SNX20

SNX20 is a sorting nexin of the SNX-PXB sub-family that directly binds the cytoplasmic domain of PSGL-1 and, through its phosphoinositide-binding PX domain, targets the SNX20–PSGL-1 complex to endosomes, thereby cycling PSGL-1 into the endosomal compartment (PMID:18196517). Loss of SNX20 in a murine knockout model does not impair PSGL-1-dependent leukocyte adhesion or signaling, indicating that SNX20 functions specifically in PSGL-1 endosomal sorting rather than in adhesion or signal transduction (PMID:18196517).

1. 2008 Medium

   Identification of SNX20 as a direct PSGL-1 cytoplasmic-domain interactor established that a sorting nexin participates in PSGL-1 trafficking, answering the question of how PSGL-1 is routed to endosomes.

   Evidence Yeast two-hybrid screen, reciprocal co-immunoprecipitation, and motif mapping in human cells; PX-domain-dependent colocalization to endosomes; murine knockout with adhesion and signaling assays

   PMID:18196517

   Open questions at the time

   • Single-lab study; independent replication in a second system is lacking
   • Structural basis of SNX20–PSGL-1 interaction is unresolved
   • Functional consequence of disrupted PSGL-1 endosomal sorting (e.g., surface turnover kinetics) was not quantified
2. 2018 Low

   Classification of SNX20 within the SNX-PXB sub-family suggested that its PXB domain serves as an endosome-associated scaffold, extending the mechanistic model from simple phosphoinositide binding to a platform for additional protein–protein interactions.

   Evidence Bioinformatic domain analysis and functional analogy drawn from quantitative proteomics of paralog SNX21

   PMID:30072438

   Open questions at the time

   • No direct experimental data on the SNX20 PXB domain; inference is based solely on SNX21 analogy
   • Interactors recruited by the PXB domain of SNX20 have not been identified
   • No structural or mutagenesis data for the SNX20 PXB domain

• It remains unknown whether SNX20 sorts additional cargo beyond PSGL-1, what the physiological consequence of disrupted PSGL-1 endosomal sorting is in vivo, and whether the PXB domain recruits specific effectors to endosomes.
• No unbiased cargo identification screen has been performed for SNX20
• In vivo phenotype of SNX20 loss beyond leukocyte adhesion has not been examined
• No structural model of SNX20 exists