism: (a) it is attenuated in thermotolerant cells or in cells overexpressing the appropriate chaperones (b) it is enhanced in energy-deprived cells. Covalent modifications may also rapidly alter protein function. Changes in protein glycosylation, methylation, acetylation, farnesylation, ubiquitination have been found to occur during stress. But protein phosphorylation is the most studied modification. Several protein kinase cascades are activated, among which the various mitogen activated protein kinase (MAP kinase) cascades which are also triggered by a wide range of stimuli. As a possible consequence, stress modifies the phosphorylation status and the activity of components from the transcriptional and translational apparatuses. The same kinases also target key enzymes of the cellular metabolism. Protein denaturation results in constitutive hsp titration, this titration is a signal to trigger the heat-shock gene transcription and to activate some of the protein kinase cascades.

Rat fibroblasts transfected with the human 70-kDa heat shock gene exhibit altered translation and eukaryotic initiation factor 2 alpha phosphorylation following heat shock

Heat shock inhibits translation in a wide variety of cells. After heating, eukaryotic initiation factor 2-alpha (eIF-2 alpha) becomes phosphorylated which prevents the binding of Met-tRNA to the 40s ribosomal subunit inhibiting initiation of translation. Thermotolerant cells demonstrate resistance to inhibition of translation by additional heating suggesting that heat shock proteins may help to maintain translational integrity following thermal stress. Here we have examined the effects of increased intracellular levels of hsp70 protein on translation and eIF-2 alpha phosphorylation using rat fibroblasts stably transfected with a cloned human hsp70 gene. We observed a decrease in the rate of translational inhibition following heat shock in both hsp70-transfected and thermotolerant cells. Upon recovery at 37 degrees C, both hsp70-transfected and thermotolerant cells exhibit a faster rate of translational recovery. Utilizing slab gel isoelectric focusing coupled with immunoblotting we demonstrate that 45 degrees C heat shock leads to a rapid 4-5-fold increase in eIF-2 alpha phosphorylation, with little difference seen between control cells and hsp70-transfected cells. However, dephosphorylation of eIF-2 alpha occurs faster in the hsp70-transfected cells. These results suggest that hsp70 may play a role in facilitating the dephosphorylation of eIF-2 alpha as well as reversing the inhibition of translation following heat shock.

A laser densitometer for selective spot analysis on dot blots and two-dimensional gel autoradiograms

We describe an inexpensive densitometer, employing a small HeNe laser and an IBM-compatible personal computer that performs accurate measurements of selected spots on two-dimensional gel autoradiograms or chromatograms with an accuracy and a sensitivity equal or superior to those of many commercial instruments. Our open-table design allows the operator to visually monitor the scanning process in room lighting, and provides great flexibility in both the size and the nature of items to be scanned. The instrument has two moving parts (a prism and a small motor). A commercially available software package (ASYST) acquires digital data, graphs the data on the TV monitor, converts the data to optical density or to radioactive incorporation (cpm), subtracts background, integrates peak areas, and stores data on disk. The total time for these operations is 20-30 s per spot. The instrument has a dynamic range of 0.25 to 3.0 OD units and can measure a 10,000-fold range of 14C or 35S isotope concentrations on autoradiograms. The complete device can be assembled with a hobbyist's knowledge of electronics, moderate programming abilities (no machine language required), and a cost of less than $3000, not including the IBM PC.

Novel ATP-binding heat-inducible protein of Mr = 37,000 that is sensitive to transformation in BALB/3T3 cells

Using affinity chromatography on ATP-agarose, we have identified a major ATP-binding protein in Nonidet P-40 extracts of avian and mammalian cells labeled with [35S]methionine. After washing ATP-agarose beads with high-ionic-strength buffer (0.4 M NaCl), the 37-kD protein was shown to be one of the major ATP-binding proteins while p72 and grp78, which are members of the hsp70 family, also bound to ATP-agarose. This protein consisted of several spots on two-dimensional gel electrophoresis. The isoelectric point of the most basic spot was approximately 9.2 in chick embryo fibroblasts, whereas it was about 8.8 in mouse 3T3 cells. The identities of these proteins in mouse and chick cells were confirmed by peptide mapping. After heat-shock treatment of BALB/3T3 cells, the major heat-shock protein, hsp70, was shown to be induced very rapidly after heat shock and was recovered in the ATP-binding fraction. Besides hsp70, a 37-kD protein was also found to be induced by heat shock. This protein was drastically induced by treating the cells with alpha,alpha'-dipyridyl, an iron chelating reagent, but not with sodium arsenite, calcium ionophore, or tunicamycin. The synthesis and the total amount of this ATP-binding protein increased in mouse 3T3 cells transformed by simian virus 40, methylcholanthrene, or activated c-Ha-ras oncogene compared to their normal counterparts. The incorporation of [32P]orthophosphate was not detected in either normal or transformed cells. These studies established that a major ATP-binding protein of Mr = 37,000 is a heat-inducible protein and that the synthesis of this protein is regulated by malignant transformation.

2-Aminopurine selectively blocks the transcriptional activation of cellular genes by virus, double-stranded RNA and interferons in human cells

Interferons, double-stranded RNA and viruses induce the transcription of partly overlapping sets of cellular genes. We have studied the regulation of 11 interferon-inducible genes by these agents and found that four of them were also directly inducible by virus and double-stranded RNA, and two by virus only. We have investigated whether an inhibitor of interferon-beta gene activation, 2-aminopurine, would block this induction process. Induction of these genes by virus and double-stranded RNA was indeed blocked by 2-aminopurine. Since a single cis-acting element can confer inducibility both to interferons, and to virus and double-stranded RNA, we tested the effect of 2-aminopurine on gene activation by interferon-alpha and interferon-gamma. Remarkably, in all the cell lines tested, these induction processes and the establishment of an antiviral state were blocked by the drug. These observations contrast with previous reports. The inhibitory effect of this drug on gene induction was exerted in a selective fashion and at the transcriptional level. This indicate that for the virus-, double-stranded-RNA-, and interferons-mediated gene induction, an early and similar step in signal transduction was affected by 2-aminopurine.

Isolation and characterization of the promoter and flanking regions of the gene encoding the human protein-synthesis-initiation factor 2 alpha

The promoter region of the gene (eIF-2 alpha) for eukaryotic initiation factor 2 alpha (eIF-2 alpha) was isolated from a human genomic library and its structure was determined by restriction mapping and nucleotide (nt) sequence analysis. The promoter region and twelve in vivo transcriptional start points (tsp) have been identified by endonuclease S1 mapping and their location confirmed by primer-extension analysis, using RNA isolated from human cells. The untranslated leader is 102 to 140 nt long depending upon the tsp, and the 5' region of the mRNA has the potential for forming stable stem-loop structures. The nt sequence of the regions upstream and downstream from the tsp contains neither a 'TATA box' nor a 'CAAT box', but does contain several direct and inverted repeats, as well as palindromic sequences near the tsp. In addition, multiple consensus binding sites for a wide variety of regulatory proteins are present throughout upstream and downstream tsp-flanking regions.