A normal mitochondrial protein is selectively synthesized and accumulated during heat shock in Tetrahymena thermophila

Chaperonin studies: faith, luck, and a little help from our friends

Basic cellular research is a trail. One follows one’s nose toward what might be new understanding. When that leads to a need to employ unfamiliar or novel technology, it’s both exciting and very worthwhile to form collaborations. Our early studies of chaperonins support such a philosophy, as detailed in the two stories that follow, written in deep appreciation of recognition by the E.B. Wilson Medal of the American Society for Cell Biology.

Calcium-dependent mitochondrial extrusion in ciliated protozoa

Here we demonstrate that ciliated protozoa can jettison mitochondria as intact organelles, releasing their contents to the extracellular space either in a soluble form, or in association with membrane vesicles at the cell periphery. The response is triggered by lateral clustering of GPI-anchored surface antigens, or by heat shock. In the first instance, extrusion is accompanied by elevated levels of intracellular calcium and is inhibited by Verapamil and BAPTA-AM arguing strongly for the involvement of calcium in triggering the response. Cells survive mitochondrial discharge raising the interesting possibility that extrusion is an early evolutionary adaptation to cell stress.

Occurrence of chaperonin 60 and chaperonin 10 in primary and secondary bacterial symbionts of aphids: implications for the evolution of an endosymbiotic system in aphids

All aphids harbor symbiotrophic prokaryotes ("primary symbionts") in a specialized-abdominal cell, the bacteriocyte. Chaperonin 60 (Cpn60, symbionin) and chaperonin 10 (Cpn10), which are high and low molecular weight heatshock proteins, were sought in tissues of more than 60 aphid species. The endosymbionts were compared immunologically and histologically. It was demonstrated that (1) there are two types of aphids in terms of the endosymbiotic system: some with only primary symbionts and others with, in addition, secondary symbionts; (2) the primary symbionts of various aphids are quite similar in morphology whereas the secondary symbionts vary; and (3) irrespective of the aphid species, Cpn60 is abundant in both the primary and secondary symbionts, while Cpn10 is abundant in the secondary symbionts but present in small amounts in the primary ones. Based on these results, we suggest that the primary symbionts have been derived from a prokaryote that was acquired by the common ancestor of aphids whereas the secondary symbionts have been acquired by various aphids independently after divergence of the aphid species. In addition, we point out the possibility that the prokaryotes under intracellular conditions have been subject to some common evolutionary pressures, and as a result, have come to resemble cell organelles.

Function of the maize mitochondrial chaperonin hsp60: specific association between hsp60 and newly synthesized F1-ATPase alpha subunits

Mitochondria contain a protein, hsp60, that is induced by heat shock and has been shown to function as a chaperonin in the assembly of mitochondrial enzyme complexes composed of proteins encoded by nuclear genes and imported from the cytosol. To determine whether products of mitochondrial genes are also assembled through an interaction with hsp60, we looked for association between hsp60 and proteins synthesized by isolated mitochondria. We have determined by electrophoretic, centrifugal, and immunological assays that at least two of those proteins become physically associated with hsp60. In mitochondrial matrix extracts, this association could be disrupted by the addition of Mg-ATP. One of the proteins that formed a stable association with hsp60 was the alpha subunit of the multicomponent complex F1-ATPase. We have not identified the other protein. These results indicate that hsp60 can function in the folding and assembly of mitochondrial proteins encoded by both mitochondrial and nuclear genes.

Function of the maize mitochondrial chaperonin hsp60: specific association between hsp60 and newly synthesized F1-ATPase alpha subunits

Mitochondria contain a protein, hsp60, that is induced by heat shock and has been shown to function as a chaperonin in the assembly of mitochondrial enzyme complexes composed of proteins encoded by nuclear genes and imported from the cytosol. To determine whether products of mitochondrial genes are also assembled through an interaction with hsp60, we looked for association between hsp60 and proteins synthesized by isolated mitochondria. We have determined by electrophoretic, centrifugal, and immunological assays that at least two of those proteins become physically associated with hsp60. In mitochondrial matrix extracts, this association could be disrupted by the addition of Mg-ATP. One of the proteins that formed a stable association with hsp60 was the alpha subunit of the multicomponent complex F1-ATPase. We have not identified the other protein. These results indicate that hsp60 can function in the folding and assembly of mitochondrial proteins encoded by both mitochondrial and nuclear genes.

Hsp70 proteins, similar to Escherichia coli DnaK, in chloroplasts and mitochondria of Euglena gracilis

The heat-shock response of Euglena gracilis was studied by pulse-labeling cells with [35S]sulfate at both the normal growth temperature (21 degrees C) and an elevated temperature (36 degrees C). Analysis of the labeled proteins by polyacrylamide gel electrophoresis indicated that the rate of synthesis of at least 3 major and 15 minor polypeptides increased in cells grown at the higher temperature. Three of the proteins appear to be immunologically related to the ubiquitous approximately 70-kDa heat-shock protein (Hsp70) family. One protein of 68 kDa was found in the cytoplasm (P68cyt) and was the major heat-shock protein in Euglena gracilis. Two other proteins, 68 and 70 kDa, were localized in mitochondria (P68mit) and chloroplasts (P70chl), respectively, and they crossreacted with a polyclonal antibody raised against the Escherichia coli heat-shock protein DnaK. Like DnaK, P68mit and P70chl could be phosphorylated in vitro with [gamma-32P]ATP in a reaction that was stimulated by Ca2+. A protein with characteristics similar to those of P70chl was also found in chloroplasts isolated from maize and spinach.

Primary structure of a human mitochondrial protein homologous to the bacterial and plant chaperonins and to the 65-kilodalton mycobacterial antigen

The complete cDNA for a human mitochondrial protein designated P1, which was previously identified as a microtubule-related protein, has been cloned and sequenced. The deduced amino acid sequence of P1 shows strong homology (40 to 50% identical residues and an additional 20% conservative replacements) to the 65-kilodalton major antigen of mycobacteria, to the GroEL protein of Escherichia coli, and to the ribulose 1,5-bisphosphate carboxylase-oxygenase (rubisco) subunit binding protein of plant chloroplasts. Similar to the case with the latter two proteins, which have been shown to act as chaperonins in the posttranslational assembly of oligomeric protein structures, it is suggested that P1 may play a similar role in mammalian cells. The observed high degree of homology between human P1 and mycobacterial antigen also suggests the possible involvement of this protein in certain autoimmune diseases.

Primary structure of a human mitochondrial protein homologous to the bacterial and plant chaperonins and to the 65-kilodalton mycobacterial antigen

The complete cDNA for a human mitochondrial protein designated P1, which was previously identified as a microtubule-related protein, has been cloned and sequenced. The deduced amino acid sequence of P1 shows strong homology (40 to 50% identical residues and an additional 20% conservative replacements) to the 65-kilodalton major antigen of mycobacteria, to the GroEL protein of Escherichia coli, and to the ribulose 1,5-bisphosphate carboxylase-oxygenase (rubisco) subunit binding protein of plant chloroplasts. Similar to the case with the latter two proteins, which have been shown to act as chaperonins in the posttranslational assembly of oligomeric protein structures, it is suggested that P1 may play a similar role in mammalian cells. The observed high degree of homology between human P1 and mycobacterial antigen also suggests the possible involvement of this protein in certain autoimmune diseases.

The groES and groEL heat shock gene products of Escherichia coli are essential for bacterial growth at all temperatures

The products of the groES and groEL genes of Escherichia coli, constituting the groE operon, are known to be required for growth at high temperature (42 degrees C) and are members of the heat shock regulon. Using a genetic approach, we examined the requirement for these gene products for bacterial growth at low temperature (17 to 30 degrees C). To do this, we constructed various groES groEL heterodiploid derivative strains. By inactivating one of the groE operons by a polar insertion, it was shown by bacteriophage P1 transduction that at least one of the groE genes was essential for growth at low temperature. Further P1 transduction experiments with strains that were heterodiploid for only one of the groE genes demonstrated that both groE gene products were required for growth at low temperature, which suggested a fundamental role for the groE proteins in E. coli growth and physiology.

A highly evolutionarily conserved mitochondrial protein is structurally related to the protein encoded by the Escherichia coli groEL gene

We recently reported that a Tetrahymena thermophila 58-kilodalton (kDa) mitochondrial protein (hsp58) was selectively synthesized during heat shock. In this study, we show that hsp58 displayed antigenic similarity with mitochondrially associated proteins from Saccharomyces cerevisiae (64 kDa), Xenopus laevis (60 kDa), Zea mays (62 kDa), and human cells (59 kDa). Furthermore, a 58-kDa protein from Escherichia coli also exhibited antigenic cross-reactivity to an antiserum directed against the T. thermophila mitochondrial protein. The proteins from S. cerevisiae and E. coli antigenically related to hsp58 were studied in detail and found to share several other characteristics with hsp58, including heat inducibility and the property of associating into distinct oligomeric complexes. The T. thermophila, S. cerevisiae, and E. coli macromolecular complexes containing these related proteins had similar sedimentation characteristics and virtually identical morphologies as seen with the electron microscope. The distinctive properties of the E. coli homolog to T. thermophila hsp58 indicate that it is most likely the product of the groEL gene.

A highly evolutionarily conserved mitochondrial protein is structurally related to the protein encoded by the Escherichia coli groEL gene

We recently reported that a Tetrahymena thermophila 58-kilodalton (kDa) mitochondrial protein (hsp58) was selectively synthesized during heat shock. In this study, we show that hsp58 displayed antigenic similarity with mitochondrially associated proteins from Saccharomyces cerevisiae (64 kDa), Xenopus laevis (60 kDa), Zea mays (62 kDa), and human cells (59 kDa). Furthermore, a 58-kDa protein from Escherichia coli also exhibited antigenic cross-reactivity to an antiserum directed against the T. thermophila mitochondrial protein. The proteins from S. cerevisiae and E. coli antigenically related to hsp58 were studied in detail and found to share several other characteristics with hsp58, including heat inducibility and the property of associating into distinct oligomeric complexes. The T. thermophila, S. cerevisiae, and E. coli macromolecular complexes containing these related proteins had similar sedimentation characteristics and virtually identical morphologies as seen with the electron microscope. The distinctive properties of the E. coli homolog to T. thermophila hsp58 indicate that it is most likely the product of the groEL gene.