Structure and biological activity of a D3G mutation in the human mitochondrial hsp60 chaperonin
Proteins are essential components in all cellular activities, including signal transduction, gene expression regulation, immune response, structural support and many others. Following their synthesis on the ribosome, proteins need to fold into their precise three-dimensional structure to obtain functionality. If they fold incorrectly they are prone to aggregation and cause a wide range of neurodegenerative diseases. Some proteins are capable of folding without assistance but others need help from protein complexes known as chaperonins. Chaperonins are a family of proteins that assemble into barrel-shaped cages the create a protected chamber for misfolded proteins to refold into their biologically active state. A missense mutation (D3G) on the gene encoding for human mitochondrial chaperonin hsp60 has been implicated in the neurodegenerative disease termed Mitchap60 disease. Research has confirmed a link between neurodegenerative disorders and dysfunctional chaperonins. In 2008 a study on an Israeli family, who carried MitCHAP-60, revealed a mutation on the gene HSPD1, which encodes the mitochondrial chaperonin hsp60. Patients with the disease showed symptoms of nystagmus (repetitive, uncontrolled eye movements) and psychomotor retardation during the first months of life that later progressed to muscle hypotonia (weakness) and limb spasticity (rigidity). Most investigations mainly focus on the activity of mutant proteins instead of structural changes caused by mutations. Nevertheless, a fully characterized protein structure will lead to a comprehensive understanding of protein function and the mechanism underlying the onset of the neurodegenerative disorder. This will later contribute to the development of a new therapeutic intervention aimed at treatment or prevention of the neurodegenerative disease. Therefore, this research project aims at resolving the three-dimensional structure of the D3G hsp60 mutant protein and its biological activity using negative stain single particle reconstruction and the phosphate ATP hydrolysis assay, respectively. Our results indicate that the hsp60 D3G mutant is unable to bind to the co-chaperonin hsp10 and that nucleotide binding causes the entire tetradecameric complex to dissociate. This has given us tremendous insight into the cause of the neurodegenerative disorder MitCHAP-60. It is known that the β-subunit of the ATP synthase requires protein-folding assistance from the mitochondrial hsp60/10 protein-folding complex. In the absence or even diminished activity of the hsp60/10 chaperonin, the ATP synthase becomes non-functional and ATP is not synthesized in the mitochondria. Organs that demand high levels of ATP include the brain (neuronal cells) and muscle cells. Decreased ATP levels perfectly explain the symptoms exhibited by sufferers of the MitCHAP-60 disease. The single point mutation in the mitochondrial hsp60/10 chaperonin system is an indirect cause of the MitCHAP-60 disease because it is not folding the ATP synthase β-subunit, which in turn results in low ATP levels for the entire cell.
Li, Jihui, "Structure and biological activity of a D3G mutation in the human mitochondrial hsp60 chaperonin" (2016). ETD Collection for University of Texas, El Paso. AAI10246898.