MRPL9

MRPL9 is a conserved structural component of the mitochondrial ribosome large subunit (mt-LSU), essential for mitochondrial translation and oxidative phosphorylation. The protein was identified biochemically as one of 48 proteins of the mammalian 39S mt-LSU and resolved at near-atomic resolution by cryo-EM within both the mature mt-LSU and late-stage assembly intermediates, where it occupies a position homologous to bacterial ribosomal protein L9 (PMID:11551941, PMID:25278503, PMID:28892042). Beyond its canonical ribosomal role, MRPL9 overexpression drives cell proliferation and tumorigenesis: in vivo CRISPRa screening identified Mrpl9 as a hepatocellular carcinoma driver gene whose activation alters mitochondrial function-related gene expression programs (PMID:39761726), while in papillary thyroid cancer cells MRPL9 physically interacts with GGCT to activate MAPK/ERK signaling and promote proliferation, migration, and metastasis (PMID:36233293). Knockdown of MRPL9 in hepatocellular carcinoma and lung cancer cells consistently suppresses proliferation, migration, and invasion, with downstream effects on cell cycle progression and c-MYC/ZEB1-mediated epithelial–mesenchymal transition (PMID:36684217, PMID:37343379).

1. 1992 High

   Cloning of the yeast ortholog YmL9 established that the MRPL9 gene family encodes a mitochondrial ribosomal protein homologous to bacterial L3, and gene disruption demonstrated its essentiality for mitochondrial function — answering whether this nuclear-encoded protein is required for organellar translation.

   Evidence Gene cloning, sequencing, and disruption in S. cerevisiae

   PMID:1597181

   Open questions at the time

   • Mammalian MRPL9 not yet identified or characterized
   • Precise position within the ribosome unknown
   • Mechanism of mitochondrial import not defined
2. 2001 High

   Direct biochemical identification of MRPL9 in purified bovine 39S subunits confirmed it as a bona fide mammalian mt-LSU component, resolving whether the bacterial homolog was retained in the heavily remodeled mammalian mitoribosome.

   Evidence Proteolytic digestion and LC-MS/MS of purified bovine 39S mitoribosomal subunits

   PMID:11551941

   Open questions at the time

   • No structural information on MRPL9's position within the subunit
   • Functional contribution to translation not directly tested in mammalian cells
3. 2014 High

   Cryo-EM structures of the human mt-LSU at 3.4 Å placed MRPL9 within the architectural context of the mature ribosome, revealing how mitoribosomal remodeling accommodates hydrophobic nascent chains — the first direct structural view of MRPL9 in situ.

   Evidence Single-particle cryo-EM at 3.4 Å resolution of human mt-LSU

   PMID:25278503

   Open questions at the time

   • Timing of MRPL9 incorporation during ribosome assembly unknown
   • Functional contacts with rRNA or neighboring proteins not individually characterized
4. 2017 High

   Resolution of native late-stage assembly intermediates showed that MRPL9 is present in pre-mature mt-LSU complexes, establishing its incorporation timing during mitoribosome biogenesis.

   Evidence Cryo-EM of native mt-LSU assembly intermediates at ~3 Å from human cells

   PMID:28892042

   Open questions at the time

   • Specific assembly factors that recruit MRPL9 not identified
   • Consequence of MRPL9 depletion on assembly intermediate accumulation not tested
5. 2022 Medium

   Discovery of a physical MRPL9–GGCT interaction linked MRPL9 to MAPK/ERK signaling and cancer cell proliferation/metastasis, raising the question of whether MRPL9 has extra-ribosomal oncogenic functions.

   Evidence Co-immunoprecipitation, immunofluorescence co-localization, xenograft and lung metastasis models in papillary thyroid cancer

   PMID:36233293

   Open questions at the time

   • Whether the MRPL9–GGCT interaction occurs in the mitochondrial matrix or cytosol is unclear
   • Whether ERK activation is a direct consequence of the interaction or secondary to altered mitochondrial function is unresolved
   • Single-lab finding not yet independently replicated
6. 2023 Medium

   Functional studies in HCC and lung cancer cells demonstrated that MRPL9 knockdown consistently suppresses proliferation, migration, and invasion, with evidence linking MRPL9 to c-MYC/ZEB1-mediated EMT in lung cancer — broadening the cancer types where MRPL9 overexpression is functionally relevant.

   Evidence siRNA knockdown with proliferation, migration, invasion, and cell cycle assays in HCC and lung cancer cell lines; patient serum ELISA; Western blot for c-MYC and ZEB1

   PMID:36684217 PMID:37343379

   Open questions at the time

   • The c-MYC/ZEB1 link relies partly on bioinformatic inference and indirect knockdown experiments
   • Whether pro-tumorigenic effects depend on MRPL9's ribosomal function or an independent activity is unknown
   • Elevated serum MRPL9 in HCC patients not validated in independent cohorts
7. 2025 Medium

   In vivo CRISPRa screening identified Mrpl9 as a bona fide hepatocellular carcinoma driver whose gain-of-function alters mitochondrial function gene programs and accelerates tumorigenesis, directly linking MRPL9's mitochondrial biology to its oncogenic capacity.

   Evidence In vivo CRISPRa screen in mouse liver with Myc co-activation, RNA-seq of tumors, independent validation cohorts

   PMID:39761726

   Open questions at the time

   • Whether MRPL9-driven tumorigenesis requires intact mitoribosomal incorporation or occurs through a ribosome-independent mechanism is unresolved
   • Specific mitochondrial pathways altered (OXPHOS, ROS, metabolites) not dissected at protein level
   • Cooperation with Myc complicates attribution of tumor phenotype to MRPL9 alone

• It remains unknown whether the pro-tumorigenic effects of MRPL9 overexpression are mediated through enhanced mitochondrial translation, altered mitochondrial metabolism, extra-ribosomal protein interactions, or a combination of these mechanisms.
• No separation-of-function mutants distinguishing ribosomal from extra-ribosomal roles
• No structural basis for the MRPL9–GGCT interaction
• Effects of MRPL9 loss on mitochondrial translation in mammalian cells not directly measured