UNC119B

UNC119B is a myristoyl-binding chaperone that solubilizes and traffics N-myristoylated cargo proteins to the cilium (PMID:22085962). It captures monomyristoylated cargo such as NPHP3, GNAT1, and Cystin1 within a hydrophobic β-sandwich pocket lined by conserved phenylalanines, binding only mono-N-myristoylated peptides and not doubly lipidated or isoprenylated termini (PMID:22085962, PMID:26503308). Binding affinity ranges from subnanomolar to micromolar and is set by the residues at the +2 and +3 positions relative to the myristoylated glycine; swapping these residues reverses affinity and mislocalizes cargo (PMID:27481943). Cargo release is governed by an ARL GTPase switch: high-affinity ciliary cargo is displaced specifically by ARL3-GTP, whereas low-affinity cargo can also be released by ARL2-GTP, and efficient ciliary targeting of NPHP3 requires both ARL3 and its GAP RP2 (PMID:22085962, PMID:27481943). Consistent with a role in ciliary lipidated-protein trafficking, UNC119B expression partially rescues cilia defects caused by CEP290 deficiency in zebrafish and human patient cells (PMID:34155518). Beyond cilia, UNC119B engages the prenylated C-terminus of Rac1 to inhibit insulin-stimulated Rac1 activation and GLUT4 translocation, with its loss improving insulin sensitivity in rodent models (PMID:40754229), and it acts as an allosteric activator of PSKH1 kinase through direct engagement of the kinase domain (PMID:39964718).

1. 2011 High

   Established that UNC119B, distinct from UNC119A, is a myristoyl-binding chaperone required for ciliary delivery of myristoylated cargo, answering how lipidated cargo such as NPHP3 reaches the cilium.

   Evidence Proteomic pulldown, structural modeling with mutagenesis of β-sandwich phenylalanines, and siRNA knockdown in IMCD3 cells with ciliary localization and ARL3-GTP release readouts

   PMID:22085962

   Open questions at the time

   • Cargo repertoire beyond NPHP3 not defined
   • Atomic structure of UNC119B-myristate complex not determined in this study
2. 2012 Medium

   Consolidated UNC119B as a lipid-binding chaperone with myristoyl specificity whose depletion blocks NPHP3 ciliary trafficking, reinforcing its direct trafficking role.

   Evidence siRNA knockdown with ciliary localization readout and biochemical lipid-binding characterization (review consolidation)

   PMID:23000199

   Open questions at the time

   • Largely confirmatory of prior primary work
   • No new mechanistic detail on release step
3. 2015 Medium

   Defined the lipid-recognition rule, showing the chaperone reads mono-N-myristoylation exclusively and rejects double-lipidated or isoprenylated termini.

   Evidence Binding assays of synthetic lipid-modified peptides to purified chaperone

   PMID:26503308

   Open questions at the time

   • In vitro peptide assays only; cellular consequence of discrimination not tested
   • Single lab
4. 2016 High

   Resolved how cargo affinity is encoded and how the ARL GTPase switch sorts cargo, explaining selective ARL3-GTP-dependent release of high-affinity ciliary cargo.

   Evidence Fluorescence polarization affinity measurements, X-ray crystallography of myristoylated NPHP3 peptide complex, and +2/+3 residue swap mutagenesis with cellular mislocalization assay

   PMID:27481943

   Open questions at the time

   • Structure solved with UNC119a paralog rather than UNC119B
   • In vivo affinity thresholds for additional cargo not mapped
5. 2021 Medium

   Placed UNC119B within the CEP290-dependent ciliary trafficking pathway by showing it can compensate for CEP290 loss.

   Evidence Morpholino knockdown rescue by ectopic unc119b expression in zebrafish and analysis of human Joubert syndrome CEP290 patient cells

   PMID:34155518

   Open questions at the time

   • Epistatic order relative to CEP290 not fully resolved
   • Rescue only partial for photoreceptor defects
6. 2025 Medium

   Extended UNC119B function beyond cilia, identifying it as a negative regulator of Rac1-driven GLUT4 translocation and insulin signaling.

   Evidence Co-immunoprecipitation, in vitro binding competition with the prenylated Rac1 C-terminus and compound C59, and genetic knockdown/overexpression in cells and rodent insulin-resistance models

   PMID:40754229

   Open questions at the time

   • How prenyl-cargo binding reconciles with myristoyl specificity not explained
   • Single lab
7. 2025 Medium

   Identified a kinase-regulatory role, with UNC119B acting as an allosteric activator of PSKH1.

   Evidence Mass spectrometry interactome analysis and kinase activity assays with direct kinase-domain engagement

   PMID:39964718

   Open questions at the time

   • Structural basis of allosteric activation unknown
   • Physiological context of UNC119B-PSKH1 signaling undefined

• How UNC119B reconciles myristoyl-specific cargo capture with binding to a prenylated Rac1 terminus, and how its ciliary, metabolic, and kinase-regulatory functions are coordinated in vivo, remains unresolved.
• No unified structural model spanning myristoyl and prenyl cargo
• Tissue- and pathway-specific partitioning of functions not defined