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The cryo-EM structure of human PNPase in the closed conformation
Structural highlights
DiseasePNPT1_HUMAN Combined oxidative phosphorylation defect type 13;Autosomal recessive nonsyndromic sensorineural deafness type DFNB. Combined oxidative phosphorylation deficiency 13 (COXPD13) [MIM:614932: A mitochondrial disorder characterized by early onset severe encephalomyopathy, dystonia, choreoathetosis, bucofacial dyskinesias and combined mitochondrial respiratory chain deficiency. Nerve conductions velocities are decreased. Levels of plasma and cerebrospinal fluid lactate are increased. Note=The disease is caused by mutations affecting the gene represented in this entry. Deafness, autosomal recessive, 70 (DFNB70) [MIM:614934: A form of non-syndromic deafness characterized by severe, bilateral hearing impairment with prelingual onset, resulting in inability to acquire normal speech. Note=The disease is caused by mutations affecting the gene represented in this entry. FunctionPNPT1_HUMAN RNA-binding protein implicated in numerous RNA metabolic processes. Hydrolyzes single-stranded polyribonucleotides processively in the 3'-to-5' direction. Mitochondrial intermembrane factor with RNA-processing exoribonulease activity. Component of the mitochondrial degradosome (mtEXO) complex, that degrades 3' overhang double-stranded RNA with a 3'-to-5' directionality in an ATP-dependent manner. Required for correct processing and polyadenylation of mitochondrial mRNAs. Plays a role as a cytoplasmic RNA import factor that mediates the translocation of small RNA components, like the 5S RNA, the RNA subunit of ribonuclease P and the mitochondrial RNA-processing (MRP) RNA, into the mitochondrial matrix. Plays a role in mitochondrial morphogenesis and respiration; regulates the expression of the electron transport chain (ETC) components at the mRNA and protein levels. In the cytoplasm, shows a 3'-to-5' exoribonuclease mediating mRNA degradation activity; degrades c-myc mRNA upon treatment with IFNB1/IFN-beta, resulting in a growth arrest in melanoma cells. Regulates the stability of specific mature miRNAs in melanoma cells; specifically and selectively degrades miR-221, preferentially. Plays also a role in RNA cell surveillance by cleaning up oxidized RNAs. Binds to the RNA subunit of ribonuclease P, MRP RNA and miR-221 microRNA.[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] Publication Abstract from PubMedHuman polynucleotide phosphorylase (hPNPase) is a 3'-to-5' exoribonuclease located in mitochondria, where it plays crucial roles in RNA degradation and RNA import. Mutations in hPNPase can impair these functions, leading to various mitochondrial dysfunctions and diseases. However, the mechanisms by which hPNPase switches between its roles as an RNA-degrading enzyme and an RNA carrier, as well as how disease-associated mutations may affect these distinct functions, remain unclear. In this study, we present cryo-electron microscopy structures of hPNPase, highlighting the flexibility of its S1 domains, which cap the ring-like RNA-degradation chamber and shift between two distinctive open and closed conformations. We further demonstrate by small-angle X-ray scattering and biochemical analyses that the disease-associated mutations P467S and G499R impair hPNPase's stem-loop RNA-binding and degradation activities by limiting the S1 domain's ability to transition from an open to closed state. Conversely, the D713Y mutation, located within the S1 domain, does not affect the RNA-binding affinity of hPNPase, but diminishes its interaction with Suv3 helicase for cooperative degradation of structured RNA. Collectively, these findings underscore the critical role of S1 domain mobility in capturing structured RNA for degradation and import, as well as its involvement in mitochondrial degradosome assembly. Our study thereby reveals the molecular mechanism of hPNPase in RNA binding and degradation, and the multiple molecular defects that could be induced by disease-linked mutations in hPNPase. Structural insights into human PNPase in health and disease.,Li YC, Wang CH, Patra M, Chen YP, Yang WZ, Yuan HS Nucleic Acids Res. 2025 Feb 8;53(4):gkaf119. doi: 10.1093/nar/gkaf119. PMID:39997218[13] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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