DNAJC21

DNAJC21 is a J-domain co-chaperone that functions in late cytoplasmic maturation of the 60S ribosomal subunit, a role conserved from its yeast ortholog Jjj1 to humans (PMID:17242366, PMID:27346687). The protein associates with pre-60S ribosomal particles and stimulates the ATPase activity of the cytosolic Hsp70 Ssa (HSPA8), coupling chaperone power to ribosome biogenesis (PMID:17242366, PMID:27346687). Mechanistically, it acts cooperatively with the biogenesis factor Rei1 to drive removal and recycling of the nuclear export factor Arx1/PA2G4 from pre-60S particles near the polypeptide tunnel exit; loss of either factor blocks Arx1 dissociation and traps shuttling factors in the cytoplasm (PMID:17652132, PMID:23142985). The direct Jjj1–Rei1 interaction is mediated by a C-terminal segment containing a charged-residue region flanked by C2H2-type zinc fingers, and deletion of this region produces 60S biogenesis defects (PMID:19901025, PMID:15067379). In humans, loss-of-function mutations impair interactions with cofactors PA2G4, HSPA8, and ZNF622, causing cytoplasmic PA2G4 accumulation, aberrant ribosome profiles, and increased cell death, and underlie a bone marrow failure syndrome (PMID:27346687). Beyond its canonical ribosome role, DNAJC21 deficiency in zebrafish impairs hematopoietic differentiation and disrupts nucleotide metabolism, with nucleoside supplementation restoring neutrophil counts (PMID:39138265).

1. 2004 Medium

   Before functional characterization, the human gene needed to be defined at the sequence level, establishing it as a DnaJ-domain protein with a C2H2 zinc finger.

   Evidence cDNA library sequencing, RT-PCR expression analysis, and chromosomal mapping

   PMID:15067379

   Open questions at the time

   • No functional assay linking the protein to ribosome biogenesis
   • Tissue expression described but cellular role unknown
2. 2007 High

   The yeast ortholog Jjj1 was shown to be a 60S-associated co-chaperone whose deletion phenocopies loss of Rei1, defining its place in a late cytosolic step of subunit biogenesis.

   Evidence Genetic deletion with growth readout, in vitro Hsp70 Ssa ATPase stimulation assay, and ribosomal particle co-sedimentation in yeast

   PMID:17242366

   Open questions at the time

   • Substrate of the chaperone activity not identified
   • Mechanism of cooperation with Rei1 not resolved
3. 2007 High

   The cellular consequence of Jjj1 loss was defined as failure to recycle the shuttling export factors Arx1 and Alb1, linking the co-chaperone to nucleo-cytoplasmic factor recycling.

   Evidence Sucrose gradient sedimentation, two-hybrid interaction, and fluorescence microscopy of shuttling factors in yeast deletion strains

   PMID:17652132

   Open questions at the time

   • Direct physical contact with Rei1 not mapped
   • How recycling is mechanistically achieved unclear
4. 2009 High

   The Jjj1–Rei1 interaction was localized to a defined C-terminal charged region flanked by zinc fingers and shown to be jointly required for Arx1 dissociation, providing the structural basis for cooperative action.

   Evidence Co-immunoprecipitation, C-terminal deletion mutagenesis, ribosome association assays, and in vivo growth assays in yeast

   PMID:19901025

   Open questions at the time

   • Atomic structure of the interaction surface not resolved at this stage
5. 2012 High

   Cryo-EM placed Jjj1 spatially near Arx1 and Rei1 at the polypeptide tunnel exit, establishing the structural geometry of the Arx1-recycling network on the pre-60S particle.

   Evidence Cryo-EM at 8.1 Å of yeast 60S complexes with Arx1, Rei1, and Jjj1, plus biochemical characterization

   PMID:23142985

   Open questions at the time

   • Resolution insufficient for atomic-detail modeling
   • Dynamics of factor release not captured
6. 2015 Medium

   Domain dissection distinguished Jjj1 from the related Zuo1, showing a zuotin homology domain governs ribosome association and that Jjj1 binds primarily pre-60S rather than mature subunits.

   Evidence Domain truncation, ribosome co-sedimentation, and genetic rescue/complementation in yeast

   PMID:25639645

   Open questions at the time

   • Single-lab study
   • Precise binding determinants on the pre-60S not defined
7. 2016 High

   Human loss-of-function mutations were tied to a ribosome biogenesis defect and disease, confirming the conserved 60S maturation role and identifying PA2G4, HSPA8, and ZNF622 as cofactors.

   Evidence Exome sequencing, reciprocal co-IP, rRNA association, ribosome profiling, and actinomycin D sensitivity in patient-derived lymphoblastoid cells

   PMID:27346687

   Open questions at the time

   • In-cell mechanism of PA2G4 release in humans not structurally resolved
   • Cell-type basis of marrow failure not addressed
8. 2024 High

   An in vivo vertebrate model connected DNAJC21 loss to hematopoietic failure and revealed a nucleotide-metabolism role beyond ribosome biogenesis, with nucleoside supplementation as a functional rescue.

   Evidence Zebrafish dnajc21 knockout with flow cytometry, DNA damage assays, transcriptomics/metabolomics, nucleoside rescue, and tp53 epistasis

   PMID:39138265

   Open questions at the time

   • Molecular link between ribosome biogenesis and nucleotide metabolism unclear
   • Whether the nucleotide phenotype is direct or downstream of impaired translation not resolved

• How DNAJC21's chaperone activity is mechanistically coupled to nucleotide metabolism, and whether its human disease phenotype reflects ribosomal versus metabolic dysfunction, remains unresolved.
• No mechanism connecting 60S maturation to nucleotide pools
• Human atomic-resolution structure of the recycling complex lacking