CDC42BPB

CDC42BPB (MRCKβ) is a broadly expressed serine/threonine protein kinase that acts as a downstream effector of CDC42 to regulate the actin cytoskeleton, epithelial polarity, and membrane morphogenesis (PMID:10198171, PMID:31113848). Through phosphorylation of nonmuscle myosin light chain it links CDC42 activation to actin-myosin contractility, and in epithelial cells it coordinates apical membrane morphogenesis, lumen formation, and junction maturation in response to localized CDC42 signaling (PMID:10198171, PMID:31113848). In the renal collecting duct, the kinase localizes to the apical plasma membrane where its abundance changes upon vasopressin stimulation, and it interacts with and phosphorylates the urea channel UT-A1, implicating it in signal-induced F-actin and membrane-transport regulation (PMID:24085853, PMID:29046292). CDC42BPB also has substrate roles in disease contexts: it phosphorylates the E3 ligase Siah2 at Ser6/Thr279 to stabilize Siah2 and promote tumorigenicity, while Siah2 reciprocally ubiquitinates MRCKβ to drive its proteasomal degradation (PMID:33536006), and it phosphorylates AURKA to upregulate PD-L1 via cMYC, with its loss or inhibition restoring tumor susceptibility to T cell killing (PMID:39086134). In vivo, loss of Cdc42bpb in mice produces ventral diaphragmatic hernias and heart septal defects, establishing the gene as a genetic cause of congenital diaphragmatic hernia (PMID:42124667). Heterozygous loss-of-function and de novo missense variants in the kinase, citron homology, and coiled-coil domains cause neurodevelopmental disorders including developmental delay, intellectual disability, and autism (PMID:32031333).

1. 1999 Medium

   Established the molecular identity and core enzymatic activity of the human gene by cloning it as a serine/threonine kinase acting on nonmuscle myosin light chain.

   Evidence cDNA cloning, Northern blot, and FISH mapping of the human MRCKβ homologue

   PMID:10198171

   Open questions at the time

   • Myosin light chain phosphorylation inferred from rat ortholog, not directly re-demonstrated
   • No structural or substrate-site characterization in human protein
2. 2009 Low

   Addressed whether the kinase participates in intracellular trafficking by implicating it in mannose-6-phosphate receptor membrane transport.

   Evidence High-throughput kinase siRNA/inhibitor fluorescence imaging screen of CI-M6PR trafficking in HeLa cells

   PMID:19210549

   Open questions at the time

   • Screen hit without mechanistic follow-up specific to CDC42BPB
   • No biochemical validation of the trafficking phenotype
3. 2013 Medium

   Connected the kinase to physiological signaling by showing its apical membrane abundance in collecting duct cells changes with vasopressin, suggesting a role in signal-induced actin remodeling.

   Evidence Quantitative SILAC mass spectrometry with apical surface biotinylation in mouse cortical collecting duct cells

   PMID:24085853

   Open questions at the time

   • Abundance change does not establish a direct kinase substrate in this context
   • Functional consequence for actin dynamics not directly assayed
4. 2017 Medium

   Identified a tissue-specific substrate by showing the kinase binds and phosphorylates the urea channel UT-A1, linking it to renal transport regulation.

   Evidence Cross-linking, Co-IP/LC-MS/MS, and in vitro kinase assay on UT-A1 peptides in rat inner medullary collecting duct

   PMID:29046292

   Open questions at the time

   • Phosphorylation shown on peptides, not validated on full-length UT-A1 in vivo
   • Functional effect on channel activity not measured
5. 2020 Medium

   Established human disease relevance by linking heterozygous loss-of-function and de novo missense variants to neurodevelopmental disorders.

   Evidence Whole-exome/genome sequencing of 14 unrelated individuals with de novo confirmation and in silico domain mapping

   PMID:32031333

   Open questions at the time

   • No direct biochemical assay of variant effects on kinase activity
   • Mechanism connecting haploinsufficiency to brain phenotype unresolved
6. 2021 High

   Revealed a reciprocal kinase-ligase relationship by showing MRCKβ phosphorylates and stabilizes Siah2 while being targeted for degradation by it, defining an oncogenic feedback loop.

   Evidence Co-IP/MS, site-directed mutagenesis (S6A/T279A), MG132 rescue, and tumorigenicity assays in H. pylori-infected gastric epithelial cells

   PMID:33536006

   Open questions at the time

   • Whether this circuit operates outside gastric cancer not addressed
   • Upstream signals controlling the loop not defined
7. 2024 Medium

   Connected the kinase to tumor immune evasion by showing it phosphorylates AURKA to upregulate PD-L1 via cMYC and modulate anti-PD-1 response.

   Evidence CRISPR-Cas9 kinome screen, BDP5290 inhibition, in vitro T cell killing, and in vivo breast cancer models

   PMID:39086134

   Open questions at the time

   • Direct biochemical AURKA phosphorylation assay not detailed
   • Generality beyond breast cancer not established
8. 2026 Medium

   Established CDC42BPB as a genetic cause of congenital diaphragmatic hernia through in vivo loss-of-function and patient-variant modeling.

   Evidence Mouse knockout and CRISPR/Cas9 patient-variant knock-in with embryonic phenotyping of diaphragm, heart, and lung (preprint)

   PMID:42124667

   Open questions at the time

   • Preprint, not yet peer-reviewed
   • Cellular mechanism linking kinase loss to diaphragm defect not resolved

• The direct in-cell substrate repertoire that mediates the cytoskeletal, developmental, and disease phenotypes, and how CDC42 activation is mechanistically transduced through MRCKβ in each tissue, remain undefined.
• No unifying mechanistic link between actin regulation and the diverse disease phenotypes
• No structural model of substrate recognition
• Variant effects on kinase activity not biochemically tested