LARP4B

LARP4B is an RNA-binding protein that selects a defined set of cellular mRNAs through their 3' UTRs and acts post-transcriptionally to stabilize them and promote their translation (PMID:26001795). Target recognition is mediated by its La module (the LA motif plus adjacent RRM), which binds an AU-rich sequence motif and is required both for direct mRNA association and for LARP4B's effects on cell growth (PMID:26001795, PMID:26933087). LARP4B is integrated into the translational machinery through two protein contacts: its N-terminal variant PAM2w motif engages the MLLE domain of the poly(A)-binding protein PABPC1 in a structurally distinct PAM2–MLLE mode (PMID:32517187), and its conserved region-2 directly binds the ribosome-associated protein RACK1, an interaction that enhances stabilization and translation efficiency of ARE-containing mRNAs [PMID:bio_10.1101_2024.11.01.621267]. Through these activities LARP4B controls cell and organ size and cell cycle progression: in Drosophila its growth-suppressive effect depends on the LA motif and RRM and operates by lowering dMyc protein levels (PMID:29462618), while in human tumor cells its depletion deregulates cell-cycle regulators including Cyclins B1 and E2, Aurora B, and E2F1 (PMID:38532886). In cancer contexts LARP4B stabilizes specific oncogenic transcripts—SPINK1 to drive SPINK1–EGFR signaling in hepatocellular carcinoma and WNK1 to engage the Keap1/NRF2/GCH1/BH4 axis and suppress ferroptosis in pancreatic cancer—linking its mRNA-stabilizing function to downstream signaling outputs (PMID:38693111, PMID:41014504).

1. 2013 Low

   An early loss-of-function test asked whether LARP4B is required for hematopoietic progenitor function, establishing a context-dependent rather than universal cellular requirement.

   Evidence Lentiviral shRNA knockdown in mouse Lin(-) progenitor cells with colony-forming, apoptosis, and cell-cycle readouts

   PMID:23815932

   Open questions at the time

   • Negative result from a single colony-forming assay; does not exclude roles in other cell types or molecular functions
   • No molecular target or binding activity tested
   • No protein-level or stability readout
2. 2015 High

   Defined LARP4B's core molecular activity by identifying its transcriptome-wide mRNA targets and showing it stabilizes them and promotes their translation, establishing it as a 3' UTR-binding post-transcriptional regulator.

   Evidence PAR-CLIP transcriptome-wide mapping, motif analysis, in vitro binding, and knockdown with mRNA-stability and proteome readouts

   PMID:26001795

   Open questions at the time

   • Did not resolve which protein partners couple binding to stabilization/translation
   • AU-rich motif defined but structural basis of recognition not determined
3. 2016 Medium

   Localized the RNA-binding and growth-regulatory function to the La module, linking a specific domain to direct mRNA association and a cellular phenotype.

   Evidence La module deletion mutagenesis and RNA association assays with proliferation/apoptosis readout in glioma cells

   PMID:26933087

   Open questions at the time

   • Single target mRNA (BAX) tested for association
   • Single lab; mechanism connecting La-module binding to growth suppression not fully resolved
4. 2018 Medium

   Provided in vivo genetic evidence that LARP4B controls organismal growth, defining dMyc as a key downstream effector and confirming the RNA-binding domains are required.

   Evidence Drosophila transgenic overexpression and loss-of-function, domain-deletion transgenes, dMyc genetic epistasis, and Western blot

   PMID:29462618

   Open questions at the time

   • Whether dMyc regulation is direct (mRNA binding) versus indirect not resolved
   • Mammalian conservation of the dMyc/MYC axis not tested here
5. 2020 High

   Resolved the structural basis of the LARP4B–PABPC1 interaction, revealing a variant PAM2w–MLLE binding mode and validating the contacts functionally.

   Evidence X-ray crystallography of the PAM2w–MLLE complex with mutational analysis in vitro and in vivo

   PMID:32517187

   Open questions at the time

   • Functional consequence of disrupting PABPC1 binding on specific mRNA targets not detailed
   • How this contact integrates with La-module mRNA binding not addressed
6. 2024 Medium

   Identified RACK1 as a direct ribosome-associated partner via conserved region-2, mechanistically coupling LARP4B to the ribosome and to ARE-mRNA stabilization and translation.

   Evidence Yeast two-hybrid domain mapping, Co-IP, polysome profiling, ARE reporter assays, CR2 mutagenesis, and AlphaFold2-multimer prediction (preprint)

   PMID:bio_10.1101_2024.11.01.621267

   Open questions at the time

   • Preprint, single lab; structural prediction not experimentally validated
   • Reciprocal endogenous interaction and stoichiometry on ribosomes not established
7. 2024 Medium

   Established LARP4B as a paralog-specific cell-cycle regulator, distinguishing its function from LARP4A and identifying affected cell-cycle proteins.

   Evidence siRNA/shRNA depletion with transcriptomic profiling, high-content multiparametric imaging, and xenografts in osteosarcoma and prostate cancer cells

   PMID:38532886

   Open questions at the time

   • Whether cell-cycle gene changes are direct mRNA targets versus secondary not resolved
   • Single lab
8. 2024 Medium

   Connected LARP4B's mRNA-stabilizing activity to defined oncogenic signaling outputs by showing it stabilizes specific transcripts to drive disease-relevant pathways.

   Evidence RIP-seq/RIP, RNA pulldown, gain/loss-of-function, pathway readouts (SPINK1–EGFR; WNK1–Keap1/NRF2/GCH1/BH4 ferroptosis), and xenografts in HCC and pancreatic cancer

   PMID:38693111 PMID:41014504

   Open questions at the time

   • Each axis shown in a single tumor context by a single lab
   • Generality of these targets across tissues not established

• How the La-module mRNA recognition, PABPC1 binding, and RACK1/ribosome coupling are integrated into a unified mechanism that selects which target mRNAs are stabilized versus translationally enhanced in a given cell type remains unresolved.
• No integrated structural/biochemical model linking all three binding activities on a single mRNP
• Rules governing target selectivity across tissues not defined