PLP2

PLP2 is a MARVEL-domain transmembrane protein that operates at the interface of cytoskeletal protein folding, ER membrane trafficking, and cell migration (PMID:36921056, PMID:24278019, PMID:34409455). Within the cytosolic chaperonin pathway it acts as a co-chaperone for TRiC/CCT, occupying one chaperonin ring and coordinating substrate translocation on the CCT6 hemisphere to stabilize and facilitate the folding of tubulin and actin across the ATPase cycle (PMID:36921056). At the ER, PLP2 is required for membrane protein egress: it is essential for the KSHV E3 ubiquitin ligase K5 to exit the ER and ubiquitinate plasma membrane immunoreceptors, and its loss causes defective ER trafficking, elevated basal ER stress, and heightened susceptibility to ER stress–induced neuronal death in vivo (PMID:24278019, PMID:26512060). PLP2 also promotes cell migration through direct association with membrane and junctional partners—binding the cytoplasmic tail of the chemokine receptor CCR1 to enhance agonist-induced migration, and associating with ZO-1 to position it at the leading edge and remodel actin via Rac1 during collective migration (PMID:15474493, PMID:34409455). A regulatory promoter variant (-113C>A) that disrupts an ELK1 binding site reduces PLP2 expression and, in patient fibroblasts, links PLP2 loss-of-function to defective ER homeostasis in the setting of X-linked intellectual disability (PMID:26512060, PMID:17416750).

1. 2004 Medium

   Established the first physical and functional partner for PLP2/A4, linking it to chemokine-receptor signaling and cell motility rather than to a purely structural role.

   Evidence Yeast two-hybrid, reciprocal co-IP, colocalization, and migration assays in HOS/CCR1 cells

   PMID:15474493

   Open questions at the time

   • Single-lab interaction without structural mapping of the CCR1-tail contact
   • Mechanism connecting CCR1 binding to migration not resolved
2. 2007 Medium

   Defined a cis-regulatory basis for PLP2 dosage by showing a promoter variant disrupts an ELK1 site and lowers expression, connecting PLP2 to a heritable neurological phenotype.

   Evidence Luciferase reporter assay of -113C>A, X-chromosome microarray and expression validation

   PMID:17416750

   Open questions at the time

   • Reporter assay does not establish the downstream cellular consequence of reduced PLP2
   • Brain expression pattern is descriptive, not mechanistic
3. 2013 High

   Revealed an ER-export function by showing PLP2 is required for a viral E3 ligase to leave the ER, establishing PLP2 as a component of membrane protein trafficking.

   Evidence Haploid forward genetic screen in KBM7 cells plus plasma membrane proteomics

   PMID:24278019

   Open questions at the time

   • Whether PLP2 acts directly on the trafficking machinery or via cargo not defined
   • Host cargo dependent on PLP2 not enumerated
4. 2014 Low

   Extended the PLP2 interaction network to TPD52 and RAB5C, hinting at links to endosomal/vesicular machinery.

   Evidence Yeast two-hybrid and pull-down with interaction domain mapping

   PMID:24604726

   Open questions at the time

   • Pull-down only, no reciprocal co-IP or functional follow-up
   • No cellular consequence of the interactions tested
5. 2015 High

   Connected the trafficking defect to physiology by showing PLP2 loss disrupts ER homeostasis and sensitizes neurons to ER stress and hypoxia-ischemia, providing a mechanistic basis for its disease link.

   Evidence Plp2 knockout mouse, XLID patient fibroblasts, ER trafficking/stress assays, neonatal hypoxia-ischemia model, overexpression rescue

   PMID:26512060

   Open questions at the time

   • Molecular step in ER trafficking that PLP2 supports not pinpointed
   • Relationship between ER role and chaperonin role not integrated
6. 2015 Low

   Proposed a proliferation/invasion signaling role via PI3K-AKT activation in melanoma.

   Evidence Cited as established in a miR-664/melanoma study without primary experimental detail

   PMID:26287415

   Open questions at the time

   • No direct experimental demonstration of PI3K binding in this report
   • Not independently validated within the timeline
7. 2021 Medium

   Defined a leading-edge migration mechanism in which PLP2 directs ZO-1 to drive Rac1-dependent actin remodeling and collective migration in colorectal cancer.

   Evidence Co-IP, wound-healing/collective migration assays, ZO-1 imaging, Rac1 activity assay, exosome isolation

   PMID:34409455

   Open questions at the time

   • Single-lab; direct vs indirect PLP2–ZO-1 contact not structurally defined
   • How an ER/chaperonin protein reaches the leading edge and exosomes unclear
8. 2022 Medium

   Showed a PLP2-derived peptide perturbs actin dynamics and ER-calcium to trigger necrotic death, providing functional evidence that PLP2 sequences engage the actin/calcium axis.

   Evidence In vitro cytotoxicity, F-actin polymerization, calcium imaging, PARP-1 cleavage western, in vivo syngeneic melanoma model

   PMID:35190586

   Open questions at the time

   • Peptide behavior may not reflect full-length PLP2 function
   • Direct molecular target of the Rb4 peptide not identified
9. 2023 High

   Resolved the molecular function of PLP2 as a TRiC/CCT co-chaperone that coordinates tubulin and actin folding, unifying its links to cytoskeletal biology.

   Evidence Cryo-EM of TRiC–plp2–tubulin/actin complexes across ATPase cycle states

   PMID:36921056

   Open questions at the time

   • How the chaperonin role relates to PLP2's ER-membrane and migration functions not established
   • Cellular requirement for PLP2 in folding native tubulin/actin not quantified in human cells here

• It remains unknown how PLP2's distinct roles—cytosolic TRiC co-chaperone, ER-resident trafficking factor, and membrane/junctional migration regulator—are reconciled within one transmembrane protein.
• No model linking topology/localization to the multiple activities
• Substrate or cargo specificity in the ER-trafficking role undefined