MRPL28

MRPL28 is a structural component of the 39S large subunit of the human mitochondrial ribosome, identified by mass spectrometry of purified subunits and positioned within the complex by cryo-EM at 3.4 Å resolution as one of 48 proteins in the remodeled mammalian mitoribosome (PMID:11551941, PMID:25278503). As part of the mitoribosome, MRPL28 contributes to mitochondrial translation of hydrophobic membrane proteins essential for oxidative phosphorylation; its knockdown in pancreatic cancer cells reduces oxygen consumption and shifts metabolism toward glycolysis, paradoxically accelerating xenograft tumor growth by relieving oxygen as a rate-limiting substrate for proliferation (PMID:19753307).

1. 2001 High

   The identity of MRPL28 as a bona fide subunit of the human 39S mitoribosomal large subunit was established, resolving the composition of the mammalian mitoribosome and assigning MRPL28 as the homolog of bacterial L28.

   Evidence LC-MS/MS peptide sequencing of purified human 39S subunits

   PMID:11551941

   Open questions at the time

   • No structural context for where MRPL28 sits within the 39S subunit
   • Functional contribution of MRPL28 to translation not tested
2. 2009 Medium

   MRPL28 loss was shown to impair mitochondrial oxidative phosphorylation and increase glycolysis, linking mitoribosomal function to metabolic reprogramming and demonstrating that reduced oxygen consumption can paradoxically promote tumor growth in vivo.

   Evidence shRNA knockdown in pancreatic cancer cells with oxygen consumption, glycolysis, and xenograft growth measurements

   PMID:19753307

   Open questions at the time

   • Single cell line and lab; not independently replicated
   • Whether MRPL28 knockdown specifically impairs translation of particular mitochondrial mRNAs was not assessed
   • Mechanism by which reduced oxygen consumption accelerates in vivo growth (e.g., angiogenesis independence vs. diffusion advantage) was not resolved
3. 2014 High

   Near-atomic cryo-EM resolved MRPL28's structural position within the 39S subunit, confirming it as an integral part of the extensively remodeled mammalian mitoribosome specialized for membrane protein translation.

   Evidence Single-particle cryo-EM of the human 39S subunit at 3.4 Å resolution

   PMID:25278503

   Open questions at the time

   • Specific protein–protein or protein–rRNA contacts mediated by MRPL28 not individually characterized
   • Whether MRPL28 is required for ribosome assembly vs. translation catalysis is unknown

• It remains unknown whether MRPL28 plays a dedicated role in mitoribosome assembly, translational fidelity, or substrate selectivity beyond serving as a structural subunit, and whether its loss phenocopies other 39S subunit deficiencies in human disease.
• No patient mutations or Mendelian disease link reported
• No reconstitution or mutagenesis studies dissecting MRPL28-specific contacts
• Role in mitoribosome assembly pathway not characterized