Disease relevance of HSP78

• Deletion of HSP78 gene did not affect fragmentation of mitochondria upon heat stress, but significantly inhibited ability to restore mitochondrial network [1].

High impact information on HSP78

• Whereas deletion of HSP78 does not affect cell growth at temperatures up to 39 decrees C and cellular thermotolerance at 50 degrees C, Hsp78 is crucial for maintenance of respiratory competence and for mitochondrial genome integrity under severe temperature stress (mitochondrial thermotolerance) [2].
• When expressed in the cytosol, Hsp78 can substitute for the homologous heat shock protein Hsp104 in mediating cellular thermotolerance, suggesting a conserved mode of action of the two proteins [2].
• The molecular chaperone Hsp78 confers compartment-specific thermotolerance to mitochondria [2].
• Reactivation of mitochondrial protein synthesis after heat stress depends on the presence of Hsp78, though Hsp78 does not confer protection against heat-inactivation to this process [2].
• Hsp78 appears to act in concert with other mitochondrial chaperone proteins since a conditioning pretreatment of the cells to induce the cellular heat shock response is required to maintain mitochondrial functions under severe temperature stress [2].

Biological context of HSP78

• Deletion of HSP78 does not cause any detectable changes in wild type cells, but results in a petite phenotype in the ssc1-3 mutant strain carrying a temperature-sensitive allele of mt-hsp70 [3].
• Sequencing of HSP78 revealed a long open reading frame capable of encoding an 811-amino-acid, 91.3-kDa basic protein with a putative mitochondrial leader sequence and two potential nucleotide-binding sites [4].
• We report that deletion of the HSP78 gene in yeast strains with point mutations in the SSC1 gene (encoding matrix Hsp70) led to loss of mitochondrial DNA, indicating that at least one of the heat shock proteins Hsp78 and mt-Hsp70 is needed to maintain a rho+ state of the mitochondrial genome [5].
• Site-directed mutagenesis of the two putative Hsp78 nucleotide-binding domains suggest that the first nucleotide-binding domain is responsible for ATP hydrolysis and the second one for protein oligomerization [6].
• Gene expression profiling in model organisms like yeast has identified hundreds of genes that are up-regulated in response to various stressors, including several that are well characterized (e.g., hsp78, metallothionein, superoxide dismutase) [7].

Associations of HSP78 with chemical compounds

• The steady-state levels of HSP78 transcripts and protein varied in response to both thermal stress and carbon source with an approximately 30-fold difference between repressed levels in cells growing fermentatively on glucose at 30 degrees C and derepressed levels in heat-shocked cells growing on a nonfermentable carbon source [4].

Physical interactions of HSP78

• However, unlike misfolded proteins associated with mt-hsp70, hsp78-bound polypeptides are not efficiently degraded by the ATP-dependent PIM1 protease [3].

Other interactions of HSP78

• The mitochondrial heat shock protein Hsp78 is a member of the Hsp104/Clp family with unknown function [5].
• A bichaperone (Hsp70-Hsp78) system restores mitochondrial DNA synthesis following thermal inactivation of Mip1p polymerase [8].
• In this report we employed a novel quantitative assay system to assess the role of Hsp78 and Mcx1 in protein degradation within the matrix [9].
• Mitochondrial DNA instability mutants of the bifunctional protein Ilv5p have altered organization in mitochondria and are targeted for degradation by Hsp78 and the Pim1p protease [10].
• In contrast, Hsp78/Ssc1/Mdj1/Mge1, the corresponding protein system from yeast mitochondria, cannot activate the TrfA replication protein [11].

References